The principal quantum number is the first in a set of four quantum numbers used in quantum mechanics to describe atomic orbitals, or the arrangement of electrons around a nucleus. The principal quantum number represents the overall energy level of the orbital and is often called the "shell." It is denoted with the symbol "n" and is always valued with a positive integer (n=1, 2, 3, etc.). In general, the higher the n value means the higher the energy level of the electron and the more likely it is to be located further from the nucleus. As an example, when the electronic configuration of the single electron in a hydrogen atom is written as 1s^1, the first "1" denotes the principal quantum number of the electron (in other words it is in the first atomic shell).