# quantum

A quantum is a discrete quantity of energy *hν* associated with a wave of frequency *ν*, where *h* is Planck's
constant. It is the smallest amount of energy that can be absorbed or
radiated by matter at that frequency.

Fundamental to quantum theory is the notion that energy is quantized: that material systems can exist
only in certain discrete and defined energy states and that transitions
between these states can only occur by the gain or loss or particular quantities
of energy. In the accompanying diagram a system in energy state E_{2} changes to energy state E_{1}, emitting electromagnetic radiation
in the process. The frequency (*ν*) of this radiation is determined
by the energy change (E_{2} – E_{1}) involved in the transition
according to the equation E_{2} – E_{1} = *hν*.