Answer: the first ionization energy decreases.
Justification:
1) The group 15 is formed by N, P, As, Sb, Bi, and Mc.
2) The first ionization energy is defined as the energy needed to remove an electron from the neutral atom in the gas state.
3) The elements of the group 15 have the following general electron configuration for the valence shell: ns² np³. Where n is the principal quantum number (the same number of the row in which the element is).
4) As you go down in the group, n increases, and the valence electrons are further away of the nucleous, meaning that those electrons are lessen attracted to the nucleous.
Consequently, as you go down in the group, the electrons will be removed more easily, i.e less energy will be required to get them removed.
5) That permits you to predict this order in the first ionization energies: N > P > As > Sb > Bi > Mc.
And that agrees with the data that you can find in a table of first ionization energies.
