How Can You See an Atom?

It’s hard to imagine just how tiny atoms are. 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 onto it has to be roughly the same size as 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. Don’t forget to hit the subscribe button for weekly chemistry awesomeness.