How do I know that atoms really exist?

In the early years of the twentieth century, atoms were still theoretical entities. Strong circumstantial evidence suggested that atoms existed, but no one had ever actually seen an atom. Today, we can not only "see" individual atoms on surfaces- we can also move them from place to place, and build things from them. The strongest evidence for the reality of atoms comes from scanning tunneling microscopy, studies of Brownian motion, and by the remarkably good agreement for experimental estimates of Avogadro's Number.

Scanning tunneling microscopy. Pictures of atoms can be taken by placing the tip of very sharp needle just above a sample surface. A very small voltage is applied to cause electrons to "tunnel" from the tip to the surface. As the tip is dragged over an atom on the surface, more electrons tunnel and the needle is rigged so that the increase in current causes the tip to move up. As the tip is dragged off the atom, the distance between the tip and the surface widens, causing a decrease in current which moves the tip down. The tip height can be monitored to build an image of the actual shape and features of the sample surface on the atomic scale.
Seeing is believing. The STM Image Gallery at IBM's Almaden Research Center contains many pictures of atoms taken with a the scanning tunneling microscope.
Brownian movement. Botanist Robert Brown noticed that tiny objects like pollen grains shook and moved erratically when viewed under a microscope. Nearly seventy years later, Albert Einstein explained this "Brownian motion" as the result of bombardment by molecules. His explanation was quantitative; he was able to mathematically estimate the average distance that the particles should be moved over time as a result of molecular bombardment. A few years later Jean Perrin painstakingly measured the displacements of particles of a resin suspended in water; his experimental average displacement was in excellent agreement with Einstein's theoretical prediction.
Avogadro's number. Avogadro's number is the number of molecules in one mole of any compound. There are dozens of different experimental methods for measuring Avogadro's number. All give the same result. The fact that Avogadro's number seems to be independent of any particular method implies that it actually has meaning- and so is strong circumstantial evidence that molecules actually exist.

Author: Fred Senese senese@frostburg.edu

General Chemistry Online! How do I know that atoms really exist?