How does ionisation energy vary along a period and down a group ?

Explanation

Along the period
Ionization enthalpy increases from left to right along the period in the periodic table. This is because along the period, nuclear charge increases at every step. With increasing nuclear charge, electrons in outer shells are attracted with greater force at every step. So binding force of electron to nucleus is increasing from left to right along the period. Therefore, ionization energy increases regularly along the period.
Down the group
Ionization enthalpy decreases down the group. It is because as we move down the group, size of the atoms increases. Screening effect also increases. As a result, outer shell electrons are held by nucleus with lesser force. Therefore, less energy is required to remove the electrons. So, ionization energy decreases down the group. Some exceptions are there: I.E. of `Be gt I.E`. of B
Similarly I.E. of `NgtO`
This is due to completely filled and half filled electronic configurations of Be and N respectively.