Atoms and Molecules - What Are They

In the fifth century BC, a Greek philosopher named Democritus theorized that the smallest particle in nature was an atom.  The theory essentially died when Aristotle disputed it. 

Then, in the nineteenth century, John Dalton revisited the issue, concluding that Democritus was right.  Thereafter, the study of atoms became a focus of intense scrutiny, ultimately leading to revolutionary breakthroughs and the age of technology.

Kevin J. Todeschi, in Edgar Cayce on Vibrations, provides further background (at page x):

It was the Greeks who first theorized the concept of atoms as the building blocks of all matter, believing that different forms of matter were made up of different types and shapes of atoms.  The word atom comes from the Greek word atomos, which means "indivisible."  The Greek philosopher Democritus (ca. fifth century BC) contended that if any form of matter were repeatedly subdivided, eventually a point would be reached whereby that matter could no longer be divided - that point would be an atom.

This video clip, from a now-rare Disney production entitled Our Friend the Atom starring Dr. Heinz Haber, uses animation and other graphics to explain the make-up of atoms and molecules. 

Move the clip ahead, to 3:07, to begin the relevant section.

Credits

Video clip from Our Friend the Atom, a 1957 Disney production, online courtesy banyt's channel at YouTube.

Quoted passage from Edgar Cayce on Vibrations, by Kevin J. Todeschi, page x - online, courtesy Google Books.