The Order of the Elements Chemistry: A Volatile History

In 1869, a wild-haired Russian chemist had an extraordinary vision.

He'd been struggling with a mystery that had perplexed scientists for generations.

And for the very first time, he'd glimpsed nature's building blocks,

the elements, arranged in their natural order.

His name was Dmitri Mendeleev,

and he was on the brink of cracking the secret code of the Cosmos,

what was to become one of man's most beautiful creations -

the Periodic Table of Elements.

This is the story of those elements,

the building blocks that make up the universe...

..the remarkable tale of their discovery,

and how they fit together, reveals how the modern world was made.

My name is Jim Al-Khalili.

And ever since I started studying the mysteries of matter,

I've been fascinated by chemistry's explosive history...

Ho-ho! Brilliant!

'..I've discovered some exciting elements...'

That's fantastic!

'..and I've seen how chemistry was forged

'in the furnaces of the alchemists.

'Now I'm going to continue my journey.

'I'll take up the quest of the chemical pioneers...'

Well, my arm's burning up.

'...as they struggled to make sense of elemental chaos

'and conquer our fundamental fear of disorder.

'Could there be a grand plan underlying the elements?

'I'll take part in some volatile experiments...'

Now we're going to drop in the potassium.

Wow, look at that! Wahey!

'..and witness some fiery reactions.'

And I'll find out how the hidden order of the natural world

was revealed in all its glory - the order of the elements.

As a nuclear physicist, I've spent a lifetime studying the sub-atomic world,

the basic building blocks of matter.

But to do that, I need to understand the ingredients of OUR world...

..the elements.

Our planet was created from just 92 elements.

The ground we walk on, the air that we breathe,

the stars we gaze at,

even us.

Our bodies are entirely made of elements.

We now know the name and number

of every naturally-occurring element in existence.

But 200 years ago,

those elements were only just beginning to give up their secrets.

At the beginning of the 19th century,

only 55 had been discovered,

from liquid mercury

to dazzling magnesium...

..and volatile iodine.

Scientists had no idea how many more they might find,

or whether there could be an infinite number.

But the big question was, how did they fit together?

Were they random stars,

or was the elemental world born of order and logic?

Solving the puzzle would prove to be a daunting challenge.

And the first glimmerings of an answer came from an unlikely source.

John Dalton was an intelligent, modest man,

and he had one very British passion - the weather.

He was born here in the Lake District in 1766.

He was so clever, that as a young boy, just 12 years old,

he was already teaching other kids at a school that he set up.

Walking home, he loved watching the weather systems

sweeping across the fells.

He was so obsessed that he kept a meteorological diary for 57 years,

and every single day, come rain or shine,

he entered his precise observations - 200,000 of them.

Dalton was a quiet, retiring man with modest habits.

He was a lifelong bachelor, with not much in the way of a social life.

His only recreation was a game of bowls once a week,

every Thursday afternoon.

He was certainly a creature of habit,

and he might sound a bit dull.

But actually, Dalton was an avid reader and a deep thinker.

Underneath his mild-mannered exterior,

his head was teeming with radical ideas.

Now scientists had recently discovered something very important

about the way elements combine to form compounds.

When they do so, they always combine in the same proportions.

Dalton would have known that table salt, sodium chloride,

is always made up of one part sodium and one part chlorine.

So it doesn't matter whether the salt comes from Salt Lake City

or Siberia, it's always in the same proportion by weight, every time.

Dalton reckoned for this to happen,

each element had to be made up of its own unique building blocks,

what he called "ultimate particles", atoms.

It was a blinding illumination, completely left field.

Everything, he suggested, the entire universe,

was made up of infinitesimally small particles.

The Greeks had hit on the idea of the atom 2,000 years earlier,

but abandoned it.

Now, Dalton took up the baton with his own theory of matter.

What Dalton was describing was revolutionary.

He had struck on the foundations of atomic theory,

foreshadowing research that wouldn't be proved until a century later.

He proposed that there are as many kinds of atoms

as there are elements.

And just as each element is different,

so each element's atom has a different weight -

a unique atomic weight.

Every element has its own signature atomic weight,

whether it be a solid, a liquid, or even a gas.

These three balloons are each filled with a different gas.

