Why is an element's oxidation number related to its group on the periodic table

alkali metals (group one atoms) have an oxidation number of +1 because they have 1 valence electron and to create a full energy shell it is easier for them to lose one electron than to gain 7 electrons. Losing an electron gives them a charge of +1.

alkali earth metals (group two atoms) have an oxidation number of +2 because they have 2 valence electrons and to create a full energy shell it is easier for them to lose two electrons than to gain 6 electrons. Losing two electrons gives them a charge of +2.

Group thirteen atoms generally have an oxidation number of +3 because they have 3 valence electrons and to create a full energy shell it is easier for them to lose three electrons than to gain 5 electrons. Losing three electrons gives them a charge of +3.

You get the idea.

Although oxidation number and charge aren't directly related they are similar because both come into play in bonds. Charge is simply the charge of an ion (useful in ionic bonds). Oxidation number on the other hand indicates the number of electrons lost or gained in a bond.

I hope that helps.

It's not.
Its CHARGE is related to it's group on the table. This is not always the same as oxidation #.