Question

Why can't an atom be further divided?

Answer 1

The notion of an atom is much older than the modern discovery of atoms. The idea goes back to Democritus, around 400 BCE, who believed that the world was made of tiny, indivisible particles. This is about more than just thinking that there is a smallest division of matter. It also meant that there were only a smallish number of different kinds of atoms in the world, and that the whole complexity of the world could be understood in terms of the interactions of large numbers of simple particles.

It was inspired guesswork, coming from philosophy rather than any physical observation. But when 18th and 19th century scientists showed that there were in fact tiny particles that formed the basic elements of matter, it seemed to vindicate Democritus' idea. It was natural to use his Greek word, "atom", which also gave its name to the Atomist school of which Democritus was part. Atoms were very much like the original philosophical conception of them: fundamental, irreducible, and unchangable.

The idea lasted only about a century or so, when it was discovered that atoms were indeed made of smaller parts that could be cut and rearranged. Still, the similarity of atoms to the Greek conception is almost uncanny; besides, the term was already widespread.

Because of quantum mechanics, subatomic particles are less "un-cuttable" than atoms. They seem almost more like waves than particles, and actually aren't either one. And atoms really are pretty atomic under most circumstances: splitting an atom is very difficult, and you can do a lot of chemistry by rounding that up to "impossible".