As the first five elements in group 15 are
considered in order of increasing atomic number,
first ionization energy
(1) decreases
(2) increases
(3) decreases, then increases
(4) increases, then decreases

The correct answer is option 1. The first ionization energy, in group 15 elements in increasing atomic number, decreases. Ionization energy is the amount of energy requires to detach one electron from the atom. In the periodic table, the ionization energy decreases as one goes down.

You need to divided twenty four by a dozen them by the temperature it is getting cook by