Question

How can we use the valence electrons and the octet rule to predict the charge an element will obtain?

Answer 1

Since the octet rule states that eight valence electrons is a stable state for atoms, look at the difference between eight and the number of valence electrons in the atom.

Explanation:

Without going into a (lengthy) explanation of the reasons behind it, the octet rule tells us that there is a strong tendency for atoms to gain or lose electrons until the number of electrons in the outer shell of the atom becomes eight.

If you are considering a metal, the atom will lose all of its existing valence electrons (although there are exceptions!) thereby emptying its valence shell, and having the full shell below as the "new" valence shell.

As an example, aluminum (or aluminium, depending on where you live!) has three valence electrons. When all three are removed - in a reaction with something like chlorine, for example - the ion that forms has a +3 charge.

With a non-metal, the atom tends to gain additional electrons until the current valence shell reaches eight .

For example, sulfur has six valence electrons. Once it gains two more - which it will acquire through reaction with a metal for instance - it has eight outer electrons, and a charge of -2.

Care must be taken with the metalloids (the semi-metals) such as carbon and silicon, as these can be difficult to classify as electron-gainer or electron-loser. Here, you likely need to consider each case separately, looking at the reaction it is involved in.