Atomic physics is a branch of physics devoted to the study of the material composition and energetic processes of atoms and molecules.

Concepts of the atom and of atomic structure date to the ancient Greek philosopher Democritus who proposed that all things are made of atoms and that, although they are too small to be seen in their ceaseless motion, they could collide and accumulate.

The contemporary study of physics (i.e., the study of matter and energy and the interaction between them) is roughly divided into two categories: classical physics and modern physics. Classical physics includes the traditional branches of physics (e.g. mechanics) developed before the beginning of the 20th century. Most of classical physics is concerned with behavior of energy and matter on the macroscopic scale of observation. On the other hand, atomic, particle, and nuclear physics comprise areas of modern physics. Unlike...