0:00:06 > 0:00:11In 1869, a wild-haired Russian chemist had an extraordinary vision.
0:00:14 > 0:00:19He'd been struggling with a mystery that had perplexed scientists for generations.
0:00:21 > 0:00:26And for the very first time, he'd glimpsed nature's building blocks,
0:00:26 > 0:00:29the elements, arranged in their natural order.
0:00:30 > 0:00:33His name was Dmitri Mendeleev,
0:00:33 > 0:00:38and he was on the brink of cracking the secret code of the Cosmos,
0:00:38 > 0:00:43what was to become one of man's most beautiful creations -
0:00:43 > 0:00:46the Periodic Table of Elements.
0:00:49 > 0:00:51This is the story of those elements,
0:00:51 > 0:00:54the building blocks that make up the universe...
0:00:55 > 0:00:58..the remarkable tale of their discovery,
0:00:58 > 0:01:03and how they fit together, reveals how the modern world was made.
0:01:05 > 0:01:07My name is Jim Al-Khalili.
0:01:07 > 0:01:11And ever since I started studying the mysteries of matter,
0:01:11 > 0:01:16I've been fascinated by chemistry's explosive history...
0:01:16 > 0:01:18Ho-ho! Brilliant!
0:01:18 > 0:01:21'..I've discovered some exciting elements...'
0:01:21 > 0:01:23That's fantastic!
0:01:23 > 0:01:27'..and I've seen how chemistry was forged
0:01:27 > 0:01:30'in the furnaces of the alchemists.
0:01:30 > 0:01:33'Now I'm going to continue my journey.
0:01:36 > 0:01:41'I'll take up the quest of the chemical pioneers...'
0:01:41 > 0:01:43Well, my arm's burning up.
0:01:43 > 0:01:47'...as they struggled to make sense of elemental chaos
0:01:47 > 0:01:51'and conquer our fundamental fear of disorder.
0:01:51 > 0:01:56'Could there be a grand plan underlying the elements?
0:01:57 > 0:02:00'I'll take part in some volatile experiments...'
0:02:00 > 0:02:04Now we're going to drop in the potassium.
0:02:04 > 0:02:07Wow, look at that! Wahey!
0:02:07 > 0:02:11'..and witness some fiery reactions.'
0:02:11 > 0:02:15And I'll find out how the hidden order of the natural world
0:02:15 > 0:02:21was revealed in all its glory - the order of the elements.
0:02:41 > 0:02:46As a nuclear physicist, I've spent a lifetime studying the sub-atomic world,
0:02:46 > 0:02:48the basic building blocks of matter.
0:02:48 > 0:02:53But to do that, I need to understand the ingredients of OUR world...
0:02:54 > 0:02:57..the elements.
0:02:57 > 0:03:01Our planet was created from just 92 elements.
0:03:01 > 0:03:06The ground we walk on, the air that we breathe,
0:03:06 > 0:03:08the stars we gaze at,
0:03:08 > 0:03:10even us.
0:03:12 > 0:03:15Our bodies are entirely made of elements.
0:03:17 > 0:03:20We now know the name and number
0:03:20 > 0:03:24of every naturally-occurring element in existence.
0:03:24 > 0:03:25But 200 years ago,
0:03:25 > 0:03:29those elements were only just beginning to give up their secrets.
0:03:32 > 0:03:35At the beginning of the 19th century,
0:03:35 > 0:03:37only 55 had been discovered,
0:03:37 > 0:03:39from liquid mercury
0:03:39 > 0:03:42to dazzling magnesium...
0:03:44 > 0:03:46..and volatile iodine.
0:03:50 > 0:03:54Scientists had no idea how many more they might find,
0:03:54 > 0:03:57or whether there could be an infinite number.
0:04:01 > 0:04:05But the big question was, how did they fit together?
0:04:05 > 0:04:07Were they random stars,
0:04:07 > 0:04:11or was the elemental world born of order and logic?
0:04:19 > 0:04:23Solving the puzzle would prove to be a daunting challenge.
0:04:23 > 0:04:29And the first glimmerings of an answer came from an unlikely source.
0:04:31 > 0:04:35John Dalton was an intelligent, modest man,
0:04:35 > 0:04:40and he had one very British passion - the weather.
0:04:40 > 0:04:45He was born here in the Lake District in 1766.
0:04:45 > 0:04:49He was so clever, that as a young boy, just 12 years old,
0:04:49 > 0:04:54he was already teaching other kids at a school that he set up.
0:04:54 > 0:04:57Walking home, he loved watching the weather systems
0:04:57 > 0:04:59sweeping across the fells.
0:05:04 > 0:05:10He was so obsessed that he kept a meteorological diary for 57 years,
0:05:10 > 0:05:13and every single day, come rain or shine,
0:05:13 > 0:05:19he entered his precise observations - 200,000 of them.
0:05:30 > 0:05:36Dalton was a quiet, retiring man with modest habits.
0:05:41 > 0:05:46He was a lifelong bachelor, with not much in the way of a social life.
0:05:46 > 0:05:50His only recreation was a game of bowls once a week,
0:05:50 > 0:05:52every Thursday afternoon.
0:05:55 > 0:05:57He was certainly a creature of habit,
0:05:57 > 0:05:59and he might sound a bit dull.
0:05:59 > 0:06:04But actually, Dalton was an avid reader and a deep thinker.
0:06:04 > 0:06:07Underneath his mild-mannered exterior,
0:06:07 > 0:06:10his head was teeming with radical ideas.
0:06:17 > 0:06:21Now scientists had recently discovered something very important
0:06:21 > 0:06:25about the way elements combine to form compounds.
0:06:25 > 0:06:29When they do so, they always combine in the same proportions.
0:06:29 > 0:06:33Dalton would have known that table salt, sodium chloride,
0:06:33 > 0:06:38is always made up of one part sodium and one part chlorine.
0:06:38 > 0:06:42So it doesn't matter whether the salt comes from Salt Lake City
0:06:42 > 0:06:48or Siberia, it's always in the same proportion by weight, every time.
0:06:48 > 0:06:50Dalton reckoned for this to happen,
0:06:50 > 0:06:55each element had to be made up of its own unique building blocks,
0:06:55 > 0:06:59what he called "ultimate particles", atoms.
0:07:02 > 0:07:07It was a blinding illumination, completely left field.
0:07:07 > 0:07:11Everything, he suggested, the entire universe,
0:07:11 > 0:07:15was made up of infinitesimally small particles.
0:07:18 > 0:07:22The Greeks had hit on the idea of the atom 2,000 years earlier,
0:07:22 > 0:07:24but abandoned it.
0:07:24 > 0:07:29Now, Dalton took up the baton with his own theory of matter.
0:07:32 > 0:07:36What Dalton was describing was revolutionary.
0:07:36 > 0:07:38He had struck on the foundations of atomic theory,
0:07:38 > 0:07:44foreshadowing research that wouldn't be proved until a century later.
0:07:44 > 0:07:47He proposed that there are as many kinds of atoms
0:07:47 > 0:07:49as there are elements.
0:07:49 > 0:07:51And just as each element is different,
0:07:51 > 0:07:55so each element's atom has a different weight -
0:07:55 > 0:07:57a unique atomic weight.
0:08:05 > 0:08:09Every element has its own signature atomic weight,
0:08:09 > 0:08:13whether it be a solid, a liquid, or even a gas.
0:08:13 > 0:08:17These three balloons are each filled with a different gas.
0:08:17 > 0:08:19Now they are roughly the same size,
0:08:19 > 0:08:22so they should each have about the same number of atoms in.
