Period

A horizontal row of the periodic table. The period number is the principal quantum number $n$ of the valence shell, and across a period $Z_{\text{eff}}$ rises, so radius shrinks and ionization energy grows.

Example

Period 3 runs from Na ( $[\mathrm{Ne}]\,3s^1$ ) to Ar ( $[\mathrm{Ne}]\,3s^2\,3p^6$ ): 8 elements, because $3s$ and $3p$ together hold 8 electrons.

See alsoGroup Principal quantum number Effective nuclear charge Atomic radius