Now they are roughly the same size,

so they should each have about the same number of atoms in.

Dalton reckoned that different atoms have different atomic weights.

So these three balloons should each weigh different amounts.

So this red balloon is filled with helium gas.

And if I release it,

it floats.

Helium is very light.

This second balloon is filled with argon gas.

And if I release it,

it sinks slowly.

Argon is heavier than helium.

The third balloon is filled with krypton gas. And if I let it go,

it falls like a stone.

So Dalton was on the right lines -

different atoms of different elements have different weights.

Based on this theory, and working completely alone,

Dalton made one of the first attempts

to impose some order on the unruly world of the elements.

This wonderfully mystical set of symbols is Dalton's line-up

of the elements arranged by weight.

Now there are some elements here that I don't even recognise,

but he does start with hydrogen at one.

Then you go down to oxygen at seven,

and all the way down to mercury at 167.

As it turned out, Dalton didn't get all of his weights right.

But he had made a huge theoretical leap

working purely from his mind's eye.

Two hundred years ago,

John Dalton was using his imagination as a microscope.

But today, we have the technology to see the contours of individual atoms

with this scanning tunnelling microscope.

It's not like a normal microscope because it doesn't use light.

Atoms are less than one millionth of a millimetre across,

which is smaller than the wavelength of visible light.

This microscope uses electrons

to scan across the surface of materials,

picking out individual atoms.

The images it produces are striking.

These are atoms of shining silicon.

These are carbon atoms.

This is what gold atoms look like.

And these are atoms of copper.

Copper is a lustrous metal, essential for life.

It fuelled the move out of the Stone Age into the Bronze Age.

Copper nuggets can be found on the earth's surface,

but it usually needs to be extracted from ores.

And copper compounds run in the veins of some animals.

The blood of the octopus is blue, along with snails, and spiders.

John Dalton's idea in the early 1800s,

that elements had different atomic weights,

was dismissed by many scientists.

But one man believed in him -

Swedish chemist Jons Jakob Berzelius.

Berzelius was obsessed with imposing some kind of order on the elements.

He was convinced that knowing more about the weight of each element

was somehow vitally important.

And when he heard about Dalton's theory,

he came up with an ambitious plan.

It was a gargantuan task.

In fact, it seems almost mad.

This lone Swedish chemist set out

to measure precisely the atomic weight of every single element,

and this without a shred of proof that atoms even existed.

But before Berzelius could start, he was going to have to purify,

dilute, filter each element incredibly accurately.

And that was far from straightforward.

At the time,

very little of the crucial chemical apparatus

needed for work of this precision had even been invented.

But that wasn't going to stop a man like Berzelius.

He was on a mission.

So Berzelius set out to make his own lab equipment.

-Ah, Liam.

-Hi, Jim. Nice to meet you. Come through to the hotshop.

'Liam Reeves, a professional glassblower

'at the Royal College of Art will show me how Berzelius did it.

'Glassblowing is physically demanding,

'and calls for working at punishingly high temperatures.

'Berzelius must have been very dedicated.'

I'm getting the glass out now,

which is at about 1,000 degrees centigrade.

I'm using a wooden block just to cool and shape the glass.

What is it you're making?

It will be a round-bottomed flask,

which would have been part of the basic chemistry equipment

that Berzelius would have used. Now I'm going to introduce some air,

which I'll trap in the pipe and the heat makes expand.

Wow! How hard would it have been for Berzelius to learn to do this?

They say it takes 12 years to kind of...to really master glass.

He was a very skilled glassblower

from the evidence that I've seen of his work.

What he was making was high-precision apparatus,

so that must have made it far more difficult

than your average vase or tumbler.

From the pictures that I've seen, I've got no idea how he made it.

-Really?

-Yeah. No idea. So I'm just making the top of the bottle now.

Right, so that's a basic round-bottomed flask

very much like one that Berzelius would have made.

Glassblowing isn't something theoretical physicists like me normally do.

But I want to find out for myself

just how hard it is to master this new skill.

OK, just turn a little bit slower.

Come back ever so slightly.

Ah! Well, my arm's burning up.

-I'll shield you, actually.

-Oh, that's better.

'It's going rather well.'

SNAP!

Oh-h!

Oh, well.