0:08:22 > 0:08:26Dalton reckoned that different atoms have different atomic weights.
0:08:26 > 0:08:29So these three balloons should each weigh different amounts.
0:08:29 > 0:08:34So this red balloon is filled with helium gas.
0:08:34 > 0:08:36And if I release it,
0:08:36 > 0:08:37it floats.
0:08:37 > 0:08:39Helium is very light.
0:08:43 > 0:08:47This second balloon is filled with argon gas.
0:08:47 > 0:08:49And if I release it,
0:08:49 > 0:08:51it sinks slowly.
0:08:51 > 0:08:53Argon is heavier than helium.
0:08:56 > 0:09:00The third balloon is filled with krypton gas. And if I let it go,
0:09:00 > 0:09:03it falls like a stone.
0:09:03 > 0:09:05So Dalton was on the right lines -
0:09:05 > 0:09:09different atoms of different elements have different weights.
0:09:14 > 0:09:18Based on this theory, and working completely alone,
0:09:18 > 0:09:21Dalton made one of the first attempts
0:09:21 > 0:09:26to impose some order on the unruly world of the elements.
0:09:26 > 0:09:31This wonderfully mystical set of symbols is Dalton's line-up
0:09:31 > 0:09:35of the elements arranged by weight.
0:09:35 > 0:09:39Now there are some elements here that I don't even recognise,
0:09:39 > 0:09:41but he does start with hydrogen at one.
0:09:41 > 0:09:45Then you go down to oxygen at seven,
0:09:45 > 0:09:49and all the way down to mercury at 167.
0:09:50 > 0:09:54As it turned out, Dalton didn't get all of his weights right.
0:09:54 > 0:09:58But he had made a huge theoretical leap
0:09:58 > 0:10:02working purely from his mind's eye.
0:10:03 > 0:10:05Two hundred years ago,
0:10:05 > 0:10:09John Dalton was using his imagination as a microscope.
0:10:09 > 0:10:14But today, we have the technology to see the contours of individual atoms
0:10:14 > 0:10:17with this scanning tunnelling microscope.
0:10:17 > 0:10:22It's not like a normal microscope because it doesn't use light.
0:10:22 > 0:10:25Atoms are less than one millionth of a millimetre across,
0:10:25 > 0:10:28which is smaller than the wavelength of visible light.
0:10:28 > 0:10:31This microscope uses electrons
0:10:31 > 0:10:34to scan across the surface of materials,
0:10:34 > 0:10:37picking out individual atoms.
0:10:40 > 0:10:44The images it produces are striking.
0:10:45 > 0:10:49These are atoms of shining silicon.
0:10:49 > 0:10:53These are carbon atoms.
0:10:53 > 0:10:57This is what gold atoms look like.
0:10:57 > 0:11:00And these are atoms of copper.
0:11:05 > 0:11:10Copper is a lustrous metal, essential for life.
0:11:10 > 0:11:16It fuelled the move out of the Stone Age into the Bronze Age.
0:11:18 > 0:11:22Copper nuggets can be found on the earth's surface,
0:11:22 > 0:11:25but it usually needs to be extracted from ores.
0:11:25 > 0:11:30And copper compounds run in the veins of some animals.
0:11:30 > 0:11:37The blood of the octopus is blue, along with snails, and spiders.
0:11:41 > 0:11:44John Dalton's idea in the early 1800s,
0:11:44 > 0:11:47that elements had different atomic weights,
0:11:47 > 0:11:51was dismissed by many scientists.
0:11:51 > 0:11:54But one man believed in him -
0:11:54 > 0:11:57Swedish chemist Jons Jakob Berzelius.
0:11:59 > 0:12:04Berzelius was obsessed with imposing some kind of order on the elements.
0:12:04 > 0:12:08He was convinced that knowing more about the weight of each element
0:12:08 > 0:12:11was somehow vitally important.
0:12:11 > 0:12:14And when he heard about Dalton's theory,
0:12:14 > 0:12:16he came up with an ambitious plan.
0:12:16 > 0:12:19It was a gargantuan task.
0:12:19 > 0:12:20In fact, it seems almost mad.
0:12:20 > 0:12:23This lone Swedish chemist set out
0:12:23 > 0:12:28to measure precisely the atomic weight of every single element,
0:12:28 > 0:12:31and this without a shred of proof that atoms even existed.
0:12:31 > 0:12:37But before Berzelius could start, he was going to have to purify,
0:12:37 > 0:12:41dilute, filter each element incredibly accurately.
0:12:41 > 0:12:44And that was far from straightforward.
0:12:44 > 0:12:45At the time,
0:12:45 > 0:12:48very little of the crucial chemical apparatus
0:12:48 > 0:12:52needed for work of this precision had even been invented.
0:12:52 > 0:12:56But that wasn't going to stop a man like Berzelius.
0:12:56 > 0:12:57He was on a mission.
0:12:59 > 0:13:02So Berzelius set out to make his own lab equipment.
0:13:03 > 0:13:07- Ah, Liam.- Hi, Jim. Nice to meet you. Come through to the hotshop.
0:13:07 > 0:13:09'Liam Reeves, a professional glassblower
0:13:10 > 0:13:14'at the Royal College of Art will show me how Berzelius did it.
0:13:21 > 0:13:23'Glassblowing is physically demanding,
0:13:23 > 0:13:27'and calls for working at punishingly high temperatures.
0:13:27 > 0:13:29'Berzelius must have been very dedicated.'
0:13:29 > 0:13:31I'm getting the glass out now,
0:13:31 > 0:13:35which is at about 1,000 degrees centigrade.
0:13:39 > 0:13:43I'm using a wooden block just to cool and shape the glass.
0:13:43 > 0:13:45What is it you're making?
0:13:45 > 0:13:47It will be a round-bottomed flask,
0:13:47 > 0:13:50which would have been part of the basic chemistry equipment
0:13:51 > 0:13:54that Berzelius would have used. Now I'm going to introduce some air,
0:13:54 > 0:13:57which I'll trap in the pipe and the heat makes expand.
0:14:03 > 0:14:08Wow! How hard would it have been for Berzelius to learn to do this?
0:14:08 > 0:14:11They say it takes 12 years to kind of...to really master glass.
0:14:11 > 0:14:13He was a very skilled glassblower
0:14:13 > 0:14:16from the evidence that I've seen of his work.
0:14:16 > 0:14:20What he was making was high-precision apparatus,
0:14:20 > 0:14:23so that must have made it far more difficult
0:14:23 > 0:14:26than your average vase or tumbler.
0:14:26 > 0:14:30From the pictures that I've seen, I've got no idea how he made it.
0:14:30 > 0:14:34- Really?- Yeah. No idea. So I'm just making the top of the bottle now.
0:14:52 > 0:14:54Right, so that's a basic round-bottomed flask
0:14:54 > 0:14:57very much like one that Berzelius would have made.
0:14:57 > 0:15:03Glassblowing isn't something theoretical physicists like me normally do.
0:15:03 > 0:15:05But I want to find out for myself
0:15:05 > 0:15:08just how hard it is to master this new skill.
0:15:08 > 0:15:10OK, just turn a little bit slower.
0:15:10 > 0:15:12Come back ever so slightly.
0:15:15 > 0:15:17Ah! Well, my arm's burning up.
0:15:17 > 0:15:20- I'll shield you, actually. - Oh, that's better.
0:15:24 > 0:15:26'It's going rather well.'
0:15:29 > 0:15:30SNAP!
0:15:30 > 0:15:32Oh-h!
