Intuition The big idea in one lineLone pairs are greedy — they hug the central atom more tightly than bonding pairs, so they push bonding pairs closer together , shrinking bond angles and hiding in the shape you actually see.
Definition Electron domains vs molecular shape
An electron domain = one region of electron density around the central atom = a bond (single/double/triple counts as ONE) OR a lone pair .
Electron geometry = arrangement of all domains (bonds + lone pairs).Molecular geometry = arrangement of only the atoms — we don't "see" lone pairs, so the reported shape ignores them.
The key move: count domains to get the base geometry, then delete the lone-pair positions to name the shape.
Central atom
Bond pairs (BP)
Lone pairs (LP)
Electron geometry
Molecular shape
Angle
C (CH₄)
4
0
tetrahedral
tetrahedral
109.5°
N (NH₃)
3
1
tetrahedral
pyramidal 107°
O (H₂O)
2
2
tetrahedral
bent 104.5°
Intuition The greedy electron cloudA bonding pair is shared between two nuclei — it is stretched out between them, so its cloud is thin and localized along the bond.
A lone pair is held by only one nucleus, so its cloud is fatter and spreads out closer to the central atom.
A fatter cloud takes more angular space → it repels harder than a bond pair.
So the repulsion ordering (Feynman-verify this feels right) is:
LP–LP > LP–BP > BP–BP \text{LP–LP} \;>\; \text{LP–BP} \;>\; \text{BP–BP} LP–LP > LP–BP > BP–BP
Recall Why does that orderingfeel right?
Because both electrons of a lone pair sit near the SAME atom (one attraction center), the cloud bulges out; two electrons of a bond are split between two atoms, pulled thin. More bulge near the central atom = more elbowing of its neighbors.
Consequence: every lone pair pushes the remaining bonds together, so each lone pair typically knocks the bond angle down by ~2–2.5° from the ideal 109.5°.
109.5 ° → 1 LP (NH 3 ) 107 ° → 2 LP (H 2 O ) 104.5 ° 109.5° \xrightarrow{\text{1 LP (NH}_3)} 107° \xrightarrow{\text{2 LP (H}_2\text{O})} 104.5° 109.5° 1 LP (NH 3 ) 107° 2 LP (H 2 O ) 104.5°
Derivation of the electron count (from scratch, no memorizing):
Central atom brings V V V
Each single bond uses 1 electron from the central atom to pair with 1 from the ligand.
Electrons of the central atom left over after making bonds must pair up as lone pairs.
Worked count — H₂O:
O has V = 6 V = 6 V = 6
Left over = 6 − 2 = 4 = 6 - 2 = 4 = 6 − 2 = 4 = 4 / 2 = 2 = 4/2 = 2 = 4/2 = 2
SN = 2 + 2 = 4 = 2 + 2 = 4 = 2 + 2 = 4 bent .
Why this step? We subtract bonding electrons and halve the rest because electrons must be spin-paired to occupy a lone-pair orbital.
Worked count — NH₃:
N has V = 5 V = 5 V = 5
Left over = 5 − 3 = 2 = 1 = 5 - 3 = 2 = 1 = 5 − 3 = 2 = 1 = 3 + 1 = 4 = 3 + 1 = 4 = 3 + 1 = 4 pyramidal .
Worked example Example 1 — Why is H₂O 104.5°, not 90° or 180°?Step 1: SN(O) = 2 bonds + 2 LP = 4 → base is tetrahedral (109.5°).
Why? Four domains spread to maximize distance = tetrahedron.
Step 2: Two lone pairs replace two "corners." The two LP–LP + LP–BP repulsions squeeze the two O–H bonds.
Why? Two fat lone-pair clouds shove the bonds together more than in NH₃.
Answer: angle drops from 109.5° → 104.5° , shape = bent .
Worked example Example 2 — NH₃ vs NH₄⁺
NH₃: SN = 4 (3 BP + 1 LP) → pyramidal, 107°.NH₄⁺: N donates its lone pair to H⁺, now 4 BP, 0 LP → SN = 4 → perfect tetrahedral, 109.5° .
Why the angle grows? Removing the lone pair removes its extra repulsion, so bonds relax back to the symmetric ideal.
Worked example Example 3 — SF₄ (a seesaw)
S: V = 6 V = 6 V = 6 6 − 4 = 2 = 1 6-4 = 2 = 1 6 − 4 = 2 = 1 Why does the LP go equatorial? In the equatorial plane it makes only two 90° neighbours instead of three, minimizing strong 90° LP–BP repulsion.
Shape = seesaw , with angles slightly under 120° and 90°.
Common mistake "H₂O should be linear like CO₂."Why it feels right: Both are AB₂ (two ligands, one central atom), and CO₂ is 180°.
