The main (radial) quantum number is an integer denoting the number of the energy level. It characterizes the energy of electrons occupying a given energy level. It is the first in a series of quantum numbers, which includes the main, orbital and magnetic quantum numbers, as well as spin. These four quantum numbers determine the unique state of an electron in an atom (its wave function). The main quantum number is denoted by n. With an increase in the principal quantum number, the orbit radius and electron energy increase. The main quantum number is equal to the period number of the element.