What is an atom and what is a molecule? Take a small drop of water. Take a very, very small drop. A tiny-weenie drop. The smallest drop you can take. A molecule of water is the smallest drop you can take with it still being water. Break this up more, and we get atoms.

 

The point is that we cannot divide matter to smaller and smaller pieces forever. There is a smallest piece.

 

Some books say that atoms are the smallest bits of matter. This is wrong, as we can split atoms. The answer is that atoms are the smallest bits of matter using chemistry. The molecules of the elements are atoms. We use other things to split atoms into protons and electrons, such as atomic beams.