The fix: CO₂ carbon has SN = 2 (no lone pairs), so it's linear. Water's O has 2 lone pairs (SN = 4) that occupy space → bent. Always count lone pairs, not just atoms.
Common mistake "NH₃ is trigonal planar because it has 3 bonds."Why it feels right: 3 bonds → sounds like BF₃ (trigonal planar).
The fix: BF₃ boron has no lone pair (SN 3); NH₃ nitrogen has 1 lone pair (SN 4). That lone pair pushes the three N–H bonds down into a pyramid .
Common mistake "Double bonds count as two domains."Why it feels right: A double bond has more electrons, so seems like "more directions."
The fix: All bonds to the same atom point in one direction → one domain . Extra electron density only makes that one domain repel a bit harder.
Why does a lone pair repel more than a bonding pair? A lone pair is held by only one nucleus, so its electron cloud is fatter and closer to the central atom, occupying more angular space.
Order the repulsion strengths of electron pairs. LP–LP > LP–BP > BP–BP.
What is the steric number and how do you find it? SN = (atoms bonded) + (lone pairs on central atom); it fixes the electron geometry.
Electron geometry and shape of H₂O? Tetrahedral electron geometry; bent molecular shape; 104.5°.
Electron geometry and shape of NH₃? Tetrahedral electron geometry; trigonal pyramidal shape; 107°.
Why is CH₄ 109.5° but NH₃ 107° and H₂O 104.5°? Each added lone pair (0→1→2) squeezes the bond angle by ~2.5° due to stronger lone-pair repulsion.
Why is NH₄⁺ exactly 109.5° while NH₃ is 107°? NH₄⁺ has 0 lone pairs (all 4 domains are bonds), so bonds relax to the symmetric tetrahedral ideal.
Number of lone pairs on O in water, from electron count? O has 6 valence e⁻; 2 used in bonds; (6−2)/2 = 2 lone pairs.
Why is H₂O bent and not linear like CO₂? Water's O has 2 lone pairs (SN 4); CO₂'s C has none (SN 2), so only water gets bent.
Where does the lone pair go in SF₄ (SN 5) and why? Equatorial position, to minimise the number of strongly-repulsive 90° neighbours.
Mnemonic Remember the shrinking angle"Lonely pairs push people apart... but crowd their friends together."
Count lone pairs (0, 1, 2) on C, N, O → angles fall 109.5 → 107 → 104.5 (roughly "minus 2.5 each lone pair").
VSEPR Theory — the parent framework this note applies.Hybridization — sp³ on N and O explains the tetrahedral base geometry.Bond angle and electronegativity — ligand electronegativity also tweaks angles.Dipole moment — bent H₂O and pyramidal NH₃ are polar because lone pairs break symmetry.Steric number and molecular shape — the counting recipe generalized.Trigonal bipyramidal geometry — where lone pairs prefer equatorial sites (SF₄, ClF₃).
delete lone pair positions
LP equals V minus bonds over 2
CH4 tetrahedral NH3 pyramidal H2O bent
Intuition Hinglish mein samjhoDekho, idea simple hai: har central atom ke aas-paas jitne bhi electron ke "groups" hote hain — chahe wo bond pair ho ya lone pair — wo ek doosre se door bhagte hain (repulsion). Yehi VSEPR ka funda hai. Lekin twist ye hai ki lone pair zyada jagah ghairta hai kyunki wo sirf ek hi nucleus se attach hota hai, toh uska cloud thoda phaila hua aur mota hota hai. Isliye lone pair, bond pairs ko zyada dhakka deta hai.
CH₄ mein carbon par koi lone pair nahi — chaaro bond barabar phailte hain, angle poora 109.5° . Ab NH₃ dekho: nitrogen par 1 lone pair hai, wo teeno N–H bonds ko neeche daba deta hai, angle gir ke 107° , aur shape ban jaata hai pyramidal . H₂O mein oxygen par 2 lone pairs hain, double dhakka, angle aur gir ke 104.5° , shape bent . Yaad rakho: har lone pair roughly 2.5° angle kaat deta hai .
Ek common galti: log kehte hain "H₂O bhi CO₂ jaisa linear hoga" — nahi bhai. CO₂ ke carbon par lone pair hai hi nahi (SN 2), isliye linear. Water ke oxygen par 2 lone pairs hain (SN 4), isliye bent. Hamesha lone pairs count karo, sirf atoms mat ginno.
Yeh cheez important isliye hai kyunki shape se hi molecule ki polarity, dipole moment, aur reactivity decide hoti hai. Water bent hai isiliye polar hai, isiliye wo "universal solvent" hai — agar linear hota toh dipoles cancel ho jaate. Toh lone pairs ka geometry par effect samajhna = poori chemistry ka base pakadna.
