Covalent Bonding (Cambridge (CIE) AS Chemistry): Revision Note

Defining Covalent Bonding

• Covalent bonding occurs between two non-metals

• A covalent bond involves the electrostatic attraction between nuclei of two atoms and the bonding electrons of their outer shells

• No electrons are transferred but only shared in this type of bonding

• Non-metals are able to share pairs of electrons to form different types of covalent bonds

• Sharing electrons in the covalent bond allows each of the 2 atoms to achieve an electron configuration similar to a noble gas

  • This makes each atom more stable

Covalent bonds & shared electrons table

• Single / C–C bond

  • 2 electrons shared

• Double / C=C bond

  • 4 electrons shared

• Triple / C$\equiv$C bond

  • 6 electrons shared

Examples of Covalent Bonding

Dot & cross diagrams

• Dot and cross diagrams are used to represent covalent bonding

• They show just the outer shell of the atoms involved

• To differentiate between the two atoms involved, dots for electrons of one atom and crosses for electrons of the other atom are used

• Electrons are shown in pairs on dot-and-cross diagrams

Single covalent bonding 

Hydrogen, H₂ 

• Each hydrogen atom has one outer electron

• By sharing their outer electrons, the two hydrogen atoms are able to form a hydrogen molecule

  • The molecule contains a single covalent bond due to one shared pair of electrons

Covalent bonding in hydrogen, H₂

Chlorine, Cl₂ 

• Each chlorine atom has seven outer electrons

  • Six electrons are paired and one electron is unpaired

• By sharing their unpaired outer electrons, the two chlorine atoms are able to form a chlorine molecule

  • The molecule contains a single covalent bond due to one shared pair of electrons

Covalent bonding in chlorine, Cl₂

Hydrogen chloride, HCl 

• The hydrogen atom has one outer electron and the chlorine atom has seven outer electrons

  • The chlorine atom has six paired electrons and one unpaired electron

• When the hydrogen atom pairs its outer electron with the unpaired electron from chlorine, the two atoms are able to form a hydrogen chloride molecule

  • The molecule contains a single covalent bond due to one shared pair of electrons

Covalent bonding in hydrogen chloride, HCl

Ammonia, NH₃ 

• The hydrogen atoms have one outer electron and the nitrogen atom has five outer electrons

  • The nitrogen atom has one lone pair of electrons and three unpaired electrons

• When each hydrogen atom pairs its outer electron with each of the unpaired electrons from nitrogen, the nitrogen and hydrogen atoms are able to form an ammonia molecule

  • The molecule contains three single covalent bonds due to three shared pairs of electrons

Covalent bonding in ammonia, NH₃

Methane, CH₄ 

• The hydrogen atoms have one outer electron and the carbon atom has four outer electrons

  • The carbon atom has four unpaired electrons

• When each hydrogen atom pairs its outer electron with each of the unpaired electrons from carbon, the carbon and hydrogen atoms are able to form a methane molecule

  • The molecule contains four single covalent bonds due to four shared pairs of electrons

Covalent bonding in methane, CH₄

Ethane, C₂H₆ 

• The hydrogen atoms have one outer electron and the carbon atoms have four outer electrons

  • The carbon atom has four unpaired electrons

• Each carbon atom shares one of its unpaired electrons with the other carbon atom

  • This results in the formation of a single covalent bond between the two carbon atoms

• When each hydrogen atom pairs its outer electron with the remaining unpaired electrons from carbon, the carbon and hydrogen atoms form six single covalent C-H bonds

  • This results in an ethane molecule, which contains six single covalent C-H bonds and one single covalent C-C bond 

Covalent bonding in ethane, C₂H₆

Double covalent bonding

Oxygen, O₂ 

• Each oxygen atom has six outer electrons

• By sharing two of their outer electrons, the two oxygen atoms are able to form an oxygen molecule

  • The molecule contains a double covalent bond due to two shared pair of electrons

Covalent bonding in oxygen, O₂

Carbon dioxide, CO₂ 

• Each oxygen atom has six outer electrons and the carbon atom has four outer electrons

• Each oxygen atom shares two of its outer electrons with the carbon atom, which forms a carbon dioxide molecule

  • The molecule contains two double covalent bonds due to two sets of two shared pairs of electrons

Covalent bonding in carbon dioxide, CO₂

Ethene, C₂H₄ 

• Each hydrogen atom has one outer electron and the carbon atoms have four outer electrons

• Each carbon atom shares two of its outer electrons with the other carbon atom

  • This results in the formation of a double covalent bond between the two carbon atoms

• When each hydrogen atom pairs its outer electron with the remaining unpaired electrons from carbon, the carbon and hydrogen atoms form four single covalent C-H bonds

  • This results in an ethene molecule, which contains four single covalent C-H bonds and one double covalent C=C bond 

Covalent bonding in ethene, C₂H₄

Triple covalent bonding

Nitrogen, N₂ 

• Each nitrogen atom has five outer electrons

• By sharing three of their outer electrons, the two nitrogen atoms are able to form a nitrogen molecule

  • The molecule contains a triple covalent bond due to three shared pairs of electrons

Covalent bonding in nitrogen, N₂

• In some instances, the central atom of a covalently bonded molecule can accommodate more or less than 8 electrons in its outer shell

• Being able to accommodate more than 8 electrons in the outer shell is known as ‘expanding the octet rule’

• Accommodating less than 8 electrons in the outer shell means than the central atom is ‘electron deficient’

• Some examples of this occurring can be seen with Period 3 elements

Sulfur dioxide, SO₂ 

• Each oxygen atom has six outer electrons and the sulfur atom, also, has six outer electrons

• Each oxygen atom shares two of its outer electrons with the sulfur atom, which forms a sulfur dioxide molecule

  • The molecule contains two double covalent bonds due to two sets of two shared pairs of electrons

• Sulfur now has an expanded octet as it has a share of 10 electrons

Sulfur dioxide, SO₂ – dot and cross diagram

Phosphorus pentachloride, PCl₅

• Each chlorine atom has seven outer electrons and the phosphorus atom has five outer electrons

  • The chlorine atom has six paired electrons and one unpaired electron

• When each chlorine atom pairs its unpaired outer electron with one outer electron from phosphorus, a single covalent P-Cl bond forms

  • The overall phosphorus pentachloride molecule contains five single covalent bonds 

• Phosphorus now has an expanded octet as it has a share of 10 electrons

Phosphorus pentachloride, PCl₅ – dot and cross diagram

Sulfur hexafluoride, SF₆

• Each fluorine atom has seven outer electrons and the sulfur atom has six outer electrons

  • The fluorine atom has six paired electrons and one unpaired electron

• When each fluorine atom pairs its unpaired outer electron with one outer electron from sulfur, a single covalent S-F bond forms

  • The overall sulfur hexafluoride molecule contains six single covalent bonds 

• Sulfur now has an expanded octet as it has a share of 12 electrons

Sulfur hexafluoride, SF₆ – dot and cross diagram