Lithium atom, with three electrons orbiting, the first two in the first electron shell, the third in the second electron shell
Electrons

While the protons and neutrons in an atom are found in the nucleus:

  • The electrons orbit the nucleus

The regions of space around the nucleus, that the electrons use for their orbits, are called electron shells.

There are a number of such shells around the nucleus. The number of electrons that can occupy each shell is limited.