Halogens as oxidising agents

· Halogens are oxidising agents because they gain electrons to form halide ions: X₂ + 2e⁻ → 2X⁻.
· Oxidising power decreases down Group 17: Cl₂ > Br₂ > I₂.
· Down the group, atomic radius increases and shielding increases, so the nucleus has a weaker attraction for an incoming electron.
· Therefore, it becomes harder for the halogen molecule to gain electrons, so reactivity as an oxidising agent decreases.
· Exam phrase: chlorine is a stronger oxidising agent than bromine/iodine because it gains electrons more readily.

This shows why halogens act as oxidising agents: they gain electrons and are reduced. It supports the trend that oxidising ability decreases down Group 17 as atoms become larger and more shielded. Source

Reactions of halogens with hydrogen

· Halogens react with hydrogen to form hydrogen halides: H₂ + X₂ → 2HX.
· Examples:
· H₂ + Cl₂ → 2HCl
· H₂ + Br₂ → 2HBr
· H₂ + I₂ ⇌ 2HI
· Reactivity with hydrogen decreases down the group: Cl₂ reacts more readily than Br₂, which reacts more readily than I₂.
· Explanation: down the group, halogen atoms become larger and have more shielding, so they are less able to attract electrons during reaction.
· Fluorine reacts explosively with hydrogen, chlorine reacts readily, bromine reacts less vigorously, and iodine reacts slowly/reversibly.
· Exam phrase: the reaction becomes less vigorous down Group 17 because the halogen becomes a weaker oxidising agent.

Hydrogen halides and thermal stability

· Hydrogen halides have the formula HX, where X = F, Cl, Br or I.
· Thermal stability decreases down the group: HF > HCl > HBr > HI.
· This means HI decomposes most easily on heating, while HF is most thermally stable.
· Thermal decomposition example: 2HI → H₂ + I₂.
· Explanation: down the group, the H–X bond length increases because the halogen atom is larger.
· Longer bonds have poorer orbital overlap, so bond strength decreases.
· Therefore, less energy is needed to break the bond, so the hydrogen halide is less thermally stable.
· Key trend: H–F strongest bond, H–I weakest bond.

The diagram shows that H–X bond length increases from HF to HI while bond strength decreases. This directly explains why the thermal stability of hydrogen halides decreases down Group 17. Source

Bond strength explanation for hydrogen halides

· Bond strength order: H–F > H–Cl > H–Br > H–I.
· Down Group 17, the halogen atom uses a larger outer orbital to overlap with hydrogen’s 1s orbital.
· Larger orbitals give less effective overlap with hydrogen.
· Less effective overlap means a weaker covalent bond.
· Weaker H–X bonds are easier to break by heating.
· Therefore, HI is least thermally stable and decomposes most easily.

This image helps students visualise the polar H–F bond in a hydrogen halide. Although CIE focuses on bond strength for thermal stability, the diagram is useful for recognising that hydrogen halides are covalent molecules with polar H–X bonds. Source

Key exam comparisons

· Halogen oxidising strength: Cl₂ > Br₂ > I₂.
· Halogen reactivity with hydrogen: Cl₂ > Br₂ > I₂.
· Hydrogen halide bond strength: H–F > H–Cl > H–Br > H–I.
· Hydrogen halide thermal stability: HF > HCl > HBr > HI.
· Ease of thermal decomposition: HI > HBr > HCl > HF.
· Common explanation pattern: larger atomic radius + more shielding → weaker attraction for electrons / weaker H–X bond → lower reactivity or stability.

This image helps connect the chemical trends to the actual Group 17 elements studied in CIE: chlorine, bromine and iodine. These are the halogens most commonly compared in exam questions about oxidising ability and reactions with hydrogen. Source