For elements in groups 13 to 17, how is the charge of the cation worked out?

1. For groups 13 to 17 the group number runs ten ahead of the count of valence electrons. 2. That offset of ten allows for the d subshell that fills in the fourth and later periods. 3. So subtracting ten from the group number returns the number of valence electrons available to lose. 4. Aluminium sits in group 13, and thirteen minus ten gives the three plus charge it is known for. _Source: OpenStax Chemistry (CC BY 4.0), Ch 7 "Chemical Bonding and Molecular Geometry", section Electronic Structures of Cations_