That just goes to show how difficult this is.

So it does take 12 years to do.

I think you would have managed it in seven or eight.

There's my flask dying slowly, melting away.

I mean, it just goes to prove how incredibly talented

Berzelius was - he wasn't making something basic like this,

he was making some really intricate stuff.

'And although he was searching for elemental order, there was a bonus.'

The great thing, you see, about Berzelius was that the skills

he learned as a glassblower led him to an incredible discovery.

In 1824, he discovered a new element,

because he found that one of the constituents of glass was silicon.

Silicon is a semi-metallic element... found within some meteorites.

Closer to home, it's under your feet.

The earth's crust is made primarily of silicate minerals.

Silicon is its second most abundant element, after oxygen.

It's mostly found in nature as sand or quartz.

Its man-made compounds can be heat resistant,

water resistant and non-stick.

But silicon's ultimate achievement has to be the silicon chip,

shrinking computers from room size to palm size.

Silicon was the last of four elements that Berzelius isolated,

along with thorium, cerium, and selenium.

He then spent the next decade of his life

measuring atomic weight after atomic weight after atomic weight

in an obsessive pursuit of logic

in the face of the seemingly random chaos of the natural world.

Berzelius laboriously studied over 2,000 chemical compounds

with staggering dedication.

He weighed, he measured and he agonised over the tiniest detail

until he'd found out the relative weights of 45 different elements.

Some of his results were remarkably accurate.

His weight for chlorine, a gas,

got to within a fifth of a per cent of what we know today.

But by the time Berzelius produced his results,

other scientists had started measuring atomic weights

and come up with completely different answers.

Now they were pitted against each other,

perhaps fuelled by an innate desire to find meaning in disorder.

Berzelius's quest for order was contagious.

Scientists began looking for patterns everywhere.

One of these was German chemist Johann Wolfgang Dobereiner.

He believed that the answer lay not with atomic weights

but with the elements' chemical properties and reactions.

'Dr Andrea Sella has studied Dobereiner's work

'on chemical groups.'

What Dobereiner had really spotted was that if you considered

all the elements that were known to that time,

you could often pick out three - "triads", as he called them,

which had very, very closely related chemical properties.

And as an example, we have here the alkali metals.

And I'm going to take the first and the lightest of them, lithium.

And we have to store these under oil

because they tend to react with air and moisture. So here goes lithium.

Pop it in.

Oh, look, fizzing away, yeah.

You can see it fizzing. And the fizzing is hydrogen,

flammable air, being released.

And at the same time, it's leaving a pink trail.

We've put a bit of indicator in there, which is telling us

that what's left behind is caustic.

It's actually making an alkaline solution.

I'm breathing in some caustic soda!

Well, you're getting a little bit of steam coming off,

and the reaction is very, very exothermic.

In other words, the temperature rises a lot,

and the metal has actually melted.

The second metal in this triad was sodium.

And when we drop the sodium in...

Whoa! Oh, look at that, flashes of light!

Orange sparks. And those orange sparks are the same colour

as what you get in streetlights.

-Streetlights have sodium in them.

-Right.

Well, the third one in the series is potassium.

The potassium turns out to be the tiger.

And we may need to stand back.

-Look at those flashes.

-Wow!

And you can see that lilac flame.

And one could really see trends in these triads.

-They're all doing the same thing, aren't they?

-Yes.

The fizzing is telling us that hydrogen is coming off.

We're getting the alkali being formed.

But the lithium is relatively tame,

the sodium was more excitable, the potassium starts getting scary.

Dobereiner realised that these elements must be a family

because they reacted in a similar way.

Here was the hint of a pattern.

But it only worked on a few of the elements.

It got scientists no further than atomic weights had done.

The bigger picture, the universal order of all the elements,

was still hard to see.

And that wouldn't change until a breakthrough

by one of greatest minds in 19th-century science.

In 1848, in the far west of Siberia, a massive fire destroyed a factory.

The factory manager faced destitution.

She was a widow, Maria Mendeleeva, and she made a remarkable sacrifice

for her precociously intelligent son, 14-year-old Dmitri Mendeleev.

Maria was well aware of her son's intelligence,

and with a steely determination she set out to get him an education.

