Electron shell

An electron shell, or main energy level, is the part of an atom where electrons are found orbiting the atom's nucleus.

In chemistry and atomic physics, an electron shell may be thought of as an orbit followed by electrons around an atom's nucleus. All atoms have one or more electron shell(s), all of which have varying numbers of electrons.

Each electron shell has one or more electron sub-shells, or sub-levels. Electron shells make up the electron configuration of an atom. The number of electrons that can be in a certain shell is equal to ${\displaystyle 2n^{2}}$, where ${\displaystyle n}$ is the number of a shell.

The name for electron shells came from the Bohr model, which states that electrons orbit the nucleus at certain distances so that their orbits form "shells". This term was presented by Niels Henrik David Bohr.