Cambridge O Level Chemistry · Syllabus 5070 · Atoms, Elements and Compounds
Covalent bond
What is Covalent bond?
A covalent bond is formed when a pair of electrons is shared between two atoms. The shared pair counts towards the outer shell of both atoms at once, so both reach a noble-gas electronic configuration in the molecules required at this level, with hydrogen reaching two electrons and the other atoms reaching eight. Covalent bonding occurs between non-metal atoms.
This definition is part of the Atoms, Elements and Compounds chapter in Cambridge O Level Chemistry.
Common mistakes with Covalent bond
- 6. “Covalent bonds break when a molecular substance boils.” Repair Only the weak forces between molecules are overcome. Steam is still made of \(\mathrm{H_2O}\) molecules, each with its covalent bonds intact. Taught fully in Lesson 2.5B.
- 7. “All covalent substances have low melting points.” Repair Only simple molecular covalent substances do. Diamond, graphite and silicon(IV) oxide are covalent and have very high melting points, because melting them means breaking strong covalent bonds themselves. Taught fully in Lesson 2.6A.
Questions students ask about Covalent bond
Why do simple molecular substances have low melting points if covalent bonds are strong?
Because melting or boiling a simple molecular substance does not break any covalent bonds. The strong covalent bonds are inside each molecule; between the molecules there are only weak intermolecular forces, and it is these weak forces that are overcome when the substance melts or boils, which needs little energy. Steam is still made of \(\mathrm{H_2O}\) molecules with their covalent bonds intact. Giant covalent substances such as diamond, graphite and silicon(IV) oxide are different: melting them means breaking the covalent bonds themselves, so their melting points are very high.