0:15:32 > 0:15:34Oh, well.
0:15:36 > 0:15:39That just goes to show how difficult this is.
0:15:39 > 0:15:41So it does take 12 years to do.
0:15:41 > 0:15:45I think you would have managed it in seven or eight.
0:15:45 > 0:15:49There's my flask dying slowly, melting away.
0:15:49 > 0:15:52I mean, it just goes to prove how incredibly talented
0:15:52 > 0:15:55Berzelius was - he wasn't making something basic like this,
0:15:55 > 0:15:57he was making some really intricate stuff.
0:15:57 > 0:16:04'And although he was searching for elemental order, there was a bonus.'
0:16:04 > 0:16:08The great thing, you see, about Berzelius was that the skills
0:16:08 > 0:16:13he learned as a glassblower led him to an incredible discovery.
0:16:13 > 0:16:17In 1824, he discovered a new element,
0:16:17 > 0:16:22because he found that one of the constituents of glass was silicon.
0:16:23 > 0:16:30Silicon is a semi-metallic element... found within some meteorites.
0:16:31 > 0:16:34Closer to home, it's under your feet.
0:16:35 > 0:16:40The earth's crust is made primarily of silicate minerals.
0:16:40 > 0:16:46Silicon is its second most abundant element, after oxygen.
0:16:46 > 0:16:51It's mostly found in nature as sand or quartz.
0:16:51 > 0:16:57Its man-made compounds can be heat resistant,
0:16:57 > 0:17:01water resistant and non-stick.
0:17:01 > 0:17:06But silicon's ultimate achievement has to be the silicon chip,
0:17:06 > 0:17:11shrinking computers from room size to palm size.
0:17:11 > 0:17:16Silicon was the last of four elements that Berzelius isolated,
0:17:16 > 0:17:20along with thorium, cerium, and selenium.
0:17:20 > 0:17:23He then spent the next decade of his life
0:17:23 > 0:17:27measuring atomic weight after atomic weight after atomic weight
0:17:27 > 0:17:30in an obsessive pursuit of logic
0:17:30 > 0:17:34in the face of the seemingly random chaos of the natural world.
0:17:38 > 0:17:44Berzelius laboriously studied over 2,000 chemical compounds
0:17:44 > 0:17:47with staggering dedication.
0:17:47 > 0:17:52He weighed, he measured and he agonised over the tiniest detail
0:17:52 > 0:17:57until he'd found out the relative weights of 45 different elements.
0:18:00 > 0:18:03Some of his results were remarkably accurate.
0:18:03 > 0:18:06His weight for chlorine, a gas,
0:18:06 > 0:18:11got to within a fifth of a per cent of what we know today.
0:18:13 > 0:18:16But by the time Berzelius produced his results,
0:18:16 > 0:18:20other scientists had started measuring atomic weights
0:18:20 > 0:18:24and come up with completely different answers.
0:18:24 > 0:18:27Now they were pitted against each other,
0:18:27 > 0:18:33perhaps fuelled by an innate desire to find meaning in disorder.
0:18:33 > 0:18:37Berzelius's quest for order was contagious.
0:18:37 > 0:18:40Scientists began looking for patterns everywhere.
0:18:42 > 0:18:46One of these was German chemist Johann Wolfgang Dobereiner.
0:18:49 > 0:18:52He believed that the answer lay not with atomic weights
0:18:52 > 0:18:57but with the elements' chemical properties and reactions.
0:18:59 > 0:19:03'Dr Andrea Sella has studied Dobereiner's work
0:19:03 > 0:19:05'on chemical groups.'
0:19:05 > 0:19:09What Dobereiner had really spotted was that if you considered
0:19:09 > 0:19:11all the elements that were known to that time,
0:19:11 > 0:19:15you could often pick out three - "triads", as he called them,
0:19:15 > 0:19:19which had very, very closely related chemical properties.
0:19:19 > 0:19:23And as an example, we have here the alkali metals.
0:19:23 > 0:19:28And I'm going to take the first and the lightest of them, lithium.
0:19:28 > 0:19:30And we have to store these under oil
0:19:30 > 0:19:35because they tend to react with air and moisture. So here goes lithium.
0:19:35 > 0:19:37Pop it in.
0:19:37 > 0:19:40Oh, look, fizzing away, yeah.
0:19:40 > 0:19:44You can see it fizzing. And the fizzing is hydrogen,
0:19:44 > 0:19:46flammable air, being released.
0:19:46 > 0:19:49And at the same time, it's leaving a pink trail.
0:19:49 > 0:19:52We've put a bit of indicator in there, which is telling us
0:19:52 > 0:19:54that what's left behind is caustic.
0:19:54 > 0:19:57It's actually making an alkaline solution.
0:19:57 > 0:19:59I'm breathing in some caustic soda!
0:19:59 > 0:20:02Well, you're getting a little bit of steam coming off,
0:20:02 > 0:20:04and the reaction is very, very exothermic.
0:20:04 > 0:20:07In other words, the temperature rises a lot,
0:20:07 > 0:20:09and the metal has actually melted.
0:20:09 > 0:20:14The second metal in this triad was sodium.
0:20:14 > 0:20:16And when we drop the sodium in...
0:20:17 > 0:20:22Whoa! Oh, look at that, flashes of light!
0:20:22 > 0:20:25Orange sparks. And those orange sparks are the same colour
0:20:25 > 0:20:27as what you get in streetlights.
0:20:27 > 0:20:29- Streetlights have sodium in them. - Right.
0:20:29 > 0:20:34Well, the third one in the series is potassium.
0:20:34 > 0:20:36The potassium turns out to be the tiger.
0:20:36 > 0:20:39And we may need to stand back.
0:20:39 > 0:20:41- Look at those flashes.- Wow!
0:20:41 > 0:20:43And you can see that lilac flame.
0:20:43 > 0:20:47And one could really see trends in these triads.
0:20:47 > 0:20:49- They're all doing the same thing, aren't they?- Yes.
0:20:49 > 0:20:53The fizzing is telling us that hydrogen is coming off.
0:20:53 > 0:20:55We're getting the alkali being formed.
0:20:55 > 0:20:58But the lithium is relatively tame,
0:20:58 > 0:21:03the sodium was more excitable, the potassium starts getting scary.
0:21:04 > 0:21:08Dobereiner realised that these elements must be a family
0:21:08 > 0:21:11because they reacted in a similar way.
0:21:11 > 0:21:14Here was the hint of a pattern.
0:21:14 > 0:21:17But it only worked on a few of the elements.
0:21:17 > 0:21:23It got scientists no further than atomic weights had done.
0:21:23 > 0:21:27The bigger picture, the universal order of all the elements,
0:21:27 > 0:21:28was still hard to see.
0:21:28 > 0:21:32And that wouldn't change until a breakthrough
0:21:32 > 0:21:36by one of greatest minds in 19th-century science.
0:21:40 > 0:21:48In 1848, in the far west of Siberia, a massive fire destroyed a factory.
0:21:48 > 0:21:51The factory manager faced destitution.
0:21:51 > 0:21:57She was a widow, Maria Mendeleeva, and she made a remarkable sacrifice
0:21:57 > 0:22:03for her precociously intelligent son, 14-year-old Dmitri Mendeleev.
0:22:08 > 0:22:11Maria was well aware of her son's intelligence,
0:22:11 > 0:22:16and with a steely determination she set out to get him an education.
0:22:16 > 0:22:22So, together with Dmitri, she set off on a 1,300-mile journey
0:22:22 > 0:22:25from Siberia to St Petersburg.
