What Is a periodic property ? How do the following properties change in a group and period ? Explain .
Electron affinity

Explanation

Periodic property :Periodicity means that when elements are arranged accoding to their electronic configuratiin , the properties of elements in the perioodic table reoccr at regular intervals - such properties which reoccur are called called as periodic properties
change of properties in a group and a period :

Electron affinity : The electron affinity of an element is defined as the energy liberated when an electron is added to its neutral gaseous atom . Electron affinity of an atom is also called electron gain enthalpy of that element .
Variation in group : Electron gain enthalpy values decreases as we go down in a group due to increase in atomic radius .
Variation in period Electron gain enthalpy values increase as we go from left to right in a period , due to decrease in atomic radius .