Ethene (C2H4)

Summary: Ethene (C₂H₄) is the simplest alkene — its two carbon atoms share two pairs of electrons in a carbon–carbon double covalent bond, and each carbon also forms two C–H bonds; as an unsaturated hydrocarbon it rapidly decolourises bromine water. Tags: igcse chemistry Created: 2026-07-18

Ethene (C₂H₄) is the first member of the Alkenes homologous series and the standard IGCSE example of a double covalent bond between two carbon atoms. Each carbon atom has four valence electrons and needs four more to complete its octet; in ethene, the two carbons share two pairs of electrons with each other (the C=C double bond) and each carbon shares one further pair with each of two hydrogen atoms. This sharing of electrons gives every carbon a full outer Electron Shell of 8 and every hydrogen the helium duplet, so the Molecule is stable without forming ions. Because the C=C double bond can “open up” to let more atoms add on, ethene is described as unsaturated, which makes it far more reactive than the single-bonded Alkanes. This reactivity is exploited industrially: thousands of ethene molecules join together by addition polymerisation to form poly(ethene), the most common of the Polymers, and ethene also adds water (steam) to manufacture ethanol (see Alcohols).

Bonding: Dot-and-Cross Description

In a Dot-and-Cross Diagram of C₂H₄, one carbon’s electrons are drawn as dots and the other’s as crosses (hydrogens’ electrons in a third style or as crosses):

• The C=C double bond consists of 2 shared pairs — four electrons (two dots and two crosses) drawn together in the region between the two carbon atoms.
• Each carbon also forms two C–H single bonds (one shared pair each), giving four C–H bonds in the molecule overall.
• Each carbon therefore counts 8 outer electrons: 4 from the double bond + 2 × 2 from its C–H bonds — a full octet.
• Each hydrogen counts 2 electrons — the helium duplet.
• There are no lone pairs in ethene; all 12 valence electrons are in bonding pairs.

Displayed formula: H₂C=CH₂. The molecule is flat (planar), with all six atoms in the same plane.

Key Data

Physical Properties

• A simple molecular gas with very low melting and boiling points — only weak forces act between molecules (Intermolecular Force); the strong covalent bonds are not broken on boiling.
• Colourless, almost odourless, and insoluble in water.
• Does not conduct electricity — no mobile ions or delocalised electrons.

Test for Unsaturation

Shake the gas with bromine water (orange). Ethene decolourises it rapidly because bromine adds across the C=C double bond:

C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane, colourless)

Saturated hydrocarbons such as the Alkanes leave bromine water orange (no reaction in the dark). This is the standard test in Chemical Tests for distinguishing alkenes from alkanes.

Uses and Reactions

• Addition polymerisation — many ethene molecules join to form poly(ethene), used for bags and bottles (see Polymers).
• Hydration — ethene + steam over a phosphoric acid catalyst makes ethanol (see Alcohols).
• Hydrogenation — addition of H₂ converts unsaturated compounds to saturated ones (used to harden vegetable oils).
• Ethene is produced industrially by cracking larger hydrocarbon molecules from petroleum (see Fuels and Petroleum).

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.5, Cambridge Assessment International Education

Common Misconceptions