Consider Group 5A elements: nitrogen, phosphorus, and arsenic. These elements show an increase in their atomic numbers. Which element has the highest ionization energy?

It will be nitrogen.

The energy associated with the formation of 1 mol of gaseous cations is The ionization energy. 1 mole of electrons will form 1 mole of gaseous atoms under standard conditions. The ionization energy decreases as you go down a group on the periodic table.