Group 7

• Colour gets darker down the group: F₂ pale yellow → Cl₂ green/yellow → Br₂ orange/brown → I₂ grey/black solid (purple vapour).
• Volatility decreases down the group (melting/boiling points increase) because London dispersion forces get stronger between bigger molecules.
• Electronegativity and oxidising power decrease down the group — bigger atoms = more shielding = weaker pull on incoming electrons.
• Reactivity with hydrogen gets less vigorous down the group (F₂ explosive in the dark → I₂ needs heat and reaches equilibrium).
• Displacement reactions: a more reactive (higher) halogen displaces a less reactive (lower) halide ion from solution. Chlorine displaces both bromide and iodide; bromine displaces iodide.
• Redox with metals: halogens oxidise Group 1/2 metals to form ionic metal halides.
• Redox with iron: Cl₂ and Br₂ oxidise Fe²⁺ → Fe³⁺, but I₂ can't — instead Fe³⁺ oxidises I⁻ → I₂.
• Disproportionation: chlorine reacts with cold/hot alkali and with water, getting oxidised AND reduced simultaneously in the same reaction.
• Concentrated H₂SO₄ + halide ions produces different products depending on halide size — this is the classic "identify the mystery gas" topic, and iodide (the best reducing agent) produces the widest range of products.
• Reducing power of halide ions increases down the group (I⁻ > Br⁻ > Cl⁻) — the opposite trend to oxidising power of the halogens themselves.
• Silver nitrate + ammonia test identifies halide ions by precipitate colour and solubility in ammonia.

Every halogen has its own signature colour, and they get progressively darker as you move down the group. This isn't just a fun fact to memorise — it's actually how you identify which halogen has formed in a displacement reaction (more on that in Section 2), so get this table locked into memory.

Why the darkening trend? As atoms get bigger, they have more electrons that are more loosely held, which changes how they absorb and reflect visible light — the bonding electrons need less energy to get excited, shifting absorption toward longer (redder) wavelengths, giving darker colours.

Volatility just means "how easily something evaporates" — a highly volatile substance has a low boiling point (it turns to gas easily). Fluorine is the most volatile halogen; iodine is the least.

shielding