Shell: definition in chemistry
Shell: A group of electrons in an atom that share the same principal energy level, numbered n = 1, 2, 3 and so on outward from the nucleus. It is also called an energy level.
Each shell can hold up to 2n² electrons: 2 in the first, 8 in the second, 18 in the third and 32 in the fourth. Electrons in shells farther from the nucleus have more energy and are held less tightly. Shells are made of subshells (s, p, d and f), and the outermost occupied shell, the valence shell, controls how an atom bonds.
Examples
- Sodium — 2, 8, 1: three shells with one outer electron
- Chlorine — 2, 8, 7
- Argon — 2, 8, 8, with its third shell holding 8 of a possible 18
- Shells are sometimes lettered K, L, M and N instead of numbered
On the periodic table
An element's period number tells you how many shells its atoms have: potassium, in period 4, has electrons in four shells.