The modern atomic theory is a theory that all matter is composed of tiny particles called atoms. This theory is used in physics and chemistry to explain the behavior of all matter.
The modern atomic theory traces back to ancient Greece where philosophers theorized that matter could be divided down until it reached a single, indivisible part. In the 19th century, this concept was brought into mainstream thought as physicists and chemists began to find more evidence for the existence of the atom. It was initially thought that the atom was the smallest particle, until J.J. Thomson discovered the electron in 1897. Today, the theory has grown to include many smaller particles that make up the atom and all matter.