Atoms, with a typical size of 0.1 nm (10-10 m), are too small to be seen with visible light (wavelength 100-1000 nm). But what about other ways of seeing atoms?
The History of Atoms
From Democritus to Dalton, an original insight survived two thousand years to become the foundation of modern science.
Ideas for Experiments
The study of chemical reactions led to the scientific hypothesis that all things were made of atoms and that there were different kinds of atoms that had different relative weights. The case for atoms was strong but maybe not definite. For example, the question of the size of atoms was still unanswered.
The classification of elements, led to the discovery of the periodic table, establishing that there are only about 100 elements. Again this doesn't by itself prove the existence of atoms.
To strengthen the case for atoms one needs to determine their size and that means looking beyond traditional chemistry (chemical reactions and the diversity of elements). This story is in large part the story of the determination of Avogadro's Number.
Monolayers can help estimate molecular and atomic sizes. An oil layer spreads fast, but only so far. This suggests a minimum thickness is needed, a minimum size, and from there an estimate of the size of an oil molecule is possible. With an estimate for an atomic size, the case for atoms is considerably stronger.
Brownian Motion is a visually compelling and sophisticated confirmation of the existence of atoms. Under mild magnification, small blobs of a carefully chosen substance in solution can be seen drifting randomly as they are pummeled by legions of minute invisible molecules.
If the molecules were infinitely small and infinitely numerous, the imbalance would be infinitely minute and therefore would not be observable. Brownian motion therefore shows that there is a finite number of molecules in a given volume and a theory of Brownian motion can help determine that number.
Spectral lines, whether emitted or absorbed, are a wonderful way of identifying elements. They can only be understood by the existence of atoms and quantum mechanics, and therefore can be considered yet more indirect proof that atoms exist.
A Geiger Counter allows one to hear (and thus count) the radiation emitted by unstable atomic nuclei as they decay into lighter ones. The fact that the counter emits discrete clicks is a clear indication of the granular nature of the elements.
A Cloud Chamber is used to visualize the tracks of high speed sub-atomic particles as they produce contrails while passing through a super-saturated gas. These observations can leave no doubt here that matter is made of small elements.
The Bubble Chamber is the same idea as the cloud chamber except that instead of producing liquid trails in a gas, it produces gas trails in a liquid. The denser liquid is better for studying faster particles.
A Cyclotron can be used to artificially accelerate ionized atoms. Discrete properties of ions, such as charge and mass, can be measured directly by "boiling" then off a heated filament of some element.
A Scanning Tunneling Microscope (STM) is the closest one can get today to imaging atoms. Beautiful images of atoms can be produced by an STM. These should satisfy even the most hard headed skeptic as to the true existence of atoms.