0:22:25 > 0:22:29And incredibly, they walked a good part of that journey.
0:22:32 > 0:22:36I'm following in their footsteps to St Petersburg,
0:22:36 > 0:22:39then the capital of the Russian empire.
0:22:42 > 0:22:45After their arduous journey across the Russian steppes,
0:22:45 > 0:22:48mother and son finally arrived at St Petersburg.
0:22:48 > 0:22:52Maria Mendeleeva had got what she wanted,
0:22:52 > 0:22:54but the effort destroyed her.
0:22:54 > 0:22:57She died ten weeks later.
0:23:03 > 0:23:07The story goes that her last words to her son were -
0:23:07 > 0:23:12"Refrain from illusions and seek divine and scientific truth."
0:23:12 > 0:23:16And young Mendeleev promised to obey.
0:23:18 > 0:23:21He studied day and night to fulfil his mother's dream
0:23:21 > 0:23:26and became the most brilliant chemistry student of his generation.
0:23:28 > 0:23:32Chemistry had come a long way since the Greeks' idea of four elements -
0:23:32 > 0:23:37earth, air, fire and water.
0:23:37 > 0:23:40But there was still no order to the 63 elements
0:23:40 > 0:23:43that had so far been discovered.
0:23:43 > 0:23:49Now the search for a pattern gripped some of the best minds in science.
0:23:49 > 0:23:52But no-one could agree how to find it.
0:23:52 > 0:23:55Mendeleev was still a student when he attended
0:23:56 > 0:24:00the world's first ever international chemistry conference.
0:24:00 > 0:24:03The world's chemists had gathered to settle the dispute
0:24:03 > 0:24:09that was holding back their subject, the confusion over atomic weights.
0:24:10 > 0:24:15Mendeleev watched as Sicilian chemist Stanislao Cannizzaro
0:24:15 > 0:24:16stole the show.
0:24:16 > 0:24:18Cannizzaro was still convinced
0:24:18 > 0:24:21that atomic weights held the key to the elements,
0:24:21 > 0:24:25and he'd struck on a wonderful innovation,
0:24:25 > 0:24:28a reliable new way of calculating them.
0:24:28 > 0:24:34He knew that equal volumes of gases contain equal numbers of molecules.
0:24:34 > 0:24:37So instead of working with liquids and solids,
0:24:37 > 0:24:42his breakthrough was to use the densities of gases and vapours
0:24:42 > 0:24:46to measure the atomic weights of single atoms.
0:24:48 > 0:24:52Cannizzaro gave a talk in which he presented striking new evidence
0:24:52 > 0:24:55that won over the assembled chemists.
0:24:55 > 0:24:59So whereas Berzelius's work had failed to convince anyone,
0:24:59 > 0:25:03Cannizzaro's new method set an agreed standard.
0:25:03 > 0:25:08Finally, chemists had a way of measuring atomic weights accurately.
0:25:12 > 0:25:16It was the moment everybody had been waiting for.
0:25:16 > 0:25:19Surely with precise atomic weights
0:25:19 > 0:25:24they would now be able to unravel the mystery of the elements?
0:25:24 > 0:25:28One chemist wrote, "It was as though the scales fell from my eyes
0:25:28 > 0:25:33"and doubt was replaced by peaceful clarity."
0:25:33 > 0:25:35There was a real buzz in the air.
0:25:35 > 0:25:38Finally, it seemed that the order of the elements
0:25:38 > 0:25:40may have been within science's grasp.
0:25:40 > 0:25:43Mendeleev was electrified.
0:25:45 > 0:25:50But chemists soon found that even arranged in order of atomic weight,
0:25:50 > 0:25:53the elements appeared unsystematic.
0:25:53 > 0:25:57They were still missing something vital.
0:25:57 > 0:26:01Then, in 1863, a solitary English chemist
0:26:01 > 0:26:06named John Newlands made an unusual discovery.
0:26:06 > 0:26:10Newlands noticed that when the elements are arranged by weight,
0:26:10 > 0:26:12something very strange happened.
0:26:15 > 0:26:18Imagine each element is like a key on the piano,
0:26:18 > 0:26:20arranged by their atomic weight.
0:26:20 > 0:26:22Then this will be carbon,
0:26:22 > 0:26:24followed by nitrogen,
0:26:24 > 0:26:31oxygen, fluorine, sodium, magnesium, aluminium
0:26:31 > 0:26:34and finally silicon.
0:26:34 > 0:26:37'Thinking of the elements like a musical scale,
0:26:37 > 0:26:42'Newlands reckoned that every octave, every eight notes,
0:26:42 > 0:26:46'certain properties seemed to repeat, to harmonise.'
0:26:48 > 0:26:51He called it a "law of octaves".
0:26:51 > 0:26:55It was the first real attempt to find a law of nature
0:26:55 > 0:26:57that pulled all the known elements together.
0:27:01 > 0:27:03Newlands proudly presented his idea
0:27:03 > 0:27:08to the great and the good of the Chemical Society in 1866.
0:27:08 > 0:27:10It was his big moment.
0:27:10 > 0:27:14But his music analogy didn't seem to strike a chord.
0:27:14 > 0:27:17They completely failed to see his point.
0:27:20 > 0:27:24The assembled chemists said Newlands' idea was ridiculous,
0:27:24 > 0:27:28that he might as well have arranged the elements alphabetically
0:27:28 > 0:27:30for all the insight his theory gave.
0:27:30 > 0:27:34Maybe, they even suggested with biting sarcasm,
0:27:34 > 0:27:37that Newlands could get his elements to play them a little tune.
0:27:39 > 0:27:43It must have been a shattering blow for Newlands.
0:27:50 > 0:27:54But was John Newlands really onto something
0:27:54 > 0:27:56with his curious law of octaves?
0:27:58 > 0:28:00It's such a bizarre concept
0:28:00 > 0:28:04that every eighth element will behave in a similar way.
0:28:04 > 0:28:08It's not surprising that people thought Newlands' idea was mad.
0:28:09 > 0:28:13Here are eight elements in order of their atomic weight,
0:28:13 > 0:28:18and I'm going to explore their properties by smelling them.
0:28:18 > 0:28:21The first element is chlorine.
0:28:21 > 0:28:25It's a yellowy-green gas that's highly toxic.
0:28:25 > 0:28:27If I have a sniff...
0:28:27 > 0:28:30Yep, distinctive smell of bleach.
0:28:30 > 0:28:33The second one is potassium.
0:28:33 > 0:28:36But no odour to it at all.
0:28:36 > 0:28:40'And as I smell my way through the next five elements,
0:28:40 > 0:28:45'calcium, gallium, germanium, arsenic -
0:28:45 > 0:28:47'not poisonous to smell in its pure form -
0:28:47 > 0:28:49'and selenium, there's no scent.'
0:28:49 > 0:28:54Finally number eight, bromine.
0:28:54 > 0:28:55I already see it's a gas,
0:28:55 > 0:28:58like chlorine, a reddish gas, highly toxic.
0:28:58 > 0:29:00I'm going to be very careful,
0:29:00 > 0:29:03because I don't recommend you try this at home.
0:29:03 > 0:29:08Smells very much like chlorine, only a lot worse, a lot stronger.
0:29:08 > 0:29:12And so Newlands' law of octaves seems to work here,
0:29:12 > 0:29:15because the eighth element, bromine, is similar in properties
0:29:15 > 0:29:17to the first one, chlorine.
0:29:19 > 0:29:24'Today we know Newlands' law of octaves as the law of periodicity.
