Chemistry · Chemical Bonding · NEET
There are four key ideas. (1) The shape of a molecule depends on the number of electron pairs (bonding and non-bonding) around the central atom. (2) These electron pairs repel each other because they are all negative, so they move as far apart as possible to keep repulsion low. (3) A lone pair (non-bonding pair) takes up more space than a bond pair, so lone pairs repel more strongly. (4) The repulsion order is lone pair-lone pair > lone pair-bond pair > bond pair-bond pair. This is exactly what NEET asks, so learn this order by heart.
A bond pair is shared between two atoms, so it is pulled by two nuclei and held tightly in the space between them. A lone pair belongs to only one atom, so it is not pulled away and spreads out closer to the central atom, taking up more room. Because it is fatter and closer, a lone pair pushes harder on nearby pairs. This is why lone pairs shrink bond angles.
Lone pair-lone pair (lp-lp) > lone pair-bond pair (lp-bp) > bond pair-bond pair (bp-bp). Read it as: two lone pairs repel the most, a lone pair and a bond pair repel medium, two bond pairs repel the least. NEET 2016 asked this order directly and the answer was lp-lp > lp-bp > bp-bp. This single line explains why H2O (104.5 degrees) has a smaller angle than NH3 (107 degrees), which is smaller than CH4 (109.5 degrees).
It counts electron pairs around the central atom, both bond pairs and lone pairs. The total number of electron pairs decides the basic geometry. But the visible shape only uses the positions of the atoms, because lone pairs are invisible. That is why NH3 has 4 electron pairs (tetrahedral geometry) but its shape is called pyramidal, since one corner is a lone pair, not an atom.
Geometry (or electron geometry) is the arrangement of ALL electron pairs, including lone pairs. Shape (or molecular shape) is what you actually see using only the atoms. If there are no lone pairs, geometry and shape are the same (like CH4, both tetrahedral). If there are lone pairs, they differ: H2O has tetrahedral geometry but bent shape. NEET options often mix these two words to trap you.
VSEPR treats a multiple bond as one region of electron density, so a double bond or triple bond counts as ONE bond pair for deciding shape. For example, CO2 has two double bonds but is treated as two electron regions, giving a linear shape. A multiple bond does repel a bit more than a single bond, but for finding the basic shape you count it as one pair.
Predict the correct order among the following (repulsion between electron pairs as per VSEPR theory):
Consider the molecules CH4, NH3 and H2O. Which of the given statements is false?
Amongst the following, which one will have maximum 'lone pair-lone pair' electron repulsion?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
VSEPR (Valence Shell Electron Pair Repulsion) theory was developed mainly by Ronald Gillespie and Ronald Nyholm. For NEET you do not need the history; you need the four postulates and how to use them to predict molecular shape.
No. VSEPR predicts the shape by counting and repelling electron pairs. Hybridisation explains the bonding using mixed orbitals. They often give the same shape (for example sp3 and 4 electron pairs both give tetrahedral geometry), but they are two different models. NEET may test them together in one option.
No, VSEPR gives the correct order and approximate angles, not exact values. It correctly predicts CH4 > NH3 > H2O for bond angle, but the exact numbers (109.5, 107, 104.5 degrees) come from experiment. VSEPR is a quick tool to compare and predict, which is enough for NEET MCQs.
VSEPR works well for main-group (p-block) molecules. It is less reliable for transition metal complexes because d-electrons and other factors change the shape. For NEET, apply VSEPR to molecules like CH4, NH3, H2O, PCl5, SF6, ClF3 and XeF2, not to coordination complexes.
