Generally,the first ionisation energy increases along a period. But there are some exceptions. The one which is not an exception is

The successive ionisation energy values for an element $X$ are given below. Element $X$ belongs to which group?
$1^{st} \text{ IE} = 410 \ kJ \ mol^{-1}$
$2^{nd} \text{ IE} = 820 \ kJ \ mol^{-1}$
$3^{rd} \text{ IE} = 1100 \ kJ \ mol^{-1}$
$4^{th} \text{ IE} = 1500 \ kJ \ mol^{-1}$
$5^{th} \text{ IE} = 3200 \ kJ \ mol^{-1}$

The first ionization potentials in electron volts $(eV)$ of nitrogen and oxygen atoms are respectively given by:

When the first ionization energies are plotted against atomic number,the peaks are occupied by:

What are the various factors due to which the ionization enthalpy of the main group elements tends to decrease down a group?

Consider the following changes:
$M_{(s)} \to M_{(g)}$ ........$(1)$
$M_{(s)} \to M^{2+}_{(g)} + 2e^-$ .......$(2)$
$M_{(g)} \to M^{+}_{(g)} + e^-$ .........$(3)$
$M^{+}_{(g)} \to M^{2+}_{(g)} + e^-$ .........$(4)$
$M_{(g)} \to M^{2+}_{(g)} + 2e^-$ ..........$(5)$
The second ionization energy of $M$ could be calculated from the energy values associated with: