Molecules and Compounds

Section 1 of 6More than one shared pair

Why more pairs?

• Why more pairs? Some atoms must share more than one pair of electrons to reach a full outer shell.

Double bond

• Double bond Two adjacent atoms share two pairs of electrons, forming a double covalent bond.

Triple bond

• Triple bond Two adjacent atoms share three pairs of electrons, forming a triple covalent bond.

Section 2 of 6Examples of multiple bonds

Molecule	Formula	Bonding
Oxygen	O2	Each atom has six outer electrons and shares two pairs, giving a double bond
Nitrogen	N2	Each atom has five outer electrons and shares three pairs, giving a triple bond
Ethene	C2H4	The two carbon atoms share two pairs, a double bond, and each also bonds to two hydrogens
Carbon dioxide	CO2	Carbon forms a double bond with each oxygen atom
Methanol	CH3OH	All bonds are single: carbon to three hydrogens and to oxygen, then oxygen to hydrogen

Section 3 of 6Worked example: carbon dioxide

1

Carbon is in Group IV with four outer electrons, and each oxygen is in Group VI with six.

2

Carbon needs to share four electrons and each oxygen needs to share two.

3

Each oxygen shares two pairs with the carbon, forming two double bonds in total, O=C=O.

4

Count the electrons: carbon has eight outer electrons and each oxygen has eight, including its two non-bonded pairs.

5

The molecule is neutral and has no brackets or charges.

Section 4 of 6Choosing the correct type of diagram

Ionic or covalent?

Metal and non-metal

• Metal and non-metal The compound is ionic. Draw separate ions, each in square brackets with its charge.

Non-metals only

• Non-metals only The compound is covalent. Draw overlapping shells containing one or more shared pairs.

Check the shells

• Check the shells In both cases make sure each atom or ion ends with a full outer shell.

Section 5 of 6Key terms

Term	Meaning
Double bond	A covalent bond made of two shared pairs of electrons.
Triple bond	A covalent bond made of three shared pairs of electrons.
Compound	A substance made of two or more different elements chemically joined together.
Ethene	A hydrocarbon with formula C2H4 that has a carbon-carbon double bond.
Shared pair	Two electrons, one from each atom, that sit between two nuclei and form a bond.

Section 6 of 6Exam technique

1

Drawing a double bond with only one pair of electrons. A double bond has two shared pairs, so four electrons.

2

Using the wrong type of diagram, such as ions with brackets for a molecule of non-metals.

3

Forgetting that nitrogen has non-bonded pairs as well as its triple bond.

4

Showing carbon dioxide with single bonds, which leaves carbon and oxygen with incomplete outer shells.