Effect of lone pairs on geometry (e.g. H₂O bent, NH₃ pyramidal)

2.3.9 · Chemistry › Chemical Bonding

KYA ho raha hai?

Key move yeh hai: domains count karo base geometry paane ke liye, phir lone-pair positions hatao aur shape ka naam lo.

KYUN lone pairs angle ko chhota karte hain?

Toh repulsion ordering (Feynman-verify karo ki yeh sahi lagta hai) yeh hai: $\text{LP–LP}>\text{LP–BP}>\text{BP–BP}$

Consequence: har lone pair baaki bonds ko aapas mein push karta hai, isliye har lone pair bond angle ko ideal 109.5° se ~2–2.5° kam kar deta hai.

$109.5°\stackrel{{\text{1 LP (NH}}_3)}{\to }107°\stackrel{{\text{2 LP (H}}_2\text{O})}{\to }104.5°$

KAISE koi bhi shape predict karein (recipe)

Electron count ki derivation (scratch se, kuch yaad karne ki zaroorat nahi):

1. Central atom $V$ valence electrons laata hai.
2. Har single bond central atom se 1 electron use karta hai jo ligand ke 1 electron se pair karta hai.
3. Bonds banane ke baad central atom ke jo electrons bacha jaate hain, wo lone pairs ke roop mein pair up ho jaate hain.

Worked count — H₂O:

• O ke paas $V=6$ hai. Wo 2 O–H bonds banata hai → bonds mein 2 electrons use hote hain.
• Bacha hua $=6-2=4$ electrons $=4/2=2$ lone pairs. ✔
• SN $=2+2=4$ → tetrahedral electron geometry → bent.

Yeh step kyun? Hum bonding electrons subtract karte hain aur baaki ko half karte hain kyunki electrons ko lone-pair orbital mein occupy karne ke liye spin-paired hona zaroori hai.

Worked count — NH₃:

• N ke paas $V=5$ hai, 3 N–H bonds banata hai → 3 electrons use hote hain.
• Bacha hua $=5-3=2=1$ lone pair. SN $=3+1=4$ → tetrahedral base → pyramidal.

Worked examples

Common mistakes

Flashcards

Connections

• VSEPR Theory — parent framework jis par yeh note apply hota hai.
• Hybridization — N aur O par sp³ tetrahedral base geometry explain karta hai.
• Bond angle and electronegativity — ligand electronegativity bhi angles ko thoda tweak karti hai.
• Dipole moment — bent H₂O aur pyramidal NH₃ polar hain kyunki lone pairs symmetry tod dete hain.
• Steric number and molecular shape — counting recipe generalized.
• Trigonal bipyramidal geometry — jahan lone pairs equatorial sites prefer karte hain (SF₄, ClF₃).

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