Chemistry Foundations

3.6.4 - Ions of Group 1-7 Elements

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# 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 N3- 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. N3-, P3-).

## 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 O2- 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. O2-, S2-).

## 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-).

## Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.

1/7

What relative charge do ions of group 1 elements usually have?

2/7

What relative charge do ions of group 2 elements usually have?

3/7

What relative charge do ions of group 3 elements usually have?

4/7

How are group 4 elements unusual when it comes to the formation of ions?

5/7

What relative charge do ions of group 5 elements usually have?

6/7

What relative charge do ions of group 6 elements usually have?

7/7

What relative charge do ions of group 7 elements usually have?