Why Lone Pairs Change Bond Angles: CH4 vs NH3 vs H2O
Chemistry · Chemical Bonding · NEET
All three molecules (CH4, NH3, H2O) are sp3 hybridised, so you might expect the same angle. But the bond angle drops: CH4 = 109.5°, NH3 = 107°, H2O = 104.5°. The reason is lone pairs. A lone pair pushes harder than a bond pair, so it squeezes the bonds closer together. More lone pairs = smaller angle. Memory hook: "More lone pairs, more squeeze, smaller angle" — count the lone pairs (0, 1, 2) and the angle falls each time.
All three are sp3, but lone pairs (red) push harder than bond pairs. As lone pairs rise from 0 (CH4) to 1 (NH3) to 2 (H2O), the bond angle falls from 109.5° to 107° to 104.5°.
Your doubts, answered
All three are sp3 hybridised, so why are the bond angles different?
Hybridisation only tells you the ideal angle if there were no lone pairs. Pure sp3 gives 109.5°. CH4 has no lone pairs, so it keeps the full 109.5°. But NH3 and H2O have lone pairs on the central atom, and lone pairs push the bonds closer. So the real angle drops below 109.5°. Hybridisation sets the starting shape; lone pairs then distort it.
Why does a lone pair push harder than a bond pair?
A bond pair is shared between two atoms, so it is pulled by two nuclei and stays spread out and further from the central atom. A lone pair belongs to only one atom, so it sits closer to the central atom and takes up more space. Being fatter and closer, it repels the nearby bonds more strongly. That extra push squeezes the bond angle.
What is the correct order of repulsion in VSEPR theory?
Lone pair–lone pair > lone pair–bond pair > bond pair–bond pair (lp-lp > lp-bp > bp-bp). This is a direct NCERT line and a favourite NEET question. Just remember: lone pairs repel most because they are closer to the nucleus and not shared.
Why is the bond angle of H2O less than NH3?
NH3 has 1 lone pair on nitrogen; H2O has 2 lone pairs on oxygen. Two lone pairs give more squeezing than one. So H2O (104.5°) has a smaller angle than NH3 (107°). Count the lone pairs: more lone pairs means smaller angle.
How do I remember the order CH4 > NH3 > H2O?
Line them up by lone pairs: CH4 = 0 lone pairs (109.5°), NH3 = 1 lone pair (107°), H2O = 2 lone pairs (104.5°). As lone pairs go 0 → 1 → 2, the angle falls 109.5 → 107 → 104.5. More lone pairs = smaller angle. That single rule answers the whole comparison.
Do the electron-pair geometry and the shape mean the same thing?
No. All three have a tetrahedral electron-pair arrangement (4 electron pairs). But the shape (what you see, counting only atoms) is different: CH4 is tetrahedral, NH3 is trigonal pyramidal, H2O is bent. Lone pairs are invisible in the shape but still change the angle.
⚠️ The NEET trap ✗ Thinking H2O has a bigger bond angle than CH4 because oxygen is more electronegative. ✓ CH4 (109.5°) > NH3 (107°) > H2O (104.5°). H2O has the smallest angle because oxygen has 2 lone pairs that squeeze the bonds most. 🧠 NEET 2016 asked which statement is FALSE — the false one said H2O angle is larger than CH4. Never let electronegativity trick you; count lone pairs, not electronegativity, for the angle.
Real NEET questions
NEET 2016 Phase 1
Consider the molecules CH4, NH3 and H2O. Which of the given statements is false?
A · The H–C–H, H–N–H and H–O–H bond angles are all greater than 90°
B · The H–O–H bond angle in H2O is larger than the H–C–H bond angle in CH4 ✓
C · The H–O–H bond angle in H2O is smaller than the H–N–H bond angle in NH3
D · The H–C–H bond angle in CH4 is larger than the H–N–H bond angle in NH3
Solution: All three are sp3 hybridised. The true angle order is CH4 (109.5°) > NH3 (107°) > H2O (104.5°) because lone pairs increase (0, 1, 2) and repel the bonds more (lp-bp > bp-bp). So H2O actually has a SMALLER angle than CH4, not a larger one. Option B claims H2O > CH4, which is false. Options A, C and D are all true.
NEET 2016 Phase 1
Predict the correct order among the following (repulsion between electron pairs as per VSEPR theory):
A · Lone pair–lone pair > lone pair–bond pair > bond pair–bond pair ✓
B · Lone pair–lone pair > bond pair–bond pair > lone pair–bond pair
C · Bond pair–bond pair > lone pair–bond pair > lone pair–lone pair
D · Lone pair–bond pair > bond pair–bond pair > lone pair–lone pair
Solution: Lone pairs sit closer to the central atom and are held by only one nucleus, so they take more space and repel more strongly. The NCERT order is lp-lp > lp-bp > bp-bp. This is exactly why more lone pairs squeeze bond angles smaller (H2O < NH3 < CH4).
Solved Chemical Bonding NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What are the exact bond angles of CH4, NH3 and H2O?
CH4 = 109.5°, NH3 = 107°, H2O = 104.5°. They fall as lone pairs go from 0 to 1 to 2.
Are CH4, NH3 and H2O all sp3 hybridised?
Yes. All three have 4 electron pairs around the central atom, so all are sp3. The difference in angle comes only from the number of lone pairs, not from hybridisation.
Why does CH4 keep the full 109.5° angle?
CH4 has no lone pairs on carbon. With only bond pairs, all repulsions are equal (bp-bp), so the bonds spread out to the perfect tetrahedral angle of 109.5°.
Is H2O bent because of its lone pairs?
Yes. Oxygen has 2 lone pairs. They push the two O–H bonds down to 104.5°, giving a bent (V-shaped) molecule instead of linear.
Does this idea matter for NEET?
Very much. NEET repeatedly asks to order bond angles and to give the VSEPR repulsion order (lp-lp > lp-bp > bp-bp). One rule — more lone pairs, smaller angle — earns easy marks.
