Principal quantum number

In quantum mechanics, the principal quantum number (n) of an electron in an atom indicates which electron shell or energy level it is in. Its values are natural numbers (1, 2, 3, ...).

Hydrogen and Helium, at their lowest energies, have just one electron shell. Lithium through Neon (see periodic table) have two shells: two electrons in the first shell, and up to 8 in the second shell. Larger atoms have more shells.

The principal quantum number is one of four quantum numbers assigned to each electron in an atom to describe the quantum state of the electron. The other quantum numbers for bound electrons are the total angular momentum of the orbit ℓ, the angular momentum in the z direction ℓ_z, and the spin of the electron s.