Each electron shells, designated by the letters K, L, M, N, O, P, and Q or the numbers 1, 2, 3, 4, 5, 6, and 7, are subdivided further into subshells.

The first principal shell, the K shell, has one subshell designated as the 1s shell. The second principal shell has two subshells designated as 2s and 2p. The third shell has three subshells designated 3s, 3p, and 3d. The fourth shell has four subshells, 4s, 4p, 4d, and 4f.

The subshells are further divided into orbitals, each can contain a maximum of two electrons. Therefore an s subshell, which is made up of one orbital, can contain a maximum of two electrons. A p subshell has three orbitals and can contain six electrons. A d subshell has five orbitals and can contain ten electrons. An f subshell has seven orbitals and can contain fourteen electrons.

On the average, an s electron will approach the nucleus more closely than a p of the same principal shell, a p electron more closely than a d electron, and a d electron more closely than an f electron.

The notation used in describing electron structures consists of a principal shell number in front of the subshell letter and a superscript to designate the number of electrons in that particular subshell. For example 3d4 indicates that there are four electrons in subshell d of the M shell.