Covalent BondingEdexcel GCSE Chemistry: Revision notes
Section 1
What is a covalent bond?
A covalent bond forms when a pair of electrons is shared between two atoms. Each atom contributes one electron to the shared pair, and the shared pair is attracted to both nuclei, holding the atoms together.
Covalent bonding results in the formation of molecules — groups of atoms held together by covalent bonds.
Always describe a covalent bond as a 'shared pair of electrons', not just 'sharing electrons' — examiners look for the word 'pair'.
Section 2
How small are atoms and molecules?
Atoms and small molecules are extremely tiny. The typical order of magnitude for the size (diameter) of an atom is about 1 × 10⁻¹⁰ m (0.1 nanometres). Small molecules are of a similar order of magnitude, though they can be larger depending on how many atoms they contain.
Section 3
How can simple molecular substances be represented?
Dot and cross diagrams show how electrons are shared in covalent molecules — electrons from one atom are shown as dots and electrons from the other as crosses in the overlapping shared region.
Key simple molecular substances and their bonding:
- Hydrogen (H2): one shared pair between two H atoms
- Hydrogen chloride (HCl): one shared pair between H and Cl
- Water (H2O): oxygen shares one pair of electrons with each of two hydrogen atoms
- Methane (CH4): carbon shares one pair of electrons with each of four hydrogen atoms
- Oxygen (O2): two shared pairs (a double covalent bond) between two O atoms
- Carbon dioxide (CO2): two double covalent bonds, one between carbon and each oxygen atom
In CH4, carbon has 4 outer electrons and needs 4 more — it forms four single covalent bonds, one with each hydrogen atom, giving it a full outer shell of 8 electrons.
Must Know
- A covalent bond is a shared pair of electrons between two atoms
- Covalent bonding forms molecules
- Atoms and small molecules are of the order of 1 × 10⁻¹⁰ m in size
- H2, HCl and H2O involve single covalent bonds; O2 and CO2 involve double covalent bonds
- Methane (CH4): carbon forms four single covalent bonds with hydrogen
- Dot and cross diagrams show which electrons are shared between which atoms
That's the notes covered.
Carry on to the next subtopic.