Modelling molecules
There are different ways of representing small covalent A collection of two or more atoms held together by chemical bonds., including dot and cross diagrams, structural formulae and three-dimensional structures.
Structural formulae
A structural formula shows the bonds between the The smallest part of an element that can exist. in a molecule, or in a small section of a large A large molecule formed from many identical smaller molecules known as monomers. molecule or a giant covalent structure. In these diagrams:
- each atom is shown by its chemical symbol
- each A bond between atoms formed when atoms share electrons to achieve a full outer shell of electrons. 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.
The structural formula shows the bonds between the atoms, but it does not show which atoms the electrons in the bonds have come from.
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
- Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. 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.
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.
Question
Give one advantage and one limitation of using a dot and cross diagram to represent a molecule.
A dot and cross diagram shows the pairs of outer electrons, and which atoms they have come from. It does not show how the atoms are arranged in space.
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 substance made of one type of atom only., 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.