Question

How do you calculate the number of valence electrons in an ion?

Answer 1

Well, what is the element?

Explanation:

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 `Z=11`. How did I know this? I looked at the atomic numbers on the Periodic Table, and since you are doing your chemistry homework, there must be a copy beside you.

If `Z=11`, then in the NEUTRAL element there are 11 electrons, with the standard electronic configuration of `2:8:1`. The sodium cation, `Na^+`, thus MUST have 10 electrons, i.e. 8 electrons in the full valence shell. Do you agree?

And we got the element `Cl`, `Z=17`. Since there are 17 electrons in the neutral element `(2:8:7)`, there must be 18 electrons associated with the ion `(2:8:8)`, i.e. 8 electrons in the full valence shell.