The ionisation enthalpy decreases on moving down a group and increases as we move along a period

1)ATOMIC SIZE

As we go down the group effective nuclear charge decreases as the distance of the nucleus from the outermost shell increases which decreases its tendency to lose electron. As we go along the period the ionisation enthalpy increases as effective nuclear charge increases. The increase in number of shells result in decrease in ionisation enthalpy.

2)EFFECTIVE NUCLEAR CHARGE

As we go from left to right in a period the effective nuclear charge increases and when we move down the group it decreases which results in the attraction of electrons from the outermost shell.

3) ELECTRONIC CONFIGURATION

The tendency to gain electrons depends upon the stability of an element. Elements having completely or half filled stable orbitals have very low tendency to gain electron thus they have very low electron gain enthalpy.

Group 1 elements have the lowest ionisation enthalpy as they have just one electron in the outermost shell and as we go down the group this ionisation enthalpy decreases due to increase in atomic size. As we go along the period from group 1 to group 17 elements then ionisation energy increases as they have 7 electrons in the outermost shell and a high effective nuclear charge on the outermost shell due to which it is difficult to lose an electron.