Among the elements of the main group, the first ionization energy increases

Going up the group

Explanation:

In any main group, the first ionization energy increases going up the group, while going down, it decreases.

The reason for that lies in understanding the fundamentals of the Coulombic attraction force. Going down the group, atomic radius increases. When atomic radius increases, the distance between a nucleus and valence electrons increases. According to the Coulomb's equation, force is inversely proportional to the square of the distance. This implies that the greater the radius, the lower the attraction between a nucleus and valence electrons.

Now, the lower the attraction, the lower the energy required to remove the electron.

That said, the first ionization energy increases up the group, as atomic radius decreases.