Covalent Bonds (Cambridge (CIE) O Level Chemistry): Revision Note

The Formation of Covalent Bonds

Covalent compounds

• Covalent compounds are formed when pairs of electrons are shared between atoms

• Only non-metal elements participate in covalent bonding

• As in ionic bonding, each atom gains a full outer shell of electrons, giving them a noble gas electronic configuration

• When two or more atoms are covalently bonded together, we describe them as ‘molecules’

• Dot-and-cross diagrams can be used to show the electronic configurations in simple molecules

• Electrons from one atom are represented by a dot, and the electrons of the other atom are represented by a cross

• The electron shells of each atom in the molecule overlap and the shared electrons are shown in the area of overlap

• The dot-and-cross diagram of the molecule shows clearly which atom each electron originated from

Diagram showing how a covalent bond forms between two chlorine atoms

Dot & Cross Diagrams of Simple Molecules

• Many simple molecules exist in which two adjacent atoms share one pair of electrons, also known as a single covalent bond (or single bond)

Common Examples of Simple Molecules

Hydrogen:

• Each hydrogen atom has one electron, so the two atoms share one pair and each then has a full first shell of two

Chlorine:

• Each chlorine atom has seven outer electrons, so the two atoms share one pair and each then has a full outer shell of eight, keeping three unshared pairs

Water:

• Oxygen shares one pair of electrons with each of two hydrogen atoms, giving oxygen a full outer shell of eight and each hydrogen a full shell of two, and leaving oxygen with two unshared pairs

Methane:

• Carbon shares one pair of electrons with each of four hydrogen atoms, giving carbon a full outer shell of eight and each hydrogen a full shell of two

Ammonia:

• Nitrogen shares one pair of electrons with each of three hydrogen atoms, giving nitrogen a full outer shell of eight and each hydrogen a full shell of two, and leaving nitrogen with one unshared pair

Hydrogen chloride:

• Hydrogen and chlorine share one pair of electrons, giving hydrogen a full first shell of two and chlorine a full outer shell of eight, and leaving chlorine with three unshared pairs

• Some atoms need to share more than one pair of electrons to gain a full outer shell of electrons

• If two adjacent atoms share two pairs of electrons, two covalent bonds are formed, also known as a double bond

Oxygen:

• Each oxygen atom has six outer electrons, so the two atoms share two pairs to form a double bond, giving each a full outer shell of eight and leaving each with two unshared pairs

• If two adjacent atoms share three pairs of electrons, three covalent bonds are formed, also known as a triple bond

Nitrogen:

• Each nitrogen atom has five outer electrons, so the two atoms share three pairs to form a triple bond, giving each a full outer shell of eight and leaving each with one unshared pair

Ethene:

• The two carbon atoms share two pairs of electrons with each other, forming a double bond, and each carbon shares one pair with two hydrogen atoms, so every atom reaches a full outer shell

Methanol:

• Carbon shares one pair of electrons with each of three hydrogen atoms and one pair with oxygen, and oxygen shares one further pair with a hydrogen atom, leaving oxygen with two unshared pairs

Carbon Dioxide:

• Carbon shares two pairs of electrons with each oxygen atom, forming two double bonds, so every atom reaches a full outer shell of eight, and each oxygen keeps two unshared pairs