Read the HOCl Series Before Ranking Oxidising Power

HOCl (Cl at +1) is the strongest OXIDISING agent of the series but the WEAKEST acid; HClO₄ (Cl at +7) is the weakest oxidiser but the strongest acid — more oxygen atoms stabilise the conjugate base through resonance (raising acid strength) while simultaneously making the parent acid a less aggressive oxidiser (the higher oxidation state is already closer to its most stable form).

Fluorine is the strongest oxidising agent among ALL halogens (not just within its own oxoacid series) despite having the lowest electron affinity of Cl/F/Br/I — F2's weak F-F bond and high hydration/lattice energy of F⁻ more than compensate for the modest electron affinity. HF, uniquely among the hydrohalic acids, does not precipitate with AgNO₃ (AgF is soluble) and is the weakest hydrohalic acid due to strong H-F bonding — both classic F-specific exceptions.

ICl hydrolyses unevenly: the more electropositive halogen (I) ends up as the hypohalite (HOI), while the more electronegative one (Cl) ends up as the simple hydracid (HCl) — general pattern: interhalogen AX hydrolyses to HX (from the more electronegative atom) + HOA (hypohalous acid of the less electronegative/more electropositive atom).

A more strongly oxidising halogen displaces a less strongly oxidising halide from solution (Cl₂ + 2Br- → 2Cl- + Br₂ works), but the reverse never does (Cl₂ + 2F- → 2Cl- + F₂ does NOT occur, since F₂ is the strongest oxidiser and can't be displaced by a weaker one) — always check the direction against the F > Cl > Br > I oxidising-power order before predicting a displacement.

Colour needs some way to absorb visible light by promoting an electron to a higher energy level, and halogens/silver halides get there by two genuinely different routes. Free halogens (Cl₂, Br₂, I₂) are coloured because of a HOMO-to-LUMO electronic transition: promoting an electron from the highest occupied molecular orbital (built mainly from lone-pair-type p orbitals) to the lowest unoccupied (antibonding) molecular orbital absorbs visible light, and the energy gap shrinks going down the group (larger, more diffuse orbitals overlap less effectively), which is exactly why the halogens get progressively darker/more strongly coloured from pale yellow-green (Cl₂) to deep violet-black (I₂) — this is a property of the isolated diatomic halogen molecule itself, nothing to do with any other atom nearby. Silver halides (AgCl, AgBr, AgI), by contrast, are coloured for a completely different reason: POLARISATION of the halide ion by the small, highly polarising Ag⁺ cation. As the halide ion gets larger and more polarisable going from Cl⁻ to Br⁻ to I⁻, the silver ion distorts its electron cloud more severely, pulling the compound bonding character further from purely ionic toward covalent — this increasing covalent/charge-transfer character is what deepens the colour from white/pale AgCl toward pale yellow AgBr and yellow AgI. Do not explain silver halide colour with a HOMO-LUMO argument, and do not explain free halogen colour with a polarisation argument — each family of coloured species has its own distinct mechanism.