CUET UG Chemistry Physical Chemistry Chemical Bonding VSEPR Theory Hybridisation Molecular Geometry

Must-Know (15–20 detailed bullets)

• VSEPR theory states that electron pairs around a central atom arrange themselves to minimize repulsion; example: two electron pairs → linear geometry (BeCl₂).
• Electron pair geometry considers both bond pairs and lone pairs; molecular geometry considers only bond pairs; example: NH₃ has tetrahedral electron geometry but trigonal pyramidal molecular geometry.
• Lone pair-lone pair repulsion > lone pair-bond pair repulsion > bond pair-bond pair repulsion; this affects bond angles; e.g., H₂O bond angle is 104.5° due to two lone pairs.
• Hybridisation is the mixing of atomic orbitals to form new hybrid orbitals suitable for bonding; number of hybrid orbitals = number of atomic orbitals mixed.
• sp hybridisation results from mixing one s and one p orbital; forms linear geometry with 180° bond angle; example: BeCl₂, CO₂.
• sp² hybridisation involves one s and two p orbitals; trigonal planar geometry with 120° bond angle; example: BF₃, C₂H₄.
• sp³ hybridisation results from one s and three p orbitals; tetrahedral geometry with 109.5° bond angle; example: CH₄, NH₃.
• dsp² hybridisation occurs in square planar complexes; involves one d, one s, and two p orbitals; example: [Ni(CN)₄]²⁻.
• sp³d hybridisation leads to trigonal bipyramidal geometry; example: PCl₅; axial bonds longer than equatorial due to greater repulsion.
• sp³d² hybridisation forms octahedral geometry; example: SF₆; all bond angles 90°.
• In molecules with lone pairs, hybridisation is determined by total electron pairs around central atom; e.g., H₂O has four electron pairs → sp³ hybridised.
• Central atom hybridisation can be calculated using formula: ½ [valence electrons of central atom + number of monovalent atoms – cation charge + anion charge]; verify from NCERT.
• For IF₇, geometry is pentagonal bipyramidal with sp³d³ hybridisation; seven electron pairs around iodine.
• XeF₄ has square planar molecular geometry due to two lone pairs on Xe; electron pair geometry is octahedral; hybridisation is sp³d².
• SF₄ has see-saw shape due to one lone pair; electron pair geometry trigonal bipyramidal; hybridisation sp³d.
• PCl₅ is trigonal bipyramidal; axial bond length > equatorial bond length due to greater repulsion from equatorial bonds.
• Hybridisation cannot be assigned based on geometry alone; it must correspond to electron domain count; e.g., ClO₃⁻ has tetrahedral electron geometry → sp³.
• dsp³ hybridisation is not used in main group elements; it's common in transition metal complexes; e.g., [Fe(CO)₅].
• Double bonds are treated as a single electron domain in VSEPR; e.g., CO₂ has two electron domains → linear.
• Bond angle decreases with increasing lone pair count; NH₃ (107°) < CH₄ (109.5°); H₂O (104.5°) < NH₃.

Difficulty Level

Intermediate — requires visualization of 3D geometries and understanding of hybridisation-electron domain relationship, but core principles are straightforward and formula-based.

Common CUET Traps

• Trap: Assuming molecular geometry is same as electron pair geometry.
  Avoid: Always check for lone pairs; e.g., NH₃ is trigonal pyramidal (molecular), not tetrahedral (electron geometry).

• Trap: Assigning hybridisation based on number of atoms bonded, not electron domains.
  Avoid: Count total electron pairs (bond pairs + lone pairs); e.g., SO₂ has three electron domains → sp², not sp.

• Trap: Thinking sp³d hybridisation implies d-orbital participation in main group elements.
  Avoid: Hybridisation is a model; d-orbitals in PCl₅ or SF₆ are not significantly involved; focus on geometry and electron domains as per NCERT.