0:29:26 > 0:29:31- '- But at the time, the establishment scoffed.
0:29:31 > 0:29:34- '- And Newlands never got over the slight.
0:29:35 > 0:29:39'The way was left clear for Dmitri Mendeleev,
0:29:39 > 0:29:41'who was thinking along the same lines.'
0:29:44 > 0:29:48I'm on my way to St Petersburg University
0:29:48 > 0:29:53to meet a man who will show me where Mendeleev actually worked.
0:29:56 > 0:29:58Hello, Professor Babaev.
0:29:58 > 0:30:01Hi, I'm Jim. Good to meet you. It's very exciting.
0:30:01 > 0:30:05- OK, well, the museum... - Right, well, lead on.
0:30:05 > 0:30:10'Professor Eugene Babaev is the leading expert on Mendeleev,
0:30:10 > 0:30:14'having studied his work for many years.
0:30:14 > 0:30:19'He's going take me inside Mendeleev's apartment,
0:30:19 > 0:30:23'preserved just as it was during the last years of his life.
0:30:25 > 0:30:27'This is a great honour.
0:30:27 > 0:30:31'Normally, nobody is allowed inside Mendeleev's study.'
0:30:31 > 0:30:37So this is quite a privilege, to be able to come in here.
0:30:37 > 0:30:39Look at this. Fantastic.
0:30:39 > 0:30:45'Mendeleev shut himself away in this room, brooding over the elements.
0:30:45 > 0:30:48'This would become the birthplace
0:30:48 > 0:30:54'of one of science's greatest achievements, the periodic table.'
0:30:54 > 0:30:55And I love this photo of him.
0:30:55 > 0:30:59- This is the photo of 1869, just the year when...- Ah!
0:30:59 > 0:31:03So that's what he looked like when he came up with the periodic table.
0:31:03 > 0:31:08- And these are all his original books. - These are his books, written by him.
0:31:08 > 0:31:09Oh, I see.
0:31:09 > 0:31:13When I say "his books", not owned by him.
0:31:13 > 0:31:16- These are the books that he wrote. - Thousands of volumes.
0:31:16 > 0:31:17That's impressive.
0:31:17 > 0:31:20OK, and if you look at his library, you will be surprised,
0:31:20 > 0:31:26because maybe 10% of the books are devoted to chemistry and physics
0:31:26 > 0:31:30but everything else is economics, technics, er...
0:31:30 > 0:31:36- geography, whatever. - He was a polymath.
0:31:36 > 0:31:40Yes, and his second wife was a painter,
0:31:40 > 0:31:45and one portrait here in profile is just by her work.
0:31:47 > 0:31:51'Mendeleev had such a breadth of intellectual curiosity
0:31:51 > 0:31:56'he became known as the Russian Leonardo da Vinci.'
0:31:58 > 0:32:04These are the clocks which stopped at the moment of his death in 1907.
0:32:04 > 0:32:05- 1907, at twenty past six.- Yeah.
0:32:05 > 0:32:11'It seems as if time has stood still in this room
0:32:11 > 0:32:13'for more than a century.
0:32:13 > 0:32:17'And now that I've seen the inner sanctum
0:32:17 > 0:32:21'where Mendeleev puzzled over the elements, I want to know
0:32:21 > 0:32:26'exactly how he pieced together his masterwork, the periodic table.
0:32:29 > 0:32:33'By 1869, Mendeleev had been trying to find a pattern
0:32:33 > 0:32:36'to the elements for a decade.
0:32:36 > 0:32:40'Whatever order he and the world's chemists tried to impose,
0:32:40 > 0:32:43'there were still elements that wouldn't fit.
0:32:43 > 0:32:47'A universal theory seemed out of reach.
0:32:47 > 0:32:50'But now Mendeleev hit on a new idea.
0:32:50 > 0:32:54'He made up a pack of cards and wrote an element
0:32:54 > 0:32:57'and its atomic weight on each one.'
0:32:57 > 0:32:59Strange though this might sound,
0:32:59 > 0:33:04so began the most memorable card game in the history of science.
0:33:04 > 0:33:06He called it chemical solitaire
0:33:06 > 0:33:10and began laying out cards just to see where there was a pattern,
0:33:10 > 0:33:12whether it all fitted together.
0:33:12 > 0:33:17Now, previously, chemists had grouped the elements in one of two ways,
0:33:17 > 0:33:20either by their properties, like those that react with water,
0:33:20 > 0:33:24or by grouping them by their atomic weight,
0:33:24 > 0:33:28which is what Berzelius and Cannizzaro had done.
0:33:28 > 0:33:34Mendeleev's great genius was to combine those two methods together.
0:33:49 > 0:33:52'The odds were stacked against him.
0:33:52 > 0:33:57'Little more than half the elements we now know had been discovered,
0:33:57 > 0:34:01'so he was playing with an incomplete deck of cards.'
0:34:09 > 0:34:13He stayed up for three days and three nights without any sleep,
0:34:13 > 0:34:16just thinking solidly about the problem.
0:34:16 > 0:34:18Then, on the 17th of February,
0:34:18 > 0:34:23with a snowstorm raging outside, he decided to stay at home.
0:34:23 > 0:34:28He was exhausted and he finally he dozed off.
0:34:30 > 0:34:34'The story goes he had an extraordinary dream.
0:34:34 > 0:34:37'He saw almost all of the 63 known elements
0:34:37 > 0:34:42'arrayed in a grand table which related them together.'
0:34:42 > 0:34:45It was an incredible breakthrough.
0:34:45 > 0:34:50I can imagine Mendeleev feeling like so many other scientific pioneers.
0:34:50 > 0:34:56It's that determination, even desperation, to crack a puzzle,
0:34:56 > 0:34:59and then that eureka moment of revelation.
0:35:01 > 0:35:07Mendeleev had revealed a deep truth about the nature of our world,
0:35:07 > 0:35:13that there is a numerical pattern underlying the structure of matter.
0:35:13 > 0:35:15This is the periodic table
0:35:15 > 0:35:17as we know it today,
0:35:17 > 0:35:18and it's rooted
0:35:18 > 0:35:21in Mendeleev's discovery.
0:35:21 > 0:35:27It decodes and makes sense of the building blocks of the whole world.
0:35:27 > 0:35:30Now, although it's so familiar to us,
0:35:30 > 0:35:34it's on the wall of every chemistry lab in every school in the world,
0:35:34 > 0:35:38if you really look at it, it's actually awe inspiring.
0:35:40 > 0:35:45What's so remarkable is that it reveals the relationships
0:35:45 > 0:35:47between each and every element in order.
0:35:48 > 0:35:52Mendeleev had brilliantly combined elements' atomic weights
0:35:52 > 0:35:54and properties
0:35:54 > 0:36:00into one universal understanding of all the elements.
0:36:00 > 0:36:01Reading it across,
0:36:01 > 0:36:06the atomic weights increase step by step with every element.
0:36:06 > 0:36:08But then, looking at it vertically,
0:36:08 > 0:36:12the elements are grouped together in families of similar properties.
0:36:12 > 0:36:18So over on this side are the alkali metals, from lithium to caesium.
0:36:18 > 0:36:21And then over on the far side are the halogens,
0:36:21 > 0:36:27like poisonous chlorine, bromine and iodine, all very highly reactive.
0:36:27 > 0:36:31And alongside them at the top are the elements important for life -
0:36:31 > 0:36:35carbon, nitrogen, oxygen, all non-metals.
0:36:35 > 0:36:38But in the middle, a vast swathe,
0:36:38 > 0:36:39are all the metals,
0:36:39 > 0:36:43and there are four times as many metals as non-metals.
