3.1.1 Periodicity GapFill

The periodic table is composed of columns called  classesgroupssetsseries and rows called   periodsrotationsintervalscycles. It can also be divided into the s, p, d and f  typessectionsfamiliesblocks, each of which contains the elements whose highest-energy electron lies in the corresponding subshell. The elements are arranged in order of increasing   melting pointreactivityatomic numberrelative atomic mass.

Moving down a column, the number of   neutronselectron shellsbondsprotons in each atom of an element increases; within a row, this number is the same for all elements. In a column, the larger atomic radius and greater   blockingmaskingwrappingshielding effect reduce the attraction between the nucleus and the outer electrons, meaning that first ionisation energy decreases. However, in a row, the most important change is the increase in   strong nuclear forcedensitynuclear chargevalency, leading first ionisation energy to (generally) increase.