**By **Robert M. Hazen, Ph.D.**, **George Mason University

## As scientific advances were made in the 19th and 20th centuries, the evidence for the presence of atoms began to grow stronger. Some of these ideas were born from investigations in radioactivity, Einstein’s mathematical analysis of Brownian motion and even imaging technology. Let us look at how all of these developments converged to prove the existence of atoms.

### Radioactivity

One of the first modern scientific concepts that proved the atomic theory was radioactivity. This is the process by which individual atoms emit radiation. If you think about matter as a continuum, then it’s hard to imagine how you have individual pulses of radioactivity.

If you take a piece of radioactive material, and put it in front of a phosphorescent screen that glows brightly anytime there’s one of these pulses of radiation, you’d see individual bright flashes, one after another. It was in 1903, upon seeing this sort of irregular twinkling, that even the most vocal skeptics of the atomic theory had to pause and wonder.

This is a transcript from the video seriesThe Joy of Science. Watch it now, on Wondrium.

### Brownian Motion

We enter the 20th century and look at the work of Albert Einstein. He studied a phenomenon called Brownian motion. Brownian motion is a fascinating effect. It is an atomic scale effect that’s visible in a microscope.

What you see if you have tiny particles of dust suspended in a liquid like water is they don’t just sit there in the microscope slide, but they move back and forth. They eventually may move some distance from where they were before through this little jiggling motion. Many people had seen this.

You can see it in dust particles; you can see it in pollen grains suspended in water. So many scientists had noticed the effect, but they hadn’t really described what caused it.

### Einstein’s Mathematics

What Einstein did in 1905 is he demonstrated mathematically that such motions have to arise from a force. Nothing accelerates without a force.

If you see something jiggling in your microscope slide, there has to be a force involved. And it turns out that that force was exactly what you’d expect from the randomness of collisions of atoms and molecules hitting the particles from different sides.

At any one instant, you have slightly more atoms hitting it from one side than another. That would cause the particle to move around in a random sort of way, in this Brownian motion. It was this mathematical treatment—probably more than any other single piece of data—that convinced most scientists of the atom’s reality. Einstein, in addition to all the other things he did, was a principal player in convincing people that atoms were real.

Also, because Brownian motion was understandable from a classical point of view—that you could actually see the effect and calculate it based on Newton’s laws—this work was probably Einstein’s most widely read and widely appreciated paper in the early part of 20th century.

Learn more about Newton’s laws of motion.

### Counting Atoms

Another line of evidence comes from the simple geometrical fact that if atoms are real, then there must be a large, but a finite number of them in any given volume of material. In a sense, you might even be able to count the number of atoms in that volume of material. And that number is called Avogadro’s number. It’s defined as the number of atoms in one mole of a substance.

Now a mole is one gram of hydrogen, because hydrogen is element one. Or it’s 12 grams of carbon, which has a weight of 12, and so forth. You have a number of grams that corresponds to the atomic weight of that particular element. How many atoms are contained in one mole?

### Determining Avogadro’s Number

Physicists and chemists tried different ways to determine Avogadro’s number. If atoms are real then all these different approaches to determining Avogadro’s number should come up with the same number. And indeed, by the early-20th century, estimates from studies of radioactivity, studies of Brownian motion, other methods, all converged on the same number. It’s about 6×10^{23} atoms per mole.

Now, that is admittedly a huge number, but the fact that different approaches all converged on the same number was highly suggestive and very convincing. That gave real credence to this idea of the atom. It also gave some evidence to the size of atoms and the size of the distance between atoms.

### X-Ray Crystallography

Another scientific approach that proved the reality of atoms was x-ray crystallography. This phenomenon, which involves diffraction of x-rays off planes of atoms in the surface provided a really convincing evidence to remaining skeptics.

The only way that you can get diffraction, the only way that you can get x-rays to bounce off in very specific directions and create spots in an x-ray photograph, is if the atoms are real.

Indeed with x-ray crystallography, you could not only say atoms are real, but you could say something quantitative about the way the atoms are arranged in space. Virtually all the structure models—the crystal structure models you must have seen in books or perhaps in museums, where you see the arrangement of little balls and sticks, which represent crystal structures—all those are determined by this property of x-ray diffraction.

Learn more about the evolution of 20th-century science.

### Atomic Microscopy

If any skeptics remained at all, there is the astonishing evidence from a new kind of microscopy called atomic microscopy. In 1980, the first photograph of an individual atom was taken—this was at the University of Heidelberg in Germany. Now studies of individual atoms are almost routine in certain scientific fields.

Atomic microscopes are found in many, many laboratories around the world. So, it’s safe to say since we can actually see atoms and see their arrangement in a microscope, we have to acknowledge them as being real.

### Common Questions about Scientific Evidence for Atomic Theory

**Q: How was radioactivity proof for the existence of atoms?**

The fact that radiation produced discrete pulses cast doubt upon the theory that matter was a continuum. The fact that radiation was measured in discrete packets was a strong pointer to the presence of atoms.

**Q: How did Einstein’s mathematical analysis of Brownian motion point to the presence of atoms?**

Einstein demonstrated mathematically that Brownian motion has to arise from a force. The force in Brownian motion was exactly what you’d expect from the randomness of collisions of atoms and molecules hitting the particles from different sides.

**Q: How did the search for Avogadro’s number prove the existence of atoms?**

Estimates from studies of radioactivity, studies of Brownian motion, and other methods, all converged on the same figure for Avogadro’s number. The fact that different approaches all converged on the same number gave real credence to the idea of the atom.