2.3.8
Intuition Ek sentence mein ideaEk central atom ke around electron pairs sab negatively charged hote hain, isliye woh repel karte hain ek doosre ko aur 3D space mein jitna door ho sake utna phail jaate hain — aur yahi arrangement molecule ki shape fix karti hai.
WHY: Bonding pairs aur lone pairs negative charge ke clouds hote hain. Same charges repel karte hain. Agar kai aisi clouds ek central atom se tied hoon, toh woh khud ko orient karengi total repulsion minimise karne ke liye , yaani unke beech ke angles maximise karne ke liye.
WHAT: VSEPR = V alence S hell E lectron P air R epulsion. Yeh central atom ke around electron domains (electron density ke regions) ki sankhya se molecule ki 3D geometry predict karta hai.
HOW: Domains count karo → unhe ek sphere par max-separation positions par rakho → phir decide karo ki lone pairs kahan baithe → atoms ki shape describe karo (electrons ki nahi).
Definition Key vocabulary
Electron domain / region: a lone pair, a single bond, a double bond, or a triple bond har ek ONE domain count hota hai (ek multiple bond = ek mota cloud, phir bhi ek domain).Steric number (SN): S N = ( no. of bonded atoms ) + ( no. of lone pairs on central atom ) electron geometry fix karta hai.Electron geometry: saare domains (bonds + lone pairs) ka arrangement.Molecular geometry: sirf atoms ka arrangement (lone pairs kahan point kar rahe hain yeh ignore karo, lekin jo distortion woh cause karte hain usse rakho).
Hum N
SN
Points maximally spread →
Ideal angle
Name
2
ek line ke opposite ends par
18 0 ∘ linear
3
ek flat triangle ke corners par
12 0 ∘ trigonal planar
4
ek tetrahedron ke corners par
109. 5 ∘ tetrahedral
5
3 equatorial + 2 axial
12 0 ∘ 9 0 ∘ trigonal bipyramidal
6
ek octahedron ke corners par
9 0 ∘ octahedral
Intuition Chaar pairs ke liye109. 5 ∘ 9 0 ∘ 9 0 ∘ Tetrahedron par push karne se saare chaar equidistant rehte hain 109. 5 ∘
Intuition Lone pairs angles ko distort kyun karte hainEk lone pair sirf EK nucleus se hold hota hai, isliye uska cloud bonding pair se mota aur central atom ke zyada paas hota hai (bonding pair do nuclei ke beech khincha rehta hai). Mota cloud zyada repel karta hai. Repulsion ka order:
lone–lone > lone–bond > bond–bond squeeze karke chhota kar dete hain.
Molecule
Bonds
Lone pairs
Electron geom
Molecular shape
Angle
C H 4 4
0
tetrahedral
tetrahedral 109. 5 ∘
N H 3 3
1
tetrahedral
trigonal pyramidal 10 7 ∘
H 2 O 2
2
tetrahedral
bent / V-shape 104. 5 ∘
Angle 109.5 → 107 → 104.5 Har added lone pair bonds par zyada push karta hai, unhe close karte hue. Do lone pairs (water) ek (ammonia) se zyada squeeze karte hain.
Intuition Lone pairs EQUATORIAL positions prefer karte hainEk trigonal bipyramid mein, ek axial position ke 9 0 ∘ equatorial position ke 9 0 ∘ 9 0 ∘ kam crowded equatorial spot choose karta hai.
S F 4 C l F 3 X e F 2
C O 2 Step 1 — central C, O ke saath 2 double bonds. Kyun? Ek double bond phir bhi ek domain hai. Step 2 — C par lone pairs = 0. Toh S N = 2 + 0 = 2 Step 3 — 2 domains → linear, 18 0 ∘
B F 3 Step 1 — B ke 3 bonds hain, 0 lone pairs. 0 LP kyun? B ke 3 valence e⁻ hain, saare bonds mein use ho gaye. Step 2 — S N = 3 12 0 ∘ Molecular = electron geometry kyunki koi lone pairs nahi hain.
S F 6 Step 1 — S, 6 F se bond karta hai, apne 6 valence e⁻ use karta hai, 0 lone pairs bachte hain. Step 2 — S N = 6 9 0 ∘ Octahedral kyun? 6 points maximally spread = do square pyramids ke 6 faces jo base par join hote hain.
X e F 4 Step 1 — Xe ke 8 valence e⁻ hain; 4 F bonds mein jaate hain, 4 bachte hain = 2 lone pairs . Step 2 — S N = 4 + 2 = 6 Step 3 — 2 lone pairs trans (opposite) jaate hain LP–LP repulsion minimise karne ke liye. Result: square planar . Yeh step kyun? Opposite lone pairs 18 0 ∘
Common mistake "Ek double bond do domains count hota hai"Kyun sahi lagta hai: usme sach mein zyada electrons hote hain, aur aapko electron pairs count karne ko bola jaata hai. Fix: ek multiple bond ke saare electrons ek hi direction mein point karte hain (ek region), isliye yeh ek domain hai. Sirf directions ki sankhya shape ke liye matter karti hai.
