How do you calculate the number of valence electrons in an ion?
Well, what is the element?
If the element is a metal, then it probably LOSES electrons to make a cation. If the element is a non-metal, it probably GAINS electrons to make a cation.
And if it is a main group element, then typically the resultant ion is isoelectronic with the LAST Noble Gas (metals), or the NEXT Noble Gas (non-metals), that is it possesses a FULL valence shell.
I will give you two bones. We got the element sodium. And for sodium
And we got the element