Interhalogen Compounds
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Direct answer
When two different halogens combine, the larger, less electronegative atom takes the central seat and the smaller — in practice only fluorine — surrounds it: hence XY (ICl, BrF), XY3 (ClF3, BrF3), XY5 (BrF5, IF5) and the unique XY7 (IF7). The mixed X–Y bond is weaker and polar, so interhalogens are collectively more reactive than the parent halogens, and all of them hydrolyse in water. Shapes follow VSEPR cleanly: ClF3 is T-shaped (sp3d with two lone pairs), IF5 square pyramidal (sp3d2, one lone pair), IF7 pentagonal bipyramidal (sp3d3, none).
What you must remember
- Four types with residents: XY — ClF, BrF, BrCl, ICl, IBr; XY3 — ClF3, BrF3 (ICl3 existing as the solid dimer I2Cl6); XY5 — ClF5, BrF5, IF5; XY7 — IF7 alone.
- Size rule: the central atom must be the bigger one, and the count cannot exceed what small atoms can pack around it — only fluorine fits seven times around iodine.
- Shapes to recite: ClF3 and BrF3 T-shaped (AX3E2); IF5 square pyramidal (AX5E1); IF7 pentagonal bipyramidal (AX7); the cousin anion I3− is linear (AX2E3).
- Preparations: I2 + Cl2 → 2ICl (equimolar); I2 + 3Cl2 → 2ICl3; Br2 + 3F2 (diluted with nitrogen) → 2BrF3; ClF3 + F2 on heating → ClF5.
- Reactivity logic: the heteronuclear bond is polar and weaker than the parents' homonuclear bonds, so interhalogens cleave towards electron-rich centres more readily than X2 does.
- Hydrolysis examples: ICl + H2O → HCl + HOI; BrF5 + 3H2O → 5HF + HBrO3; IF7 + 4H2O → 7HF + HIO4 — oxidation states survive intact.
- Autoionisation: 2BrF3 ⇌ BrF2+ + BrF4−, making liquid BrF3 a non-aqueous ionising solvent that Lewis acids and bases perturb.
- Nuclear credential: ClF3 fluorinates UF4 to volatile UF6 for enrichment — the industry fact that makes ClF3 worth remembering beyond VSEPR.
Shape logic without memorising
Every interhalogen shape falls out of counting domains. ClF3: chlorine holds three bonds and two lone pairs — five electron domains arranging as a trigonal bipyramid; the two lone pairs claim the equatorial seats, where each touches neighbours at 90° only twice (the minimum), and the three bonds left standing form the T. Give iodine seven fluorines and no lone pairs: IF7 is a pentagonal bipyramid, sp3d3. Between them sits IF5 with six domains, one of them a lone pair: an octahedral set with the pair occupying one apex leaves a square pyramid. The same counting answers the anion I3− — two bonds, three lone pairs, five domains; the three pairs take the equatorial positions and the two atoms line up linear. Hydrolysis then follows oxidation-state honesty: in ICl, iodine is +1 and chlorine −1, so water's hydroxide lands on iodine and proton on chlorine — HCl plus HOI, no redox. In BrF5, bromon stays +5 from beginning to end while five fluorines leave as HF and the oxygens of water build the bromate.
Interhalogen traps
JEE Main asks type-from-formula and shape-from-type: ClF3 belongs to XY3 and is T-shaped — both halves expected in one mark. Advanced likes the autoionisation of BrF3 (write the ions, then identify which added species shifts it), the hydrolysis equations with full balancing, and the size argument for why XY7 exists only as IF7. The predictable slips: counting ClF3's lone pairs as zero from the bare formula (it is AX3E2, not AX3); guessing square planar for IF5 — that geometry belongs to AX4E2 species such as XeF4; and predicting disproportionation during hydrolysis where the truth is straightforward acid formation. One more recall point: interhalogens are generally heavier liquids or solids than their parent halogens because the polar molecule polarises better — ICl3 a yellow solid, BrF3 a yellow-green liquid at room temperature — and bond dipoles lift their boiling points above the average of the parents.
Frequently asked questions
Why can fluorine never be the central atom in an interhalogen?
It is the smallest and most electronegative halogen; only it can be the terminal partner crowding a larger atom — seven fluorines fit around iodine, never the reverse.
What shapes do ClF3 and IF7 adopt?
ClF3 is T-shaped, a trigonal bipyramidal electron geometry with two equatorial lone pairs; IF7 is pentagonal bipyramidal with no lone pairs at all.
How is iodine trichloride prepared?
Direct combination with chlorine in excess: I2 + 3Cl2 → 2ICl3, whereas the equimolar mixture stops at ICl.
Why are interhalogens more reactive than the parent halogens?
The heteronuclear X–Y bond is weaker and permanently polar, so it cleaves more easily towards electron-rich substrates than either parent's homonuclear bond.
What ions does liquid BrF3 generate on self-ionisation?
2BrF3 ⇌ BrF2+ + BrF4− — a fluoride-ion equilibrium that lets liquid BrF3 act as a non-aqueous ionising solvent.