electron shells

Electron shells can be thought as an orbit that electrons follow around an atom's nucleus. The shells correspond to the principle quantum numbers. Each electron shells is composed of one or more subshells.

Electrons are organized according to their energy levels into shells. Generally, the higher the energy of a shell, the farther it is from a nucleus. Shells don't have a specific distances from the nucleus, but an electron in a higher-energy shell will spend more time farther from the nucleus than an electron in a lower-energy shell.

The shorthand description for the arrangement of electrons in an atom can be written in the form of electron configuration.