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Chemical Bonding (Ionic, Covalent and Metallic): Question 2

Syllabus 2.5

Structured Core 5 marks

A farm operates an anaerobic digester that breaks down animal waste to produce biogas, which is mostly methane, CH4\text{CH}_4. The biogas is stored and transported as a gas, and only turns into a liquid if it is cooled below −161 °C-161\text{ °C}.

(a) State the type of bonding present between the carbon atom and the hydrogen atoms in a molecule of methane, and describe, in terms of electrons, how this bonding holds the atoms together. [3]

(b) Methane is a simple molecular substance. Describe two properties of methane that are typical of simple molecular substances, referring to its boiling point and its ability to conduct electricity. [2]

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Worked solution

Part (a): Bonding in methane

Carbon and hydrogen are both non-metals, so they bond by sharing electrons rather than transferring them. This is covalent bonding.

Carbon has 4 electrons in its outer shell and needs 4 more to reach a stable arrangement of 8 (like neon). Each hydrogen atom has 1 electron and needs 1 more to reach a stable arrangement of 2 (like helium). Carbon therefore forms four separate covalent bonds, one with each hydrogen atom:

C (4 outer electrons)+4 H (1 outer electron each)→CH4\text{C (4 outer electrons)} + 4\,\text{H (1 outer electron each)} \rightarrow \text{CH}_4

In each C–H bond, one electron comes from carbon and one comes from hydrogen, and this pair of electrons is shared between the two atoms. After bonding, carbon effectively has 8 electrons around it (4 of its own ++ 4 shared from the hydrogens), and each hydrogen effectively has 2 electrons (11 of its own ++ 1 shared from carbon), both reaching stable, noble-gas-like electron arrangements.

Part (b): Properties of methane as a simple molecular substance

Methane exists as small, individual molecules, unlike a giant lattice of atoms or ions. Simple molecular substances such as methane are typically described by two properties:

  • Low melting and boiling point: methane is a gas well below room temperature, and only turns to a liquid if cooled below −161 °C-161\text{ °C}.
  • Poor electrical conductivity: methane does not conduct electricity, whether solid, liquid or dissolved in water.

Final answers

  • (a) Covalent bonding: 4 shared pairs of electrons, one per C–H bond; carbon ends with 8 outer electrons, each hydrogen ends with 2.
  • (b) Low melting/boiling point (methane is a gas at room temperature); methane does not conduct electricity.