So, together with Dmitri, she set off on a 1,300-mile journey

from Siberia to St Petersburg.

And incredibly, they walked a good part of that journey.

I'm following in their footsteps to St Petersburg,

then the capital of the Russian empire.

After their arduous journey across the Russian steppes,

mother and son finally arrived at St Petersburg.

Maria Mendeleeva had got what she wanted,

but the effort destroyed her.

She died ten weeks later.

The story goes that her last words to her son were -

"Refrain from illusions and seek divine and scientific truth."

And young Mendeleev promised to obey.

He studied day and night to fulfil his mother's dream

and became the most brilliant chemistry student of his generation.

Chemistry had come a long way since the Greeks' idea of four elements -

earth, air, fire and water.

But there was still no order to the 63 elements

that had so far been discovered.

Now the search for a pattern gripped some of the best minds in science.

But no-one could agree how to find it.

Mendeleev was still a student when he attended

the world's first ever international chemistry conference.

The world's chemists had gathered to settle the dispute

that was holding back their subject, the confusion over atomic weights.

Mendeleev watched as Sicilian chemist Stanislao Cannizzaro

stole the show.

Cannizzaro was still convinced

that atomic weights held the key to the elements,

and he'd struck on a wonderful innovation,

a reliable new way of calculating them.

He knew that equal volumes of gases contain equal numbers of molecules.

So instead of working with liquids and solids,

his breakthrough was to use the densities of gases and vapours

to measure the atomic weights of single atoms.

Cannizzaro gave a talk in which he presented striking new evidence

that won over the assembled chemists.

So whereas Berzelius's work had failed to convince anyone,

Cannizzaro's new method set an agreed standard.

Finally, chemists had a way of measuring atomic weights accurately.

It was the moment everybody had been waiting for.

Surely with precise atomic weights

they would now be able to unravel the mystery of the elements?

One chemist wrote, "It was as though the scales fell from my eyes

"and doubt was replaced by peaceful clarity."

There was a real buzz in the air.

Finally, it seemed that the order of the elements

may have been within science's grasp.

Mendeleev was electrified.

But chemists soon found that even arranged in order of atomic weight,

the elements appeared unsystematic.

They were still missing something vital.

Then, in 1863, a solitary English chemist

named John Newlands made an unusual discovery.

Newlands noticed that when the elements are arranged by weight,

something very strange happened.

Imagine each element is like a key on the piano,

arranged by their atomic weight.

Then this will be carbon,

followed by nitrogen,

oxygen, fluorine, sodium, magnesium, aluminium

and finally silicon.

'Thinking of the elements like a musical scale,

'Newlands reckoned that every octave, every eight notes,

'certain properties seemed to repeat, to harmonise.'

He called it a "law of octaves".

It was the first real attempt to find a law of nature

that pulled all the known elements together.

Newlands proudly presented his idea

to the great and the good of the Chemical Society in 1866.

It was his big moment.

But his music analogy didn't seem to strike a chord.

They completely failed to see his point.

The assembled chemists said Newlands' idea was ridiculous,

that he might as well have arranged the elements alphabetically

for all the insight his theory gave.

Maybe, they even suggested with biting sarcasm,

that Newlands could get his elements to play them a little tune.

It must have been a shattering blow for Newlands.

But was John Newlands really onto something

with his curious law of octaves?

It's such a bizarre concept

that every eighth element will behave in a similar way.

It's not surprising that people thought Newlands' idea was mad.

Here are eight elements in order of their atomic weight,

and I'm going to explore their properties by smelling them.

The first element is chlorine.

It's a yellowy-green gas that's highly toxic.

If I have a sniff...

Yep, distinctive smell of bleach.

The second one is potassium.

But no odour to it at all.

'And as I smell my way through the next five elements,

'calcium, gallium, germanium, arsenic -

'not poisonous to smell in its pure form -

'and selenium, there's no scent.'

Finally number eight, bromine.

I already see it's a gas,

like chlorine, a reddish gas, highly toxic.

I'm going to be very careful,

because I don't recommend you try this at home.

Smells very much like chlorine, only a lot worse, a lot stronger.

And so Newlands' law of octaves seems to work here,

because the eighth element, bromine, is similar in properties

to the first one, chlorine.

'Today we know Newlands' law of octaves as the law of periodicity.