0:36:43 > 0:36:45Everything is ordered.
0:36:45 > 0:36:47It's a chemical landscape
0:36:47 > 0:36:52and a perfect map of the geography of the elements.
0:36:53 > 0:36:59'Intriguingly, the periodic table didn't always look like this.
0:36:59 > 0:37:02'Professor Babaev is keen to show me a copy
0:37:02 > 0:37:05'of Mendeleev's very first manuscript.'
0:37:05 > 0:37:09So, this is the first draft of Mendeleev's periodic table.
0:37:09 > 0:37:15- You can see the date, 17th February 1869.- And it's in his handwriting.
0:37:15 > 0:37:19I can see the crossings out, you can feel his thought processes.
0:37:19 > 0:37:21Some familiar elements here.
0:37:21 > 0:37:25I see hydrogen, the lightest element, all the way to lead.
0:37:25 > 0:37:28Yeah, yeah. Now you can see some familiar groups,
0:37:28 > 0:37:30like alkali metals, halogens.
0:37:30 > 0:37:34It's got lithium, sodium, potassium.
0:37:34 > 0:37:37It's not like the periodic table that I would be familiar with,
0:37:37 > 0:37:39it's the other way round.
0:37:39 > 0:37:40It took maybe two years
0:37:40 > 0:37:43for Mendeleev to bring it to modern form.
0:37:43 > 0:37:46But it's remarkable that this is the foundations
0:37:46 > 0:37:48of the modern periodic table. It started here.
0:37:53 > 0:37:56'Mendeleev's first draft wasn't perfect.
0:37:58 > 0:38:03'To make his table work, he had to do something astonishing.
0:38:03 > 0:38:09'He had to leave spaces for elements that were still unknown.'
0:38:09 > 0:38:15This is a copy of the first published draft of the periodic table,
0:38:15 > 0:38:20and these question marks are where Mendeleev left gaps.
0:38:20 > 0:38:22You see, he was so confident about his model
0:38:22 > 0:38:24that he wouldn't fudge the results.
0:38:24 > 0:38:26So where the model didn't work,
0:38:26 > 0:38:30he left gaps for elements that had yet to be discovered.
0:38:30 > 0:38:33So, for instance, this question mark here
0:38:33 > 0:38:37he predicted was a metal slightly heavier than its neighbour calcium.
0:38:37 > 0:38:39And here two more metals.
0:38:39 > 0:38:42One he predicted would be dark grey in colour,
0:38:42 > 0:38:45and the other would have a low melting point.
0:38:45 > 0:38:49Mendeleev had the audacity to believe
0:38:49 > 0:38:51that he would, in time, be proved right.
0:38:53 > 0:38:57It's as if Mendeleev was a chemical prophet,
0:38:57 > 0:39:01foretelling the future in a visionary interpretation
0:39:01 > 0:39:03of the laws of matter.
0:39:11 > 0:39:16But before he could claim the glory, his gaps needed explaining.
0:39:16 > 0:39:22And a new way of detecting elements was invented in 1859.
0:39:22 > 0:39:26That was thanks to Gustav Kirchhoff and his colleague,
0:39:26 > 0:39:29the man who made the Bunsen burner.
0:39:29 > 0:39:34Robert Bunsen was a wonderfully intrepid experimenter.
0:39:34 > 0:39:36How's this for dedication?
0:39:36 > 0:39:39He lost his right eye in an explosion in his lab.
0:39:39 > 0:39:43Now, he knew that when different elements burned in the flame
0:39:43 > 0:39:45of his Bunsen burner,
0:39:45 > 0:39:48wonderful colours were revealed. This one is copper.
0:39:53 > 0:39:56This one contains strontium.
0:40:00 > 0:40:03And this one is potassium.
0:40:09 > 0:40:12Bunsen wondered whether every element
0:40:12 > 0:40:15might have a unique colour signature
0:40:15 > 0:40:18and so he and Kirchhoff set to work.
0:40:20 > 0:40:24Kirchhoff knew that when white light is shone through a prism
0:40:24 > 0:40:28it gets split up into all its spectral colours...
0:40:31 > 0:40:33..all the colours of the rainbow,
0:40:33 > 0:40:38from red through yellow to blue and violet.
0:40:38 > 0:40:40And he came up with this.
0:40:40 > 0:40:43It's called a spectroscope.
0:40:43 > 0:40:46It has a prism in the middle
0:40:46 > 0:40:49with two telescopes on either side.
0:40:49 > 0:40:51Bunsen and Kirchhoff then worked together
0:40:51 > 0:40:56to analyse different materials using their new piece of kit.
0:40:56 > 0:41:00So they took a compound containing sodium.
0:41:00 > 0:41:03And if I heat it up in the Bunsen burner,
0:41:03 > 0:41:08the light from the sodium passes through the first telescope
0:41:08 > 0:41:12and gets split up by the prism into its spectral lines.
0:41:12 > 0:41:16They then pass through the second telescope. And if I have a look.
0:41:16 > 0:41:18Yep, I can see the two orange lines
0:41:18 > 0:41:21which are the unique spectrum of sodium.
0:41:21 > 0:41:24No other element would give that pattern.
0:41:24 > 0:41:29Using this technique, they actually discovered two new elements,
0:41:29 > 0:41:32silvery-gold caesium, and rubidium,
0:41:32 > 0:41:37so named because of the ruby-red colour of its spectrum.
0:41:39 > 0:41:42It was this same technique that was used to test
0:41:42 > 0:41:46whether Mendeleev's prediction of gaps was right.
0:41:49 > 0:41:52He'd described in meticulous detail
0:41:52 > 0:41:55an unknown element that followed aluminium in his periodic table.
0:41:55 > 0:42:01He predicted it would be a silvery metal with atomic weight 68.
0:42:01 > 0:42:06Then, in 1875, a French chemist used a spectroscope
0:42:06 > 0:42:10to identify just such an element -
0:42:10 > 0:42:12gallium.
0:42:14 > 0:42:20Gallium is a beautiful silvery-white metal, and it's relatively soft.
0:42:20 > 0:42:24Although Mendeleev predicted its existence,
0:42:24 > 0:42:26it was actually found
0:42:26 > 0:42:31by Parisian chemist Paul Emile Lecoq de Boisbaudran.
0:42:31 > 0:42:34Gallium has a very low melting point.
0:42:34 > 0:42:40And with a boiling point of 2,204 degrees centigrade,
0:42:40 > 0:42:44it's liquid over a wider range of temperatures
0:42:44 > 0:42:46than any other known substance.
0:42:46 > 0:42:51Gallium is used to make semiconductors.
0:42:51 > 0:42:55It's found in light-emitting diodes, LEDs.
0:42:55 > 0:43:01One of gallium's compounds was shown to be effective
0:43:01 > 0:43:04in attacking drug-resistant strains of malaria.
0:43:19 > 0:43:25But even though Mendeleev had left gaps for gallium and other elements,
0:43:25 > 0:43:27his table was not complete.
0:43:29 > 0:43:32There was one group that eluded him completely,
0:43:32 > 0:43:34an entirely new family of elements.
0:43:36 > 0:43:41The story of their discovery began with an other-worldly search
0:43:41 > 0:43:44for an extraterrestrial element.
0:43:49 > 0:43:56In August 1868, a total eclipse of the sun in India was the moment
0:43:56 > 0:44:00that French astronomer Pierre Janssen had been waiting for.
