Electron shells

Definition

Discrete energy levels around the nucleus that each hold a limited number of electrons.

The innermost shell fills first with two electrons, then electrons stack outward into successive higher-energy levels until each has a place. The lowest-energy, most stable state is reached when the outermost shell is completely filled. Each period (row) of the periodic table corresponds to filling a new shell.

Related concepts

Valence electrons · Octet Rule · Periodic Table

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