-'

-But at the time, the establishment scoffed.

-'

-And Newlands never got over the slight.

'The way was left clear for Dmitri Mendeleev,

'who was thinking along the same lines.'

I'm on my way to St Petersburg University

to meet a man who will show me where Mendeleev actually worked.

Hello, Professor Babaev.

Hi, I'm Jim. Good to meet you. It's very exciting.

-OK, well, the museum...

-Right, well, lead on.

'Professor Eugene Babaev is the leading expert on Mendeleev,

'having studied his work for many years.

'He's going take me inside Mendeleev's apartment,

'preserved just as it was during the last years of his life.

'This is a great honour.

'Normally, nobody is allowed inside Mendeleev's study.'

So this is quite a privilege, to be able to come in here.

Look at this. Fantastic.

'Mendeleev shut himself away in this room, brooding over the elements.

'This would become the birthplace

'of one of science's greatest achievements, the periodic table.'

And I love this photo of him.

-This is the photo of 1869, just the year when...

-Ah!

So that's what he looked like when he came up with the periodic table.

-And these are all his original books.

-These are his books, written by him.

Oh, I see.

When I say "his books", not owned by him.

-These are the books that he wrote.

-Thousands of volumes.

That's impressive.

OK, and if you look at his library, you will be surprised,

because maybe 10% of the books are devoted to chemistry and physics

but everything else is economics, technics, er...

-geography, whatever.

-He was a polymath.

Yes, and his second wife was a painter,

and one portrait here in profile is just by her work.

'Mendeleev had such a breadth of intellectual curiosity

'he became known as the Russian Leonardo da Vinci.'

These are the clocks which stopped at the moment of his death in 1907.

-1907, at twenty past six.

-Yeah.

'It seems as if time has stood still in this room

'for more than a century.

'And now that I've seen the inner sanctum

'where Mendeleev puzzled over the elements, I want to know

'exactly how he pieced together his masterwork, the periodic table.

'By 1869, Mendeleev had been trying to find a pattern

'to the elements for a decade.

'Whatever order he and the world's chemists tried to impose,

'there were still elements that wouldn't fit.

'A universal theory seemed out of reach.

'But now Mendeleev hit on a new idea.

'He made up a pack of cards and wrote an element

'and its atomic weight on each one.'

Strange though this might sound,

so began the most memorable card game in the history of science.

He called it chemical solitaire

and began laying out cards just to see where there was a pattern,

whether it all fitted together.

Now, previously, chemists had grouped the elements in one of two ways,

either by their properties, like those that react with water,

or by grouping them by their atomic weight,

which is what Berzelius and Cannizzaro had done.

Mendeleev's great genius was to combine those two methods together.

'The odds were stacked against him.

'Little more than half the elements we now know had been discovered,

'so he was playing with an incomplete deck of cards.'

He stayed up for three days and three nights without any sleep,

just thinking solidly about the problem.

Then, on the 17th of February,

with a snowstorm raging outside, he decided to stay at home.

He was exhausted and he finally he dozed off.

'The story goes he had an extraordinary dream.

'He saw almost all of the 63 known elements

'arrayed in a grand table which related them together.'

It was an incredible breakthrough.

I can imagine Mendeleev feeling like so many other scientific pioneers.

It's that determination, even desperation, to crack a puzzle,

and then that eureka moment of revelation.

Mendeleev had revealed a deep truth about the nature of our world,

that there is a numerical pattern underlying the structure of matter.

This is the periodic table

as we know it today,

and it's rooted

in Mendeleev's discovery.

It decodes and makes sense of the building blocks of the whole world.

Now, although it's so familiar to us,

it's on the wall of every chemistry lab in every school in the world,

if you really look at it, it's actually awe inspiring.

What's so remarkable is that it reveals the relationships

between each and every element in order.

Mendeleev had brilliantly combined elements' atomic weights

and properties

into one universal understanding of all the elements.

Reading it across,

the atomic weights increase step by step with every element.

But then, looking at it vertically,

the elements are grouped together in families of similar properties.

So over on this side are the alkali metals, from lithium to caesium.

And then over on the far side are the halogens,

like poisonous chlorine, bromine and iodine, all very highly reactive.

