Question

how do you find the ionic charge of an element?

Answer 1

There are some rules/guidelines on your book. It says the common charges on some atoms/ions. Example, oxygen usually have 2-, Hydrogen usually has 1+, etc. etc. etc.
Read it now!

When you know these ones, you can find the charge on the ion (oxidation state?).
Ex.
Cr2O3, a neutral compound so Zero Net Charge.
O has 2- charge, as we know. But there are 3 O in total, so 3 times 2- charge is 6- charge.

Now, there are two chromium. Let x be the charge of Chromium.
2x charge + 6- charge must equal to Zero (since Zero Net Charge).
So, x has to be 3+. The Chromium has 3+ in this case.


For NaCl, you will know that Cl has 1- since it is in the 7th group, which means that it needs 1 more electron to become stable and it is a non-metallic atom. Non-metallic ions are anions (negative ions), so negative charges.
NaCl has Zero Net Charge, since it is neutral. Let x be the charge of Na.
There is 1 Na and 1 Cl.
1(x) + 1(1-) = 0
So, x has to be 1+. Na has 1+ charge.

Usually, we look at the anions first. Then, solve for the charge of the cations (positive ions -- for metals).

Note that some transitional elements have different charges, if that have any importance.