The history & defination of an Atom

For many Years, People have wondered what matter is made up of. Some ancient Greek Philosophers believed that all mater was made up of four substances, namely earth, water, air and fire. Ancient Chinese philosophers believed that matter was made up of gold, wood, water, fire and earth.
About 430 B.C., a Greek philosopher named Democritus suggested that matter was made up of indivisible particles called "Atoms". This came from the Greek word atomos, meaning indivisible. He believed that nothing could be smaller than an atom.

Since then, scientific experiments have shown hat all matter is indeed made up of atoms. However, they are very different from what Democritus thought them to be.

An atom is defined as the smallest particle of an element that can exist. They are the building block of all matter.