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The Full Outer Shells Rule

There is a rule that can be used to predict which ions occur naturally for each element.

The rule states that for each element we can work out which ions occur naturally by starting with an atom of that element and then finding the easiest way to form a full outer shell by gaining or losing electrons.

For example, an oxygen atom has six electrons in its outer shell (which we can tell because it is in group 6). Since the outer shell of an oxygen atom is the second shell, and the capacity of that shell is 8, the easiest way for it to form a full outer shell is to gain 2 electrons. Therefore, we can predict that the only naturally occurring ion of oxygen is the one which has 2 more electrons than an oxygen atom - an O2- ion. This prediction is correct.

The reasoning behind the rule is that having a full outer shell makes an ion more stable, and therefore ions which have full outer shells are more likely to exist than ones which do not.

However, the rule does not work perfectly. There are some naturally occurring ions which do not have full outer shells. The rule generally works well for the elements in groups 1-3, groups 5-7 and group 0. It does not work so well for the elements in group 4 or the transition elements.

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What is the rule for working out which ions occur naturally? Which elements does it work for?

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