Covalent Bonds (Cambridge O Level Chemistry)

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 electric 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

• 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:

 

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

Nitrogen:

• When 2 nitrogen atoms react they share 3 pairs of electrons to form a triple bond

Ethene:

• In ethene, the 2 carbon atoms share 2 pairs of electrons
• This is known as a double bond

Methanol:
Carbon Dioxide: