Covalent Bonds (Cambridge International IGCSE Science)

The formation of covalent bonds

Spec point — State that a covalent bond is formed when a pair of electrons is shared between two atoms leading to noble gas electronic configurations

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

• Covalently bonded substances may consist of small molecules or giant molecules

• 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 to show the formation of a covalent bond

Diagram showing how a covalent bond forms between two chlorine atoms

Exam Hint When drawing dot-and-cross diagrams for covalent compounds, make sure that the electron shell for each atom is full (remember that the 1st shell can only hold 2 electrons).

Single covalent bonds

Spec point — Describe the formation of covalent bonds in simple molecules, including H2, Cl 2, H2O, CH4, NH3 and HCl. Use dot-and-cross diagrams to show the electronic configurations in these and similar 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:

Chlorine:

Water:

Methane:

Ammonia:

Hydrogen chloride: