Modelling molecules

There are different ways of representing small covalent , including dot and cross diagrams, structural formulae and three-dimensional structures.

Structural formulae

A structural formula shows the bonds between the in a molecule, or in a small section of a large molecule or a giant covalent structure. In these diagrams:

each atom is shown by its chemical symbol
each is shown as a straight line

For example, an ammonia molecule has three covalent bonds. The diagram shows its structure.

Three-dimensional structures

The structure of a small molecule can also be shown as a three-dimensional ball-and-stick model. These models show how the atoms and bonds are arranged in space. The diagram shows a ball-and-stick model of ammonia.

Question

Give one advantage and one limitation of using a structural formula to represent a molecule.

Dot and cross diagrams

A dot and cross diagram can show the bonding in a small molecule:

the outer shell of each atom is drawn as a circle
circles overlap where there is a covalent bond
from one atom are drawn as dots, and electrons from another atom as crosses

For example, a hydrogen molecule, H₂, forms when two hydrogen atoms each share their outer electron.

Dot and cross diagram of a hydrogen bonding — Figure caption,
A dot and cross diagram to show the bonding in hydrogen

An ammonia molecule, NH₃, forms when one nitrogen atom shares its outer electrons with three hydrogen atoms. There are two types of dot and cross diagram - one without circles, and one with.

Dot and cross diagram of neumonia. — Figure caption,
These dot and cross diagrams show the bonding in ammonia

Question

Give one advantage and one limitation of using a dot and cross diagram to represent a molecule.

How many bonds?

Atoms form covalent bonds by sharing electrons to get a full outer shell. This means that the number of covalent bonds an atom can form is the same as the number of electrons needed to get a full outer shell. For most , a full outer shell is eight electrons.

The table below shows the number of bonds formed by elements in groups 4 to 7.

Group	Example	Electrons	Covalent bonds
4	Carbon, C	4	4
5	Nitrogen, N	5	3
6	Oxygen, O	6	2
7	Chlorine, Cl	7	1

Group	4
Example	Carbon, C
Electrons	4
Covalent bonds	4

Group	5
Example	Nitrogen, N
Electrons	5
Covalent bonds	3

Group	6
Example	Oxygen, O
Electrons	6
Covalent bonds	2

Group	7
Example	Chlorine, Cl
Electrons	7
Covalent bonds	1

Hydrogen atoms only have one electron and form one covalent bond as they only need one more for a full outer shell.

Question

Draw a dot and cross diagram for methane, CH₄.

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Covalent bonding - AQA SynergyModelling molecules