0:44:01 > 0:44:05He knew that it was possible to use a spectroscope
0:44:05 > 0:44:09to identify some elements in the light of the sun.
0:44:09 > 0:44:14But the intensity of sunlight meant that many elements were hidden.
0:44:14 > 0:44:18Janssen hoped to see more during a total eclipse,
0:44:18 > 0:44:20when the sun was less blinding.
0:44:22 > 0:44:24As Janssen studied the eclipse,
0:44:24 > 0:44:28he discovered a colour signature never seen before.
0:44:28 > 0:44:30He was faced with an unknown element.
0:44:30 > 0:44:37The same spectral line was confirmed by another astronomer,
0:44:37 > 0:44:38Norman Lockyer.
0:44:38 > 0:44:42He named it helium, after the Greek sun god,
0:44:42 > 0:44:46because he thought that it could only exist on the sun.
0:44:46 > 0:44:50Enter Scottish chemist William Ramsay,
0:44:50 > 0:44:55who linked extraterrestrial helium to Earth.
0:44:55 > 0:44:59Ramsay experimented with a radioactive rock called cleveite.
0:44:59 > 0:45:02By dissolving the rock in acid,
0:45:02 > 0:45:05he collected a gas with an atomic weight of 4
0:45:05 > 0:45:11and the same spectral signature that Lockyer had seen, helium.
0:45:13 > 0:45:18Helium is the second most abundant element in the universe,
0:45:18 > 0:45:20after hydrogen.
0:45:20 > 0:45:24It was one of the elements produced just after the Big Bang.
0:45:24 > 0:45:29Liquid helium is used to cool superconducting magnets
0:45:29 > 0:45:31for MRI scanners.
0:45:33 > 0:45:37Deep-sea divers rely on helium to counter the narcotic effects
0:45:37 > 0:45:43on the brain of increased nitrogen absorption.
0:45:43 > 0:45:46And it was a vital ingredient in the space race,
0:45:46 > 0:45:50used to cool hydrogen and oxygen for rocket engines.
0:45:53 > 0:45:55Before he discovered helium on Earth,
0:45:55 > 0:46:00William Ramsay had already separated a new gas from the air, argon,
0:46:00 > 0:46:03with an atomic weight of 40.
0:46:04 > 0:46:07Now Ramsay faced a puzzle.
0:46:07 > 0:46:12He realised that the new elements didn't fit the periodic table
0:46:12 > 0:46:16and suggested there must be a missing group,
0:46:16 > 0:46:18so his search began.
0:46:18 > 0:46:24He found three more gases, which he named neon, Greek for "new",
0:46:24 > 0:46:29krypton, meaning "hidden", and xenon, "stranger".
0:46:30 > 0:46:34The group became known as the noble gases
0:46:34 > 0:46:38because they were unreactive and seemed so aloof.
0:46:39 > 0:46:44This family of gases completed the rows on the periodic table.
0:46:47 > 0:46:50Now, Mendeleev may not have known about these elusive elements,
0:46:50 > 0:46:54but he'd established the unshakeable idea of elemental relationships.
0:46:54 > 0:46:58And so he made sure that there was a place on his table
0:46:58 > 0:47:02for every new element, no matter when it was discovered.
0:47:09 > 0:47:12The periodic table is a classic example
0:47:12 > 0:47:15of the scientific method at work.
0:47:18 > 0:47:22From a mass of data, Mendeleev found a pattern.
0:47:22 > 0:47:26It led him to make predictions that could be tested
0:47:26 > 0:47:29by future experiments,
0:47:29 > 0:47:32pointing the way for 20th-century scientists
0:47:32 > 0:47:35to prove him and his theory right.
0:47:38 > 0:47:41By the time he died at the age of 72,
0:47:41 > 0:47:45he was a hero in Russia and a superhero in the world of science.
0:47:48 > 0:47:52His periodic table was immortalised in stone
0:47:52 > 0:47:56here in the centre of St Petersburg,
0:47:56 > 0:48:00and he eventually had an element named after him, mendelevium,
0:48:00 > 0:48:03as well as a crater, the Mendeleev Crater,
0:48:03 > 0:48:06on the dark side of the moon...
0:48:11 > 0:48:16..fitting tributes to a man who came from the Siberian wastelands
0:48:16 > 0:48:20to become the ultimate cartographer of the elements.
0:48:27 > 0:48:33The periodic table had finally created order out of chaos.
0:48:33 > 0:48:37But it tells us nothing about WHY our world is as it is,
0:48:37 > 0:48:40why some elements are energetic,
0:48:40 > 0:48:45others are slow, some inert, others volatile.
0:48:47 > 0:48:48It would be another 40 years
0:48:48 > 0:48:53before an entirely different branch of science came up with an answer.
0:48:55 > 0:49:03In 1909, Ernest Rutherford looked inside the atom for the first time.
0:49:03 > 0:49:06Rutherford proposed that the structure of the atom
0:49:06 > 0:49:08was like a miniature solar system,
0:49:08 > 0:49:12an overwhelmingly empty space with a few tiny electrons
0:49:12 > 0:49:17orbiting randomly around a dense, positively-charged nucleus.
0:49:17 > 0:49:21But it wasn't until Niels Bohr came along, one-time goalkeeper
0:49:21 > 0:49:26for the Danish football squad and future Nobel prize-winning physicist
0:49:26 > 0:49:29that things really kicked off.
0:49:30 > 0:49:35He suggested that the electrons orbited around the nucleus
0:49:35 > 0:49:36in fixed shells.
0:49:36 > 0:49:40And it was his idea that was to lead to the discovery that these shells
0:49:40 > 0:49:44could only accommodate a set number of electrons.
0:49:49 > 0:49:54Imagine this football pitch is an atom, a single atom of an element.
0:49:54 > 0:49:56This is the nucleus.
0:49:56 > 0:50:00If this nucleus were to scale, my nearest orbiting electrons
0:50:00 > 0:50:05would be beyond the stands, so I've scaled it down.
0:50:05 > 0:50:08Here, on the shell nearest to the nucleus,
0:50:08 > 0:50:11there can be just two electrons, then it's full.
0:50:13 > 0:50:15Here in the second shell,
0:50:15 > 0:50:20there can be eight electrons, then it's fully occupied, too.
0:50:22 > 0:50:27The third shell is happy with 18 electrons. And so it goes on.
0:50:27 > 0:50:31Outer shells can accommodate an increasing number of electrons.
0:50:31 > 0:50:37So electrons sit in discrete shells, never in-between the shells.
0:50:38 > 0:50:43Bohr's theory would explain WHY elements behave as they do.
0:50:43 > 0:50:47It turns out that it's all to do with the number of electrons
0:50:47 > 0:50:49in the outermost shell.
0:50:49 > 0:50:54So, for example, Bohr's model showed that sodium has eleven electrons -
0:50:54 > 0:50:58two here, eight here and just one in its outer shell.
0:50:58 > 0:51:04And fluorine has nine - two here and seven in its outer shell.
0:51:04 > 0:51:06To be completely stable,
0:51:06 > 0:51:10atoms like to have a full outer shell of electrons.
0:51:10 > 0:51:13So a sodium atom would like to lose an electron,
0:51:13 > 0:51:15to have a completely full outer shell,
0:51:15 > 0:51:19whereas a fluorine atom has a gap in its outer shell,
0:51:19 > 0:51:22so by gaining an electron it can complete it.
0:51:22 > 0:51:28In this way, a sodium atom and a fluorine atom can stick together
0:51:28 > 0:51:32by exchanging an electron, making sodium fluoride.
