Britannica
Sources used in this answer
Why does the first period of the periodic table contain only hydrogen and helium?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
chemistry
A ball-and-stick model of molecular structure, showing atoms bonded together.
The first period of the periodic table contains only hydrogen and helium because it corresponds to the filling of the first electron shell (n = 1), which consists of only a single orbital, known as the 1s orbital.123
According to the Pauli exclusion principle, no more than two electrons can occupy any single orbital, and if two electrons do occupy the same orbital, they must have opposite spins. Because the n = 1 shell contains only this one orbital, it can hold a maximum of two electrons. Hydrogen has one electron in this orbital, and helium has two, completing the shell. Once this shell is full, any additional electrons must enter a higher energy shell (n = 2), which marks the beginning of the second period.14532