One sheet of paper is roughly half a million atoms thick.

Volume-wise, one atom is as small compared

to an apple as that apple is to the entire earth.

So you might be surprised to learn that

chemists can actually see atoms.

Not with their eyes.

With incredibly precise tools.

[Legends of Chemistry intro]

The idea of atoms stretches back to ancient Greece,

when the philosopher Democritus declared that

all matter is made of tiny particles.

The philosopher Plato even decided—wrongly—that

different substances had differently

shaped atoms, like pyramids or cubes.

The first modern evidence for atoms appeared

in the early 1800s, when British chemist

John Dalton discovered that chemicals always

contain whole-number ratios of elements.

That’s why it’s H2O and not H2.4O or H√17O.

The reason for these whole numbers, Dalton

suggested, was because you can’t have a half

an atom or point-two atoms, only whole atoms.

It’s hard to imagine chemistry today without Dalton’s insight.

But it was actually controversial in its day.

Why?

Because chemists couldn’t see atoms.

Many considered them like negative numbers

or ideal gases: useful for calculating things,

but not existing in the real world.

Even Dmitri Mendeleev, father of the periodic table,

refused to believe in atoms for many years.

Why didn’t chemists just look for atoms under microscopes?

To see something under a microscope, the wavelength

of light you’re shining through the microscope

can't be larger than whatever you’re looking at.

Unfortunately, visible light is thousands of times bigger than atoms.

So chemists had to wait for light with

shorter wavelengths, like x-rays.

X-rays were discovered in the 1890s by

German scientist Wilhelm Röntgen, who

realized that photographs taken with x-rays

allowed him to see through objects.

Roentgen thought he’d gone insane when

he saw this, but today we’re all familiar with

x-rays from trips to the dentist and doctor.

Chemists don’t use x-rays to see through things.

Instead, they bounce x-rays off things like crystals,

which are solids with layers of atoms. When x-rays

hit an atom in a crystal, they bounce back.

Others slip through and bounce off the second layer down.

Or the third layer, or deeper.

After being reflected, these x-rays strike a detector

screen, like the ball bouncing back in Pong.

And based on where the x-rays came from

and how they interacted with each other,

scientists can work backward and figure out

the arrangement of atoms in the crystal.

This reflection and interaction of light rays is called diffraction.

X-ray diffraction, sometimes called x-ray

crystallography, has led to dozens of

Nobel Prizes for chemists since the 1920s.

It also led to one of the biggest discoveries in science

history, the structure of DNA. James Watson and

Francis Crick get credit nowadays, but they based

their work on the work of Rosalind Franklin,

a crystallographer in England.

She began taking x-ray pictures of DNA in 1952,

and Watson’s glimpse of one picture — photograph 51.

— was the vital clue in determining that

DNA was a double helix.

This incident remains controversial today

Because Franklin never gave Watson

permission to view photograph 51.

If x-rays let chemists peer at the structure of atoms,

scanning tunneling microscopes

finally revealed atoms themselves.

Rather than bounce light off something,

an STM runs a sharp needle over its surface.

It’s like chemical Braille, except

the tip never quite touches.

As the tip moves along the surface, scientists

can reconstruct the atomic landscape — making

individual atoms visible at last in the early 1980s.

Lo and behold, the atoms weren’t Plato’s cubes

and pyramids, but spheres of different sizes.

By 1989 a few scientists had even adapted

STM technology to manipulate single

xenon atoms and spell out words.

We’ll let you guess what company they worked for.

Also in 1989, the chemist Ahmed Zewail moved

beyond looking at stationary atoms and

developed tools to see atoms in action.

Zewail wanted to study how atoms break bonds

and swap partners during reactions.

So he developed the world’s fastest camera,

which shoots pulses of laser light a few femtoseconds

long—a few billionths of a microsecond.

If you stretched one femtosecond to a full second,

it would be like stretching a single

second out to 32 million years.

While Zewail’s laser flashed like a strobe, his camera snapped pictures.

Zewail then ran the pictures together

like a slow-motion replay.

Since then femtochemistry had provided insight into

everything from ozone depletion to the workings

of the human retina. Zewail won a Nobel Prize

in chemistry for his work in 1999.

The ancient Greeks dreamed up fanciful shapes for atoms.

But it took 2,400 years before scientists could see

them for real and study their behavior.

Seeing truly is believing for human beings, and it was

chemists and other scientists who fulfilled this need

and finally revealed what our universe is made of.

Thanks for watching chemheads.

Be sure to check out other videos in the

Legends of Chemistry series, Like the Woman

Who Saved the U.S. Space Program, and

the crafty scientists who tricked the Nazis.

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