Practice MCQs

Q1. Which of the following species has a linear shape and sp hybridisation?
A) H₂O
B) NH₃
C) CO₂
D) SO₂

Answer: C
Explanation: CO₂ has two bond pairs, no lone pairs on C, linear shape, and sp hybridisation.
Why others fail: SO₂ has bent shape and sp² hybridisation due to lone pair on S.

Q2. What is the molecular geometry of XeF₂?
A) Linear
B) Bent
C) Trigonal planar
D) Trigonal bipyramidal

Answer: A
Explanation: XeF₂ has three lone pairs and two bond pairs; electron geometry trigonal bipyramidal, molecular geometry linear.
Why others fail: Students confuse electron geometry (trigonal bipyramidal) with molecular geometry.

Q3. The hybridisation of phosphorus in PCl₅ is:
A) sp³
B) sp³d
C) sp³d²
D) dsp²

Answer: B
Explanation: PCl₅ has five electron domains, requiring sp³d hybridisation for trigonal bipyramidal geometry.
Why others fail: sp³d² is for six domains (e.g., SF₆); students miscount electron pairs.

Q4. Which molecule has the smallest bond angle?
A) CH₄
B) NH₃
C) H₂O
D) BF₃

Answer: C
Explanation: H₂O has two lone pairs causing maximum repulsion, reducing bond angle to 104.5°.
Why others fail: NH₃ (107°) is close, but H₂O has smaller angle due to two lone pairs.

Q5. The correct order of hybridisation of carbon atoms in CH₃–CH=CH–CN is:
A) sp³, sp², sp², sp
B) sp³, sp, sp, sp
C) sp³, sp², sp, sp
D) sp³, sp², sp², sp²

Answer: A
Explanation: CH₃– (sp³), –CH= (sp²), =CH– (sp²), –CN (sp); triple bond carbon is sp hybridised.
Why others fail: Students misassign hybridisation of nitrile carbon (sp, not sp²).

Last‑Minute Revision (15–20 one‑liners)

• ⚠️ VSEPR: Lone pairs occupy more space than bond pairs → reduce bond angles.
• ⚠️ sp hybrid → 2 electron domains → linear → 180°; e.g., BeCl₂.
• ⚠️ sp² hybrid → 3 domains → trigonal planar → 120°; e.g., BF₃.
• ⚠️ sp³ hybrid → 4 domains → tetrahedral → 109.5°; e.g., CH₄.
• ⚠️ 5 domains → sp³d → trigonal bipyramidal; e.g., PCl₅.
• ⚠️ 6 domains → sp³d² → octahedral; e.g., SF₆.
• ⚠️ 7 domains → sp³d³ → pentagonal bipyramidal; e.g., IF₇.
• ⚠️ NH₃: 3 bond pairs + 1 lone pair → sp³ → trigonal pyramidal → 107°.
• ⚠️ H₂O: 2 bond pairs + 2 lone pairs → sp³ → bent → 104.5°.
• ⚠️ XeF₄: 4 bond pairs + 2 lone pairs → sp³d² → square planar.
• ⚠️ XeF₂: 2 bond pairs + 3 lone pairs → sp³d → linear.
• ⚠️ SF₄: 4 bond pairs + 1 lone pair → sp³d → see-saw.
• ⚠️ ClF₃: T-shaped → sp³d hybridisation → 2 lone pairs.
• ⚠️ CO₂: O=C=O → sp hybridised carbon → linear.
• ⚠️ C in C₂H₂: sp hybridised → triple bond → linear.
• ⚠️ Hybridisation formula: ½[V + M – C + A]; verify from NCERT.
• ⚠️ Double/triple bonds count as one electron domain in VSEPR.
• ⚠️ dsp² hybridisation → square planar → e.g., [Ni(CN)₄]²⁻.
• ⚠️ Mnemonic: "Sister Poppins Danced Spontaneously During Parties" → sp, sp², sp³, dsp², sp³d, sp³d², sp³d³.