The fact that it has valence electrons makes it "charged". That means it will try to fill its outermost electron level and become stable.
For instance- Na (sodium) and C(h)l(orine)
Sodium has one electron. the closest stable electron configurization is no electrons, so it has one more electron than it needs which means it has a +1 charge.
8 electrons is the most there can be in one layer, and chlorine has 7, so it has one empty spot and a -1 charge.
Na (+1) + Cl (-1)= NaCl (stable)
The electron from the sodium is shared with the chlorine so it will have 8 in its outer layer and the sodium will be empty.
Hope this helps
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