A quantum of energy is the smallest, indivisible amount of energy that can be emitted or absorbed by a quantum system, that is, an atomic nucleus , atom or molecule .

Previously, it was believed that in all physical phenomena, energy passes from one body to another continuously, so in small amounts. But as long as this opinion existed, it was impossible to calculate the distribution of the energy of the radiation by individual wavelengths from the radiation of a black body . In 1900, Max Planck proposed the quantum theory of energy. According to this theory, which has been fully confirmed by experience, an atom cannot emit or absorb any amount of energy, but only a precisely determined quantum of energy or multiples of that quantum. The size of one quantum of energy is determined by the frequency ν and the constant h , determined by Planck, so it is called Planck's constant .