Only Certain Ions Occur Naturally

In an atom, the number of electrons is equal to the number of protons. This means that all atoms of the same element have the same number of electrons as each other.

For example, copper's atomic number is 29, meaning that all copper atoms have 29 protons and therefore 29 electrons. (The only thing that can vary between different copper atoms is the number of neutrons).

However, in an ion, the number of electrons is not equal to the number of protons. This means that different ions of the same element can have different numbers of electrons.

For example, there are two common types of copper ion: Cu⁺ and Cu²⁺. Cu⁺ has 28 electrons (one less than the number of protons) and Cu²⁺ has 27 electrons (two less than the number of protons).

In theory we could imagine all kinds of different copper ions. The number of electrons just has to be different to 29 (since there are 29 protons). For example we could imagine a copper ion with 24 electrons (which would be Cu⁵⁺) or one with 33 electrons (which would be Cu^4-).

However, when scientists actually examine the copper ions that exist naturally, it turns out that Cu⁺ and Cu²⁺ are the only ones. It is possible to produce other copper ions in a laboratory under very unusual conditions, but Cu⁺ and Cu²⁺ are the only naturally occurring copper ions.

For every element, there are only certain ions which are found naturally. For example, for the element iron the only two ions that are found naturally are Fe²⁺ and Fe³⁺.

For many elements there is only one ion that is commonly found naturally. For example, the only one for oxygen is O^2-.

In this course, we are mostly interested in the ions that are occur naturally (rather than the ones that can only be produced under very unusual conditions). This is because the naturally occurring ions are the ones involved in the chemical reactions we are going to study. On the next few pages we will learn which ions these are.

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