0:51:32 > 0:51:35Bohr's work and that of many other scientists
0:51:35 > 0:51:37in the early part of the 20th century
0:51:37 > 0:51:41led to an explanation of every element and every compound,
0:51:41 > 0:51:44why some elements react together to make compounds
0:51:44 > 0:51:46and why others didn't,
0:51:46 > 0:51:50why the elements had the properties that they did, and this in turn
0:51:50 > 0:51:54explained why the periodic table had the shape that it did.
0:51:55 > 0:51:59Mendeleev had managed to reveal a universal pattern
0:51:59 > 0:52:02without understanding why it should be so.
0:52:02 > 0:52:06To find the answer, physicists had to delve into a subatomic world
0:52:06 > 0:52:09that Mendeleev didn't even know existed.
0:52:13 > 0:52:16This work was nothing short of a triumph.
0:52:16 > 0:52:18Even Albert Einstein was impressed.
0:52:18 > 0:52:22He wrote, "This is the highest form of musicality
0:52:22 > 0:52:24"in the sphere of thought."
0:52:24 > 0:52:29But there was still one fundamental question left to answer.
0:52:29 > 0:52:31How many elements were there?
0:52:31 > 0:52:35Could there be an infinite number between hydrogen,
0:52:35 > 0:52:37with the lightest atomic weight,
0:52:37 > 0:52:40and uranium, the heaviest known element?
0:52:43 > 0:52:48In the early 20th century, a brilliant young English physicist,
0:52:48 > 0:52:52Henry Moseley, was determined to find out.
0:52:52 > 0:52:56He speculated that the secret lay within the nucleus
0:52:56 > 0:52:59at the heart of each atom.
0:52:59 > 0:53:02Moseley developed a unique way of studying atoms.
0:53:02 > 0:53:05Scientists still use a similar technique today,
0:53:05 > 0:53:07although this X-ray spectrometer
0:53:07 > 0:53:11looks a bit different to the sort of kit Moseley that would have used.
0:53:11 > 0:53:14One of the elements that he studied was copper,
0:53:14 > 0:53:18and there's a small piece of copper inside here.
0:53:18 > 0:53:21Now, behind it is a radioactive source
0:53:21 > 0:53:25that fires high-energy radiation at the copper atoms.
0:53:25 > 0:53:27Moseley knew that the nucleus of the atom
0:53:27 > 0:53:32contains positively-charged particles we call protons.
0:53:32 > 0:53:35He also knew that surrounding the nucleus
0:53:35 > 0:53:37are negatively-charged electrons.
0:53:37 > 0:53:41Now, the radiation being fired at the copper
0:53:41 > 0:53:44is knocking some of the electrons from the atoms,
0:53:44 > 0:53:46and this had the effect
0:53:46 > 0:53:50of making the atoms give off a burst of energy, an X-ray.
0:53:50 > 0:53:53And Moseley found a way of measuring it.
0:53:53 > 0:53:55He made a startling discovery.
0:53:55 > 0:54:01He found that copper atoms always give off the same amount of energy.
0:54:01 > 0:54:04On this graph, it's shown by this spike.
0:54:04 > 0:54:07And no matter how many times I repeat this experiment,
0:54:07 > 0:54:10I will always get the spike in the same position.
0:54:10 > 0:54:12It's unique to copper.
0:54:12 > 0:54:15Moseley also experimented with other elements.
0:54:15 > 0:54:17And inside this sample there are several others.
0:54:17 > 0:54:19So if I move this on to the next one,
0:54:19 > 0:54:23which is rubidium, and run this again,
0:54:23 > 0:54:26I get another spike in a different position.
0:54:26 > 0:54:31And if I move it on again to the next one, which is molybdenum,
0:54:31 > 0:54:34I see a third spike in a new position.
0:54:34 > 0:54:38Every element has its own energy signature.
0:54:38 > 0:54:41But his stroke of brilliance was to realise
0:54:41 > 0:54:44that this is related to the number of protons.
0:54:44 > 0:54:48He was the first person to measure the number of protons
0:54:48 > 0:54:51in the nucleus of an element, the atomic number.
0:54:58 > 0:55:03Atomic numbers are whole numbers, so unlike atomic weights,
0:55:03 > 0:55:06there can't be any awkward fractions.
0:55:06 > 0:55:09For example, chlorine has an atomic weight
0:55:09 > 0:55:14that comes in an inconvenient half, 35.5,
0:55:14 > 0:55:17but a whole atomic number, 17.
0:55:17 > 0:55:20So Moseley realised that it's the atomic number,
0:55:20 > 0:55:22not the atomic weight,
0:55:22 > 0:55:25that determines the number and the order of the elements.
0:55:25 > 0:55:28And this is where it gets really clever.
0:55:28 > 0:55:32Because the atomic number goes up in whole numbers,
0:55:32 > 0:55:34there could be no extra elements
0:55:34 > 0:55:36between element number one, hydrogen,
0:55:36 > 0:55:38and number 92, uranium.
0:55:38 > 0:55:4292 elements is all there could be. There's just no more room.
0:55:44 > 0:55:49So Henry Moseley did the groundwork that enables us to say
0:55:49 > 0:55:53with absolute confidence that there are 92 elements,
0:55:53 > 0:55:58from hydrogen all the way to uranium.
0:56:00 > 0:56:03Moseley was just 26 when he completed his research,
0:56:03 > 0:56:06but his genius was lost tragically early.
0:56:08 > 0:56:13At the outbreak of World War I, he volunteered to fight,
0:56:13 > 0:56:18even though, as a scientist, he could have avoided joining up.
0:56:18 > 0:56:22He was killed in action aged just 27,
0:56:22 > 0:56:25shot through the head by a sniper.
0:56:27 > 0:56:31A colleague wrote, "In view of what he might still have accomplished,
0:56:31 > 0:56:34"his death may well have been
0:56:34 > 0:56:38"the single most costly death of the war to mankind."
0:56:44 > 0:56:50The periodic table is a wonderful fusion of chemistry and physics.
0:56:52 > 0:56:56Mendeleev and the chemists worked from the outside,
0:56:56 > 0:56:59with the chemical properties of each element,
0:56:59 > 0:57:02and the physicists worked from the inside,
0:57:02 > 0:57:05with the invisible world of the atom.
0:57:05 > 0:57:09And yet both had arrived at the same point.
0:57:11 > 0:57:16The ordered design of the natural world had finally been explained
0:57:16 > 0:57:21in a pattern of pure intellectual beauty.
0:57:21 > 0:57:23So an era that had begun
0:57:23 > 0:57:27with scientists groping towards an understanding
0:57:27 > 0:57:29of the basic building blocks of our world
0:57:29 > 0:57:33had ended with that world entirely classified
0:57:33 > 0:57:35and made clear for all to see.
0:57:35 > 0:57:39And we never looked back.
0:57:45 > 0:57:48Next time, I'll follow in the footsteps of the chemists
0:57:48 > 0:57:51who laboured to control the elements
0:57:51 > 0:57:54and combine them into the billions of compounds
0:57:54 > 0:57:57that make up the modern world.
0:57:58 > 0:58:00I'll discover how modern-day alchemists
0:58:00 > 0:58:04are attempting to push at the wildest outposts
0:58:04 > 0:58:08of the periodic table to create brand-new elements
0:58:08 > 0:58:13and I'll find out how the power of the elements was harnessed
0:58:13 > 0:58:17to release almost unimaginable forces.
0:58:40 > 0:58:43Subtitles by Red Bee Media Ltd
0:58:43 > 0:58:47E-mail subtitling@bbc.co.uk