8 Most Common Errors in Groups 16-18

Error 1: XeF2 geometry confusion

• WRONG: Students assume 2 bonds = bent/angular shape (like H2O)
• RIGHT: XeF2 is LINEAR because 3 lone pairs occupy equatorial positions of TBP, forcing 2 F atoms to axial (180° apart)
• TRIGGER: "2 bonds and lone pairs = bent" is only valid when there are 2 lone pairs (sp3, like H2O). With 3 lone pairs (sp3d), the arrangement is different.

Error 2: SO3 direct dissolution in water

• WRONG: "SO3 + H2O → H2SO4 directly in Contact process"
• RIGHT: SO3 is absorbed in conc. H2SO4 → oleum, THEN diluted with water
• TRIGGER: Acid mist formation is the reason — never absorb SO3 in water directly

Error 3: H2SO4 as reducing agent

• WRONG: Hot, concentrated H2SO4 acts as a reducing agent
• RIGHT: H2SO4 is always an OXIDIZING agent (when hot and conc.), never reducing
• TRIGGER: In reactions with HBr and HI, H2SO4 oxidizes them ($Br^{-}$/$I^{-}$ are oxidized to Br2/I2)

Error 4: HF as strong acid

• WRONG: "F is most electronegative, so HF is strongest acid"
• RIGHT: HF is the WEAKEST hydrogen halide acid
• TRIGGER: High electronegativity ≠ high acid strength here. H-F bond (568 kJ/mol) is too strong to break easily.

Error 5: XeF4 geometry

• WRONG: XeF4 is tetrahedral (like CH4, 4 bonds)
• RIGHT: XeF4 is SQUARE PLANAR because 2 lone pairs push the 4 F atoms into a plane
• TRIGGER: Always count lone pairs. 4 bonds + 2 lone pairs = 6 electron pairs = octahedral base → square planar

Error 6: Contact process catalyst temperature

• WRONG: V2O5 catalyst works at 800°C or 250°C
• RIGHT: V2O5 at 450°C (compromise between yield and rate)
• TRIGGER: Memorize: "V2O5, 450°C" as a unit — they always come together in NEET

Error 7: Oxoacid strength — F forms strong oxoacids

• WRONG: "F has highest electronegativity, so HOF would be strongest oxoacid"
• RIGHT: F forms NO oxoacids (no d-orbitals, cannot show positive oxidation states)
• TRIGGER: Electronegativity comparison is irrelevant here — the issue is d-orbital availability

Error 8: Lone pair positions in ClF3

• WRONG: Lone pairs go to axial positions in ClF3
• RIGHT: Lone pairs go to EQUATORIAL positions (less repulsion — fewer 90° repulsions)
• TRIGGER: Remember the rule: lone pairs always prefer equatorial in TBP because equatorial-equatorial angle is 120° vs axial-equatorial is 90°