Covalent Bonds and Full Outer Shells

We have learnt previously that atoms tend to gain or lose enough electrons to form a full outer shell. This is because having a full outer shell is a very stable arrangement.

We can use this information to predict how many covalent bonds an atom will form. We know that the number of outer shell electrons an atom has is equal to the group number of its element. Each single covalent bond increases the number of outer shell electrons by 1, each double bond increases it by 2 and each triple bond increases it by 3. An atom will usually keep forming covalent bonds until it has a full outer shell.

We will now go through each group of the periodic table and see how many covalent bonds the atoms in that group form. We still start with group 0 and work our way left to group 1.

The periodic table.

Atoms of group 0 elements do not form covalent bonds

Atoms of group 0 elements already have a full outer shell. Therefore they do not take part in covalent bonding at all.

Atoms of group 7 elements form 1 covalent bond

Atoms of group 7 elements have 7 electrons in their outer shell, meaning that they only need to gain 1 electron to form a full outer shell. Therefore they form 1 covalent bond.

Atoms of group 6 elements form 2 covalent bonds

Atoms of group 6 elements have 6 outer shell electrons, meaning that they need to gain 2 electrons in order to get a full outer shell.

Therefore, atoms of group 6 elements tend to form two covalent bonds. This could be two single bonds or it could be one double bond.

Atoms of group 5 elements form 3 covalent bonds

Atoms of group 5 elements have 5 electrons in their outer shell, meaning that they need to gain 3 electrons in order to form an full outer shell.

Therefore, atoms of group 5 elements tend to form three covalent bonds. This could be three single bonds, a double bond and a single bond, or one triple bond.

Atoms of group 4 elements form 4 covalent bonds

Atoms of group 4 elements have 4 electrons in their outer shell, meaning that they need to gain 4 electrons in order to form a full outer shell.

Therefore, atoms of group 4 elements tend to form four covalent bonds. This could be four single bonds, two double bonds, a double bond and two single bonds, or a triple bond and a single bond.

Atoms of group 3 elements form 3 or 4 covalent bonds

In group 3, things are a bit more complicated. Atoms of group 3 elements have 3 electrons in their outer shell, meaning that they need to gain 5 electrons in order to get a full outer shell. Therefore, we might expect them to form 5 covalent bonds.

However, it is not actually possible for them to do this. In order to form a covalent bond, each atom usually contributes one electron to form a shared pair of electrons. Since group 3 atoms only have 3 electrons to contribute, they usually only form 3 covalent bonds.

Forming 3 covalent bonds leaves a group 3 atom with 6 electrons in its outer shell. This is not a full outer shell - there is room for another pair of electrons.

Therefore, it is possible for a fourth covalent bond to form, but both of the electrons for this fourth bond have to be supplied by the other atom, because the group 3 atom has no more electrons to contribute.

Atoms of group 2 elements do not form covalent bonds

Atoms of group 2 elements do not usually take part in covalent bonding. Instead, they tend to get a full outer shell by losing electrons to become ions.

Atoms of group 1 elements do not form covalent bonds - apart from hydrogen

Atoms of group 1 elements do not tend to take part in covalent bonding either. Instead, they gain a full outer shell by losing an electron to become an ion.

The one exception to this is hydrogen, which does regularly take part in covalent bonding.

A hydrogen atom has 1 electron in its outer shell. Since its outer shell is the first shell, which has a capacity of 2, it only needs to gain 1 electron to form a full outer shell. Therefore, hydrogen atoms tend to form 1 covalent bond.

Some transition elements take part in covalent bonding

Atoms of some transition elements also form covalent bonds. For this course, you do not need to know which transition elements these are or how many covalent bonds they form.

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.