Covalent bonding

A covalent bond is a strong bond between two non-metal atoms. It consists of a shared pair of electrons. A covalent bond can be represented by a straight line or dot-and-cross diagram.

Hydrogen and chlorine can each form one covalent bond, oxygen two bonds, nitrogen three, while carbon can form four bonds.

A shared pair of electrons

You will need to understand what covalent bonding is, and to remember some of the properties of molecules that are formed in this way.

A covalent bond forms when two non-metal atoms share a pair of electrons. The electrons involved are in the highest occupied energy levels - or outer shells - of the atoms. An atom that shares one or more of its electrons will complete its highest occupied energy level.

Covalent bonds are strong - a lot of energy is needed to break them. Substances with covalent bonds often form molecules with low melting and boiling points, such as hydrogen and water.

The animation shows a covalent bond being formed between a hydrogen atom and a chlorine atom, to form hydrogen chloride.

After bonding, the chlorine atom is now in contact with eight electrons in its highest energy level - so it is stable. The hydrogen atom is now in contact with two electrons in its highest energy level - so the hydrogen is also stable.

How many bonds?

Atoms may form multiple covalent bonds - that is, share not just one pair of electrons but two or more pairs. Atoms of different elements will form either one, two, three or four covalent bonds with other atoms.

There is a quick way to work out how many covalent bonds an element will form. The number of covalent bonds is equal to eight minus the group number (you can brush up on group numbers by reading through the section in AQA GCSE Science on the Periodic Table). The table below gives more detail on this rule:

 

	Group 4	Group 5	Group 6	Group 7
Example	Carbon	Nitrogen	Oxygen	Chlorine
Number of bonds	8 - 4 = 4	8 - 5 = 3	8 - 6 = 2	8 - 7 = 1

Hydrogen forms one covalent bond. The noble gases in Group 0 do not form any

Representing covalent bonds

Covalent bonds can be represented in several different ways.

Straight lines and models

Straight lines are the most common way to represent covalent bonds, with each line representing a shared pair of electrons. 2D or 3D molecular models are especially useful for showing the relationship between atoms in multiple covalent bonds. Below are some examples of straight lines and images of 3D models.

Models for covalent bonds

Element	Formula	Chemical structure	Ball-and-stick model
Hydrogen	H2
Water	H2O
Ammonia	NH3
Methane	CH4

Double and triple bonds

Note that molecules can have a double covalent bond - meaning they have two shared pairs of electrons - or a triple covalent bond - three shared pairs of electrons. A double covalent bond is shown by a double line, and a triple bond by a triple line.

A molecule of oxygen (O₂) consists of two oxygen atoms held together by a double bond, like this: 

A molecule of nitrogen (N₂) has two nitrogen atoms held together by a triple bond, like this: 

Dot-and-cross diagrams - elements

Dot-and-cross diagrams

Dot-and-cross diagrams are used to represent covalent bonds. The shared electron from one atom is shown as a dot, while the shared electron from the other atom is shown as a cross.

When drawing dot-and-cross diagrams for covalent bonds, you only need to show the electrons in the highest occupied energy level, as only these are involved.

The animations show covalent bonds represented by both displayed formulae (which use straight lines to represent bonds) and dot-and-cross diagrams:

Covalent bonding between two hydrogen atoms to form a molecule of hydrogen gas, H₂.

Covalent bonding between two oxygen atoms to form a molecule of oxygen gas, O₂.

Elements

For your examination, you need to be able to draw dot-and-cross diagrams for hydrogen, chlorine and oxygen.

You do not need to use colours in your answers.

Dot-and-cross diagrams - compounds

You will also need to be able to draw dot-and-cross diagrams representing thecovalent bonds in the molecules of some common compounds:

Hydrogen chloride, HCl

Hydrogen atoms and chlorine atoms can each form one covalent bond. One pair of electrons is shared in a hydrogen chloride molecule (HCl).

Water, H₂O

Hydrogen atoms can each form one covalent bond, while oxygen atoms can each form two covalent bonds. Two pairs of electrons are shared in a water molecule (H₂O).

Ammonia, NH₃

Hydrogen atoms can each form one covalent bond, while nitrogen atoms can each form three covalent bonds. Three pairs of electrons are shared in an ammonia molecule (NH₃).

Methane, CH₄

Hydrogen atoms can each form one covalent bond, while carbon atoms can each form four covalent bonds. Four pairs of electrons are shared in a methane molecule (CH₄).