In chemical bonding: Arrangement of the elements. The horizontal rows of the periodic table are called periods. Each period corresponds to the successive occupation of the orbitals in a valence shell of the atom, with the long periods corresponding to the occupation of the orbitals of a d subshell.
A period in a periodic table are the atomic numbers that increase from left to right..The periods are numbered from 1 to 7 on the left-handed side of the table.
