Question

How ionisation energy and valency related?

Answer 1

Generally, the further the outer electrons are from the nucleus, the weaker the attraction, and so as you go down any column, the first ionization energy decreases. After an orbital (e.g., the s, p, of f orbital, which ever it happens to be) of the outer shell is half full, the the electrons need to pair up to fill the orbital, which increases their repulsion from one another, whereas the more protons in the nucleus the greater the attraction. Also, the number of protons and the number of electrons are the same as one another. So, if the outer orbital of the shell has one more electron than half full, it will have a lower first ionization energy, than the first ionization energy of the atom just prior to it in the same row.