And alongside them at the top are the elements important for life -

carbon, nitrogen, oxygen, all non-metals.

But in the middle, a vast swathe,

are all the metals,

and there are four times as many metals as non-metals.

Everything is ordered.

It's a chemical landscape

and a perfect map of the geography of the elements.

'Intriguingly, the periodic table didn't always look like this.

'Professor Babaev is keen to show me a copy

'of Mendeleev's very first manuscript.'

So, this is the first draft of Mendeleev's periodic table.

-You can see the date, 17th February 1869.

-And it's in his handwriting.

I can see the crossings out, you can feel his thought processes.

Some familiar elements here.

I see hydrogen, the lightest element, all the way to lead.

Yeah, yeah. Now you can see some familiar groups,

like alkali metals, halogens.

It's got lithium, sodium, potassium.

It's not like the periodic table that I would be familiar with,

it's the other way round.

It took maybe two years

for Mendeleev to bring it to modern form.

But it's remarkable that this is the foundations

of the modern periodic table. It started here.

'Mendeleev's first draft wasn't perfect.

'To make his table work, he had to do something astonishing.

'He had to leave spaces for elements that were still unknown.'

This is a copy of the first published draft of the periodic table,

and these question marks are where Mendeleev left gaps.

You see, he was so confident about his model

that he wouldn't fudge the results.

So where the model didn't work,

he left gaps for elements that had yet to be discovered.

So, for instance, this question mark here

he predicted was a metal slightly heavier than its neighbour calcium.

And here two more metals.

One he predicted would be dark grey in colour,

and the other would have a low melting point.

Mendeleev had the audacity to believe

that he would, in time, be proved right.

It's as if Mendeleev was a chemical prophet,

foretelling the future in a visionary interpretation

of the laws of matter.

But before he could claim the glory, his gaps needed explaining.

And a new way of detecting elements was invented in 1859.

That was thanks to Gustav Kirchhoff and his colleague,

the man who made the Bunsen burner.

Robert Bunsen was a wonderfully intrepid experimenter.

How's this for dedication?

He lost his right eye in an explosion in his lab.

Now, he knew that when different elements burned in the flame

of his Bunsen burner,

wonderful colours were revealed. This one is copper.

This one contains strontium.

And this one is potassium.

Bunsen wondered whether every element

might have a unique colour signature

and so he and Kirchhoff set to work.

Kirchhoff knew that when white light is shone through a prism

it gets split up into all its spectral colours...

..all the colours of the rainbow,

from red through yellow to blue and violet.

And he came up with this.

It's called a spectroscope.

It has a prism in the middle

with two telescopes on either side.

Bunsen and Kirchhoff then worked together

to analyse different materials using their new piece of kit.

So they took a compound containing sodium.

And if I heat it up in the Bunsen burner,

the light from the sodium passes through the first telescope

and gets split up by the prism into its spectral lines.

They then pass through the second telescope. And if I have a look.

Yep, I can see the two orange lines

which are the unique spectrum of sodium.

No other element would give that pattern.

Using this technique, they actually discovered two new elements,

silvery-gold caesium, and rubidium,

so named because of the ruby-red colour of its spectrum.

It was this same technique that was used to test

whether Mendeleev's prediction of gaps was right.

He'd described in meticulous detail

an unknown element that followed aluminium in his periodic table.

He predicted it would be a silvery metal with atomic weight 68.

Then, in 1875, a French chemist used a spectroscope

to identify just such an element -

gallium.

Gallium is a beautiful silvery-white metal, and it's relatively soft.

Although Mendeleev predicted its existence,

it was actually found

by Parisian chemist Paul Emile Lecoq de Boisbaudran.

Gallium has a very low melting point.

And with a boiling point of 2,204 degrees centigrade,

it's liquid over a wider range of temperatures

than any other known substance.

Gallium is used to make semiconductors.

It's found in light-emitting diodes, LEDs.

One of gallium's compounds was shown to be effective

in attacking drug-resistant strains of malaria.

But even though Mendeleev had left gaps for gallium and other elements,

his table was not complete.

There was one group that eluded him completely,

an entirely new family of elements.

The story of their discovery began with an other-worldly search

for an extraterrestrial element.

