Bound state

In physics, a bound state is a state of some system that needs additional energy to break it into pieces. The parts of a bound state act as a single object until something happens to break it apart.

A molecule is a bound state made of atoms connected by chemical bonds. An atom is a bound state of electrons around the atomic nucleus, bound by the electromagnetic force. The atomic nucleus is a bound state made of protons and neutrons bound by the strong nuclear force.

Stability

Formally, bound states must be stable. A bound state cannot break up on its own, even after a long time. Still, things that will break up over time but act like bound states until then are also often called bound states. Examples of these are excited states of atoms and radioactive atomic nuclei.