Valence Bond Theory (VBT) — hybridization (sp, sp², sp³, sp³d, sp³d²)

2.3.10 · Chemistry › Chemical Bonding

Hybridization exist kyun karti hai?

Woh problem jo VBT ko solve karni thi. Carbon ko consider karo: ground state $1 s^{2} 2 s^{2} 2 p_{x}^{1} 2 p_{y}^{1}$ . Isme sirf 2 unpaired electrons hain, isliye isko sirf 2 bonds banana "chahiye" — lekin $C H_{4}$ mein 4 identical C–H bonds hain jo $109.5°$ par hain.

Agar C seedha ek $2 s$ aur teen $2 p$ orbitals use karta, toh charon bonds identical nahi hote (ek spherical $s$ se, teen perpendicular $p$ se).
Experiment kehta hai ki charon bonds length aur energy mein equal hain, tetrahedral arrangement mein.

Hybridization jaldi kaise nikaalein (80/20 rule)

Steric number (SN) sab kuch decide karta hai:

$SN = (no. of σ bonds / atoms directly bonded) + (no. of lone pairs on central atom)$

SN	Hybridization	Geometry (electron)	Bond angle
2	$s p$	Linear	$180°$
3	$s p^{2}$	Trigonal planar	$120°$
4	$s p^{3}$	Tetrahedral	$109.5°$
5	$s p^{3} d$	Trigonal bipyramidal	$120°/90°$
6	$s p^{3} d^{2}$	Octahedral	$90°$

Figure — Valence Bond Theory (VBT) — hybridization (sp, sp², sp³, sp³d, sp³d²)

Angles aisi kyun aati hain (dual coding: geometry = repulsion)

s-character angle ko control karta hai. Zyada %s = orbital nucleus ke paas rakha, mota lobe, wider angle:

$s p : 50% s \Rightarrow 180°, s p^{2} : 33% s \Rightarrow 120°, s p^{3} : 25% s \Rightarrow 109.5°$

Worked Examples

Common mistake Common mistakes (steel-manned)

Mistake A: "Lone pairs hybridization ke liye count nahi hote." Kyun sahi lagta hai: lone pairs koi bond nahi banate, isliye students sochte hain woh invisible hain. Fix: lone pairs hybrid orbitals mein occupy hote hain — unhe SN mein zaroor count karo. O ke 2 lone pairs ignore karne par water ko galti se linear keh doge.

Mistake B: "sp³ ka matlab hamesha exactly $109.5°$ hota hai." Kyun sahi lagta hai: table yahi kehti hai. Fix: $109.5°$ ideal angle hai jab charon positions identical hoon. Lone pairs / electronegativity differences ise distort karte hain (water $104.5°$ , ammonia $107°$ ).

Mistake C: π bonds ko SN mein count karna. Kyun sahi lagta hai: double bond "zyada bonding jaisi lagti hai." Fix: sirf σ bonds + lone pairs count hote hain. Ek double bond SN mein ek contribute karta hai. Toh $C O_{2}$ (O=C=O): C ke paas 2 σ, 0 LP → SN 2 → $s p$ , linear.

Mistake D: " $S F_{6}$ mein d orbitals sach mein participate karte hain." Kyun sahi lagta hai: $s p^{3} d^{2}$ label. Fix (nuance): modern theory dikhata hai ki d-participation minimal hai; exam VBT ke liye hum $s p^{3} d^{2}$ label rakhte hain, lekin jaano ki yeh ek bookkeeping model hai, literal heavy d-orbital mixing nahi.

Recall Feynman: ek 12-saal ke bachche ko explain karo

Socho ek atom ke paas kuch alag-alag shaped "haath" hain doosre atoms ko pakadne ke liye — ek gol haath ( $s$ ) aur kuch dumbbell haath ( $p$ ). Agar woh in alag haathon se pakadta, toh bonds uneven aur wobbly hoti. Isliye atom apne sab haath blend karke identical nayi haath (hybrids) banata hai aur unhe zyada se zyada door spread karta hai, jaise ek ball par spikes. Gino kitni cheezein woh pakadta hai plus uske chhupe "resting" pairs — woh number tumhe shape batata hai: 2 = seedhi line, 3 = flat triangle, 4 = 3D tetrahedron, 5 aur 6 = fancy stars.

Forecast-then-Verify drill

Table check karne se pehle hybridization & shape predict karo, phir verify karo:

$N H_{3}$ → forecast: SN = 3σ + 1LP = 4 → $s p^{3}$ , pyramidal ( $107°$ ). ✅
$X e F_{4}$ → forecast: Xe $V = 8$ , 4 bonds, LP $= (8 - 4) /2 = 2$ → SN 6 → $s p^{3} d^{2}$ , square planar. ✅
$S O_{2}$ → forecast: S 2σ + 1LP → SN 3 → $s p^{2}$ , bent. ✅

Flashcards

Hybridization seedha kaunsa formula deta hai?

Steric number SN = (σ bonds/bonded atoms) + (lone pairs on central atom); SN 2..6 → sp, sp², sp³, sp³d, sp³d².

Carbon CH₄ mein 4 equal bonds kyun banata hai jabki sirf 2 unpaired electrons hain?

Ek 2s electron 2p par promote hota hai (4 unpaired), phir s+3p hybridize hokar chaar equivalent sp³ orbitals banate hain 109.5° par.

sp³ bond angle derive karo.

Bonds ko tetrahedron vertices par rakho (1,1,1),(1,-1,-1),(-1,1,-1),(-1,-1,1); cosθ = -1/3 → θ = 109.47°.

Water ka angle 104.5° kyun hai, 109.5° kyun nahi?

O par do lone pairs (sp³) bonding pairs se zyada repel karte hain, H–O–H angle ko ideal tetrahedral se neeche squeeze karte hain.

π bonds kaunse orbitals banate hain?

Unhybridized p orbitals jo sideways overlap karte hain — kabhi hybrid orbitals nahi.

Double bond SN mein 1 count hota hai ya 2?

Ek (sirf σ bonds + lone pairs count hote hain; π part ko SN ke liye ignore kiya jaata hai).

%s-character aur bond angle ka kya relationship hai?

Zyada s-character → wider angle: sp(50%)=180°, sp²(33%)=120°, sp³(25%)=109.5°.

XeF₄ ki hybridization aur shape kya hai?

sp³d², square planar (SN = 4 bonds + 2 lone pairs = 6).

PCl₅ mein axial bonds lambe kyun hote hain?

Axial bonds 90° par 3 equatorial neighbours face karte hain, equatorial bonds se zyada repulsion suffer karte hain.

Connections

VSEPR Theory — geometry rules jo VBT SN counting se agree karti hain.
Sigma and Pi Bonds — hybrids → σ, leftover p → π.
Molecular Orbital Theory (MOT) — alternative jo $O_{2}$ ka paramagnetism explain karta hai jahan VBT fail karta hai.
Bond Angle and Bent's Rule — refined angle predictions.
Atomic Orbitals (s, p, d shapes) — woh raw ingredients jo mix ho rahe hain.
Formal Charge and Lewis Structures — lone pairs count karne se pehle zaruri hai.