In August 1868, a total eclipse of the sun in India was the moment

that French astronomer Pierre Janssen had been waiting for.

He knew that it was possible to use a spectroscope

to identify some elements in the light of the sun.

But the intensity of sunlight meant that many elements were hidden.

Janssen hoped to see more during a total eclipse,

when the sun was less blinding.

As Janssen studied the eclipse,

he discovered a colour signature never seen before.

He was faced with an unknown element.

The same spectral line was confirmed by another astronomer,

Norman Lockyer.

He named it helium, after the Greek sun god,

because he thought that it could only exist on the sun.

Enter Scottish chemist William Ramsay,

who linked extraterrestrial helium to Earth.

Ramsay experimented with a radioactive rock called cleveite.

By dissolving the rock in acid,

he collected a gas with an atomic weight of 4

and the same spectral signature that Lockyer had seen, helium.

Helium is the second most abundant element in the universe,

after hydrogen.

It was one of the elements produced just after the Big Bang.

Liquid helium is used to cool superconducting magnets

for MRI scanners.

Deep-sea divers rely on helium to counter the narcotic effects

on the brain of increased nitrogen absorption.

And it was a vital ingredient in the space race,

used to cool hydrogen and oxygen for rocket engines.

Before he discovered helium on Earth,

William Ramsay had already separated a new gas from the air, argon,

with an atomic weight of 40.

Now Ramsay faced a puzzle.

He realised that the new elements didn't fit the periodic table

and suggested there must be a missing group,

so his search began.

He found three more gases, which he named neon, Greek for "new",

krypton, meaning "hidden", and xenon, "stranger".

The group became known as the noble gases

because they were unreactive and seemed so aloof.

This family of gases completed the rows on the periodic table.

Now, Mendeleev may not have known about these elusive elements,

but he'd established the unshakeable idea of elemental relationships.

And so he made sure that there was a place on his table

for every new element, no matter when it was discovered.

The periodic table is a classic example

of the scientific method at work.

From a mass of data, Mendeleev found a pattern.

It led him to make predictions that could be tested

by future experiments,

pointing the way for 20th-century scientists

to prove him and his theory right.

By the time he died at the age of 72,

he was a hero in Russia and a superhero in the world of science.

His periodic table was immortalised in stone

here in the centre of St Petersburg,

and he eventually had an element named after him, mendelevium,

as well as a crater, the Mendeleev Crater,

on the dark side of the moon...

..fitting tributes to a man who came from the Siberian wastelands

to become the ultimate cartographer of the elements.

The periodic table had finally created order out of chaos.

But it tells us nothing about WHY our world is as it is,

why some elements are energetic,

others are slow, some inert, others volatile.

It would be another 40 years

before an entirely different branch of science came up with an answer.

In 1909, Ernest Rutherford looked inside the atom for the first time.

Rutherford proposed that the structure of the atom

was like a miniature solar system,

an overwhelmingly empty space with a few tiny electrons

orbiting randomly around a dense, positively-charged nucleus.

But it wasn't until Niels Bohr came along, one-time goalkeeper

for the Danish football squad and future Nobel prize-winning physicist

that things really kicked off.

He suggested that the electrons orbited around the nucleus

in fixed shells.

And it was his idea that was to lead to the discovery that these shells

could only accommodate a set number of electrons.

Imagine this football pitch is an atom, a single atom of an element.

This is the nucleus.

If this nucleus were to scale, my nearest orbiting electrons

would be beyond the stands, so I've scaled it down.

Here, on the shell nearest to the nucleus,

there can be just two electrons, then it's full.

Here in the second shell,

there can be eight electrons, then it's fully occupied, too.

The third shell is happy with 18 electrons. And so it goes on.

Outer shells can accommodate an increasing number of electrons.

So electrons sit in discrete shells, never in-between the shells.

Bohr's theory would explain WHY elements behave as they do.

It turns out that it's all to do with the number of electrons

in the outermost shell.

So, for example, Bohr's model showed that sodium has eleven electrons -

two here, eight here and just one in its outer shell.

And fluorine has nine - two here and seven in its outer shell.

To be completely stable,

atoms like to have a full outer shell of electrons.

So a sodium atom would like to lose an electron,

to have a completely full outer shell,

whereas a fluorine atom has a gap in its outer shell,

so by gaining an electron it can complete it.

