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© Boardworks Ltd 20152 of 10

What is a covalent bond?

Non-metal elements usually just need one or two electrons to fill their outer shells. So how do they form a bond?

The shared electrons join the atoms together. This is called a covalent bond.

The two non-metal atoms cannot form a bond by transferring electrons from one to another. Instead, they share electrons.

Each atom now has a full, stable outer shell.

incomplete outer shellsCl

Cl

Cl

Cl

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Two common ways to represent a covalent bond are:

Only outer shells of electrons are involved in bonding, so the inner shells do not always have to be included in diagrams.

solid line

Cl Cl–

–Cl Cl

A covalent bond consists of a shared pair of electrons.

covalent bond

Cl

simplified dot and cross

diagramCl

How is a covalent bond drawn?

Cl Cl

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What are the types of covalent bonds?

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Can compounds contain covalent bonds?

Covalent bonding can also occur between atoms of different non-metals to create molecules of covalent compounds. These covalent bonds can be single, double or triple.

How is a covalent bond formed in hydrogen chloride (HCl, also represented as H–Cl)?

H ClH Cl

Hydrogen and chlorine both need one more electron to fill outer shells. By sharing one electron each, they both have a stable outer shell and a covalent bond is formed.

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How is the ratio of atoms calculated?

To calculate the ratio of atoms in a stable covalent compound:

For example, how many nitrogen and hydrogen atoms bond together in an ammonia molecule?

1. Work out how many electrons are needed by each non-metal element to complete its outer electron shell.

2. Work out the ratio of atoms that will provide enough shared electrons to fill all the outer shells.

N H

(2.5) (1)

3electrons needed

electron configuration

ratio of atoms

element

1

1 3

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H

Covalent bonding in methane

How do carbon and hydrogen atoms form covalent bonds in a molecule of methane?

CH4 or H C H

H

H

C H

(2.4) (1)

4electrons needed

electron configuration

ratio of atoms

element

1

1 4

H

H HC

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Covalent bonding in carbon dioxide

How do carbon and oxygen atoms form covalent bonds in a molecule of carbon dioxide?

C O

(2.4) (2.6)

4electrons needed

electron configuration

ratio of atoms

element

2

1 2

CO2 or O C O

O OC

double bonds

A double bond is when two pairs of electrons are shared.

In carbon dioxide there are two double bonds – one between each oxygen atom and the carbon atom.

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What are simple covalent structures?

Covalent molecules that contain only a few atoms are called simple covalent structures.

weak bonds between molecules

strong bonds within molecules

Most substances that contain simple covalent molecules have low melting and boiling points and are therefore liquids or gases at room temperature, e.g. water, oxygen, carbon dioxide, chlorine and hydrogen. Why?

The covalent bonds within these molecules are strong but the bonds between molecules are weak and easy to break.

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What are giant covalent structures?

In some substances, such as sand, diamond and graphite, millions of atoms are joined together by covalent bonds.

All the bonds are covalent, so giant covalent structures have very high melting and boiling points, and are usually hard.

The covalent bonds in these substances do not form molecules but vast networks of atoms called giant covalent structures.