Electrons are tiny, negatively charged particles that spin around the nucleus of atoms. The mass of an electron is reckoned to be 1/2000^{th} of the mass of a hydrogen atom (the lightest atom). They move in pathways called orbitals or shells. The number of electrons in an atom depends on the atom: For example, hydrogen has one electron, helium has 2 and copper has 29. The more electrons there are, the more orbitals or shells are required to hold them. The first orbital can hold 2 electrons, then it is full. The second orbital can hold 8 electrons, then it is full.

In order to calcualte how many electrons an orbital or shell can hold, use the formula: 2n^{2}, where n = the number of the orbital. For example:-

For shell 1, substitute the 1 for n, giving us 2x(1)^{2} = 2 x 1 = 2.

For shell 2, substitute the 2 for n, giving us 2x(2)^{2} =2 x 4 = 8.

For shell 3, substitute the 3 for n, giving us 2x(3)^{2} =2 x 9 = 18.