Ions of Group 1-7 Elements

We will now apply the rule of full outer shells to predict which ions occur naturally for the elements in groups 1-7.

These predictions work well for the elements in groups 1-3 and 5-7, but don't work well for elements in group 4.

Group 1 ions have a relative charge of +1

Atoms of group 1 elements have one electron in their outer shell. This means that the easiest way for them to gain a full outer shell is to lose one electron, making them into an ion with a relative charge of +1.

For example, a lithium atom has two electrons in the first shell and one electron in the second shell. It can get a full outer shell by losing one electron, as shown below:

A lithium atom has 1 outer shell electron. It forms an ion by this electron, which leaves it with a full outer shell. The ion that it becomes has a charge of +1. Its formula is Li+.

The reason that atoms of group 1 elements form ions by losing electrons rather than gaining electrons is that they can form a full outer shell by losing just one electron, whereas they would have to gain many electrons to form a full outer shell.

For example, a lithium atom would have to gain 7 electrons to form a full outer shell (since its outer shell is the second shell, which has a capacity of 8). It is much easier to lose 1 electron than it is to gain 7.

However, this does not apply to hydrogen. A hydrogen atom has 1 electron, which is in the first shell. Since the first shell only has a capacity of 2, a hydrogen atom could form a full outer shell by losing 1 electron or by gaining 1 electron. This means that hydrogen can form ions with a relative charge of either +1 or -1. This will be discussed in more detail on the next page.

In summary, the naturally occurring ions of group 1 elements usually have a relative charge of +1 (e.g. H⁺, Li⁺, Na⁺, K⁺), although there are also ions of hydrogen that have a relative charge of -1 (H^-).

Group 2 ions have a relative charge of +2

Atoms of group 2 elements have 2 electrons in their outer shells. The easiest way for them to get a full outer shell is to lose 2 electrons, which turns them into ions with a relative charge of +2.

For example, a beryllium atom has the electronic configuration 2,2. It can form a full outer shell by losing 2 electrons, turning it into a Be²⁺ ion:

A beryllium atom has 2 outer shell electrons. It forms an ion by losing both of them, which leaves it with a full outer shell. The ion that it becomes has a charge of +2.

This means that the naturally occurring ions of group 2 elements usually have a relative charge of +2 (e.g. Be²⁺, Mg²⁺, Ca²⁺).

Group 3 ions have a relative charge of +3

Atoms of group 3 elements have 3 electrons in their outer shells. The easiest way for them to get a full outer shell is to lose 3 electrons, which turns them into ions with a relative charge of +3.

For example, a boron atom has the electronic configuration 2,3. It can form a full outer shell by losing 3 electrons, turning it into a B³⁺ ion:

A boron atom has 3 outer shell electrons. It forms an ion by losing all 3 of them, which leaves it with a full outer shell. The ion that it becomes has a charge of +3.

This means that the naturally occurring ions of group 3 elements usually have a relative charge of +3 (e.g. B³⁺, Al³⁺).

Group 4 elements don't usually form ions

Atoms of group 4 elements have 4 electrons in their outer shells. Therefore, in theory, they could get a full outer shell by losing 4 electrons, turning them into ions with a relative charge of +4.

Since the second and third shells both have a capacity of 8, the group 4 elements in periods two and three (carbon and silicon) could also in theory get a full outer shell by gaining 4 electrons, which would turn them into ions with a relative charge of -4.

However, these predictions don't actually match what we observe in reality. In fact, group 4 elements do not usually form ions. In the rare instances where ions of group 4 elements are found, they usually have a relative charge of either +4 (which fits with the rule) or +2 (which does not). There are no naturally occurring group 4 ions which have a relative charge of -4.

This shows that the rule of full outer shells is not a perfect rule. It works well for many elements, but it does not work very well for the elements in group 4. In other words, it is a model which is useful in many situations, but which has limitations.

In this course, we will not come across any ions of group 4 elements, so for our purposes we can simplify things slightly by just remembering that group 4 elements do not usually form ions.

Group 5 ions have a relative charge of -3

Atoms of group 5 elements have 5 electrons in their outer shells. The easiest way for them to get a full outer shell is to gain 3 electrons, which turns them into ions with a relative charge of -3.

Note that unlike the elements in groups 1, 2 and 3, the elements in group 5 form ions by gaining electrons rather than losing them. This is because they would have to lose 5 electrons to form a full outer shell, whereas they only have to gain 3. For this reason, group 5 elements form anions.

For example, a nitrogen atom has the electronic configuration 2,5. It can form a full outer shell by gaining 3 electrons, turning it into a N^3- ion (a nitride ion):

A nitrogen atom has 5 outer shell electrons out of a possible 8. Therefore it forms an ion by gaining 3 electrons, giving it a full outer shell. The ion formed is a nitride ion with a charge of -3.

This means that the naturally occurring ions of group 5 elements usually have a relative charge of -3 (e.g. N^3-, P^3-).

Group 6 ions have a relative charge of -2

Atoms of group 6 elements have 6 electrons in their outer shells. The easiest way for them to get a full outer shell is to gain 2 electrons, which turns them into ions with a relative charge of -2.

For example, an oxygen atom has the electronic configuration 2,6. It can form a full outer shell by gaining 2 electrons, turning it into an O^2- ion (an oxide ion):

An oxygen atom has 6 outer shell electrons out of a possible 8. Therefore it forms an ion by gaining 2 electrons, giving it a full outer shell. The ion formed is an oxide ion with a charge of -2.

This means that the naturally occurring ions of group 6 elements usually have a relative charge of -2 (e.g. O^2-, S^2-).

Group 7 ions have a relative charge of -1

Atoms of group 7 elements have 7 electrons in their outer shells. The easiest way for them to get a full outer shell is to gain 1 electron, which turns them into ions with a relative charge of -1.

For example, a fluorine atom has the electronic configuration 2,7. It can form a full outer shell by gaining 1 electron, turning it into an F^- ion (a fluoride ion):

A fluorine atom has 7 electrons in its outer shell out of a possible 8. Therefore it forms an ion by gaining one electron. This gives it a full outer shell. The ion formed is a fluoride ion with a charge of -1.

This means that the naturally occurring ions of group 7 elements usually have a relative charge of -1 (e.g. F^-, Cl^-, Br^-, I^-).

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