Common mistake "NH₃ trigonal planar hai kyunki uske 3 bonds hain"Kyun sahi lagta hai: 3 bonded atoms → aap seedha SN=3 shape par jump karte ho. Fix: aap lone pair bhool gaye. S N = 4 Hamesha lone pairs ko SN mein count karo.
Common mistake "Electron geometry = molecular geometry"Kyun sahi lagta hai: C H 4 B F 3 sach mein equal hain. Fix: woh tabhi match karte hain jab koi lone pairs na hon . Warna sirf atom positions report karo (jaise, water bent hai, tetrahedral nahi).
Common mistake "Trigonal bipyramidal mein, lone pair axial jaata hai"Kyun sahi lagta hai: axial 'raaste se hataa hua' lagta hai. Fix: axial ke teen 9 0 ∘ do — equatorial worst repulsions minimise karta hai, isliye lone pairs equatorial jaate hain.
Recall Feynman: 12-saal ke bacche ko samjhaoImagine karo tum ek saath kai balloons pakde ho jo ek knot par bandhe hain. Woh automatically ek acchi round arrangement banane ke liye push apart ho jaate hain — 2 balloons ek seedhi line banate hain, 3 ek flat triangle banate hain, 4 ek chhoti pyramid banate hain. Molecules bhi apne electron "balloons" ke saath aisa hi karte hain. Aur agar ek balloon mota ho (ek lone pair — ek electron balloon jo doosre atom se share nahi ho raha), toh woh doosron ko aur paas squeeze kar deta hai. Jo shape tum dekhte ho woh bas yahi hai ki atoms kahan pahunche jab saare balloons push karna band kar dete hain.
Mnemonic SN → shape ladder yaad karo"Little Tigers Try Tackling Big Octopuses"
L inear(2) · T rigonal planar(3) · T etrahedral(4) · T rigonal B ipyramidal(5) · O ctahedral(6).
Aur tetra family mein angles ke liye: "Never Have Water" = N H 3 > H 2 O
VSEPR mein electron domain kya define karta hai? Electron density ka ek region — ek lone pair YA koi bhi single/double/triple bond (ek multiple bond = ek domain).
Steric number ka formula? SN = (bonded atoms ki sankhya) + (central atom par lone pairs ki sankhya).
SN=2 shape aur angle? Linear, 180°.
SN=3 shape aur angle? Trigonal planar, 120°.
SN=4 shape aur angle? Tetrahedral, 109.5°.
SN=5 electron geometry? Trigonal bipyramidal (90° axial, 120° equatorial).
SN=6 shape aur angle? Octahedral, 90°.
Repulsion strength ka order? lone–lone > lone–bond > bond–bond.
H₂O ka angle 104.5° kyun hai, 109.5° kyun nahi? O par do lone pairs bonding pairs se zyada repel karte hain, H–O–H angle ko squeeze karte hue.
NH₃ ki shape kya hai aur trigonal planar kyun nahi? Trigonal pyramidal; SN=4 (3 bonds + 1 lone pair), tetrahedral electron geometry.
Trigonal bipyramidal mein lone pairs kahan baithte hain aur kyun? Equatorial — kam 90° neighbours (2 vs 3), isliye kam repulsion.
SF₄ (1 lone pair, SN=5) ki molecular shape? See-saw.
ClF₃ (2 lone pairs, SN=5) ki molecular shape? T-shaped.
XeF₂ (3 lone pairs, SN=5) ki molecular shape? Linear.
XeF₄ (2 lone pairs, SN=6) ki molecular shape? Square planar.
Tetrahedral angle derive karo. cosθ = (a·b)/(|a||b|) = -1/3 → θ = arccos(-1/3) = 109.47°.
Molecular geometry electron geometry ke barabar kab hoti hai? Jab central atom par zero lone pairs hon.
Lewis Structures — lone pairs count karne ke liye pehle Lewis dot picture chahiye.Hybridization — sp/sp²/sp³/sp³d/sp³d² 1-to-1 map hota hai SN = 2/3/4/5/6 par.Bond Polarity and Dipole Moment — geometry decide karti hai ki bond dipoles cancel hote hain ya nahi (CO₂ nonpolar, H₂O polar).Molecular Orbital Theory — ek deeper model jahan VSEPR ki simple picture toot jaati hai.Formal Charge — VSEPR apply karne se pehle best Lewis structure choose karne mein help karta hai.
