Quantum numbers describe values of conserved quantities in the dynamics of the quantum system. Perhaps the most peculiar aspect of quantum mechanics is the quantization of observable quantities, since quantum numbers are discrete sets of integers or half-integers. This is distinguished from classical mechanics where the values can range continuously. Quantum numbers often describe specifically the energies of electrons in atoms, but other possibilities include angular momentum, spin etc. Any quantum system can have one or more quantum numbers; it is thus difficult to list all possible quantum numbers.