What is an atom?

In the early nineteenth century, John Dalton, an English chemist, meteorologist and physicist, stated in an atomic theory that matter was made up of tiny, indivisible particles, which he also called atoms. His laboratory work showed him that atoms could neither be divided into smaller parts nor could they be destroyed. He pictured matter as being made up of tiny, solid, spherical atoms. Today the idea of the atoms has been accepted. But further work has shown that contrary to Dalton's findings, atoms are made up of even smaller particles.