Why does reactivity increase as you go down Group 1 metals?

An atom is made in such a way that the nucleus with the positive charges (protons) is in the centre and the negative charge (electrons) are arranged in shells around it.

All group 1 metals have one electron in its outer shell. As we go down the group, the atom gets bigger. The bigger the atom, the further away the last electron. Therefore, the attraction between the nucleus and the last electron gets weaker. This makes it easier for the atom to give up the electron which increases its reactivity.