Hydrogen (H2)

Summary: Hydrogen (H₂) is the simplest covalent molecule — two hydrogen atoms share a single pair of electrons so each achieves the stable helium duplet; a colourless diatomic gas lighter than air, identified by the “squeaky pop” test with a lighted splint. Tags: igcse chemistry Created: 2026-07-18

Hydrogen (H₂) is a diatomic Molecule of the Element hydrogen and is the simplest example of Covalent Bonding on the IGCSE syllabus. Each hydrogen atom has only one electron in its single Electron Shell, one short of the stable two-electron arrangement of helium (see Group 0 Noble Gases). By sharing their electrons, two hydrogen atoms form a single covalent bond in which both atoms gain access to a full duplet, so neither atom needs to lose or gain electrons as in Ions and Ionic Bonds. The molecule is held together by this one strong shared pair, but neighbouring H₂ molecules attract each other only through a very weak Intermolecular Force, which is why hydrogen is a gas with an extremely low boiling point. Hydrogen gas is colourless, odourless and the least dense of all gases, and it burns in oxygen to form water — the basis both of its laboratory test and of its use in Fuel Cells.

Bonding: Dot-and-Cross Description

In a Dot-and-Cross Diagram of H₂, one atom’s electron is drawn as a dot and the other’s as a cross. The two symbols are placed together in the overlap region between the two H symbols:

• Each H atom contributes 1 electron (Valence Electron) to the bond.
• The molecule contains exactly 1 shared (bonding) pair — one dot and one cross between the atoms.
• There are no lone pairs anywhere in the molecule: every electron in H₂ is involved in bonding.
• After sharing, each H atom “counts” both electrons in the shared pair, giving it the duplet (2 electrons) of helium. Hydrogen follows the duplet rule, not the octet rule, because its first shell holds a maximum of 2 electrons.

Displayed formula: H—H (a single covalent bond).

Key Data

Physical Properties

• Diatomic gas made of simple molecules — very low melting and boiling points because only weak forces act between molecules (see Intermolecular Force); the covalent bond itself is strong and is not broken on boiling.
• The least dense gas: much lighter than air, so it was once used in airships.
• Colourless, odourless, and insoluble in water.
• Does not conduct electricity — there are no ions or delocalised electrons to carry charge.

Test for Hydrogen

Hold a lighted splint at the mouth of the test tube. Hydrogen burns rapidly with a squeaky pop:

2H₂(g) + O₂(g) → 2H₂O(l)

This is one of the standard gas tests listed under Chemical Tests. The product, water, can itself be confirmed with anhydrous copper(II) sulfate.

Uses

• Fuel — burns cleanly, producing only water; used in Fuel Cells to generate electricity.
• Manufacture of Ammonia in the Haber process (N₂ + 3H₂ ⇌ 2NH₃).
• Hydrogenation of vegetable oils to make margarine (addition across C=C bonds — see Alkenes).
• Hydrogen is produced industrially from methane/steam and can also be obtained at the cathode during Electrolysis of aqueous solutions.

Sources

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

Common Misconceptions