In this way, a sodium atom and a fluorine atom can stick together

by exchanging an electron, making sodium fluoride.

Bohr's work and that of many other scientists

in the early part of the 20th century

led to an explanation of every element and every compound,

why some elements react together to make compounds

and why others didn't,

why the elements had the properties that they did, and this in turn

explained why the periodic table had the shape that it did.

Mendeleev had managed to reveal a universal pattern

without understanding why it should be so.

To find the answer, physicists had to delve into a subatomic world

that Mendeleev didn't even know existed.

This work was nothing short of a triumph.

Even Albert Einstein was impressed.

He wrote, "This is the highest form of musicality

"in the sphere of thought."

But there was still one fundamental question left to answer.

How many elements were there?

Could there be an infinite number between hydrogen,

with the lightest atomic weight,

and uranium, the heaviest known element?

In the early 20th century, a brilliant young English physicist,

Henry Moseley, was determined to find out.

He speculated that the secret lay within the nucleus

at the heart of each atom.

Moseley developed a unique way of studying atoms.

Scientists still use a similar technique today,

although this X-ray spectrometer

looks a bit different to the sort of kit Moseley that would have used.

One of the elements that he studied was copper,

and there's a small piece of copper inside here.

Now, behind it is a radioactive source

that fires high-energy radiation at the copper atoms.

Moseley knew that the nucleus of the atom

contains positively-charged particles we call protons.

He also knew that surrounding the nucleus

are negatively-charged electrons.

Now, the radiation being fired at the copper

is knocking some of the electrons from the atoms,

and this had the effect

of making the atoms give off a burst of energy, an X-ray.

And Moseley found a way of measuring it.

He made a startling discovery.

He found that copper atoms always give off the same amount of energy.

On this graph, it's shown by this spike.

And no matter how many times I repeat this experiment,

I will always get the spike in the same position.

It's unique to copper.

Moseley also experimented with other elements.

And inside this sample there are several others.

So if I move this on to the next one,

which is rubidium, and run this again,

I get another spike in a different position.

And if I move it on again to the next one, which is molybdenum,

I see a third spike in a new position.

Every element has its own energy signature.

But his stroke of brilliance was to realise

that this is related to the number of protons.

He was the first person to measure the number of protons

in the nucleus of an element, the atomic number.

Atomic numbers are whole numbers, so unlike atomic weights,

there can't be any awkward fractions.

For example, chlorine has an atomic weight

that comes in an inconvenient half, 35.5,

but a whole atomic number, 17.

So Moseley realised that it's the atomic number,

not the atomic weight,

that determines the number and the order of the elements.

And this is where it gets really clever.

Because the atomic number goes up in whole numbers,

there could be no extra elements

between element number one, hydrogen,

and number 92, uranium.

92 elements is all there could be. There's just no more room.

So Henry Moseley did the groundwork that enables us to say

with absolute confidence that there are 92 elements,

from hydrogen all the way to uranium.

Moseley was just 26 when he completed his research,

but his genius was lost tragically early.

At the outbreak of World War I, he volunteered to fight,

even though, as a scientist, he could have avoided joining up.

He was killed in action aged just 27,

shot through the head by a sniper.

A colleague wrote, "In view of what he might still have accomplished,

"his death may well have been

"the single most costly death of the war to mankind."

The periodic table is a wonderful fusion of chemistry and physics.

Mendeleev and the chemists worked from the outside,

with the chemical properties of each element,

and the physicists worked from the inside,

with the invisible world of the atom.

And yet both had arrived at the same point.

The ordered design of the natural world had finally been explained

in a pattern of pure intellectual beauty.

So an era that had begun

with scientists groping towards an understanding

of the basic building blocks of our world

had ended with that world entirely classified

and made clear for all to see.

And we never looked back.

Next time, I'll follow in the footsteps of the chemists

who laboured to control the elements

and combine them into the billions of compounds

that make up the modern world.

I'll discover how modern-day alchemists

are attempting to push at the wildest outposts

of the periodic table to create brand-new elements

and I'll find out how the power of the elements was harnessed

to release almost unimaginable forces.

Subtitles by Red Bee Media Ltd

E-mail [email protected]