The periodic table is divided into rows, which are referred to as periods. The electron energy increases with the principal quantum number, each period ends with a noble gas.
The only orbital of the s-subshell can accommodate a maximum of two electrons; the first period (k-shell) is exhausted by hydrogen (1 electron) and helium (2 electrons).
The second period (l-shell) holds eight more electrons in the 2s and 2p subshells. Starting with lithium (3 electrons), the l-shell is completely filled up to neon (10 electrons).
The third period (m-shell) follows the pattern of the l-shell; it holds another eight electrons in the 3s and 3p orbitals. Elements of the 3rd period are sodium (11 electrons) to argon (18 electrons).
From the fourth period (n-shell), regularity is interrupted. After the least required energy, the occupation of the 4s- (K and Ca), 3d- (Sc–Zn) and subsequently 4d-orbitals (Ga–Kr) takes place.
In the fifth period (o-shell), filling is interrupted again. This initially takes place in the orbitals 5s (Rb and Sr) and 4d (Y–Cd), then with higher energy in 5p (In–Xe).
The sixth period (p-shell) follows the pattern of the previous periods by filling the 6s- (Cs and Ba), 5d- (La), 4f- (lanthanides), 5d- (Hf–Hg) and 6p-subshells (Tl–Rn).
In the seventh period (q-shell), following the pattern of the previous period, these are filled: 7s- (Fr and Ra), 6d- (Ac), 5f- (actinides), 6d- (Rf–Cn) and 7p-orbital (Nh–Og).