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

Sep 15, 2017

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, $N {a}^{+}$, thus MUST have 10 electrons, i.e. 8 electrons in the full valence shell. Do you agree?

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