Lone pairs squeeze bond angles: NH₃ and H₂O

5 minAS Level3.5.1

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can explain why $\text{NH}_3$ is pyramidal (107°) and $\text{H}_2\text{O}$ is non-linear (104.5°) using VSEPR theory and lone pairs.

Same number of pairs, different angles

$\text{CH}_4$ , $\text{NH}_3$ and $\text{H}_2\text{O}$ all have 4 electron pairs around the central atom. So why aren't they all 109.5°?

The difference is what kind of pairs they are: bonding pairs or lone pairs.

Lone pairs are pushier

A lone pair is held by one nucleus only, so it sits closer to the central atom and spreads out wider than a bonding pair.

So lone pairs repel more strongly:

$\text{lone pair–lone pair} > \text{lone pair–bonding pair} > \text{bonding pair–bonding pair}$

Each lone pair pushes the bonding pairs a little closer together: about 2.5° per lone pair.

The big bag on the bus seat

Bonding pairs are people sitting politely in their seats. A lone pair is the bulky bag someone has dumped on the seat: it takes up more room and squashes everyone else together.

Where it breaks down: the bag isn't just big, it's charged. Lone pairs push because they repel, not because they're physically larger.

Quick check

Fill in the blanksQuestion 1 · Recall

Lone pairs repel strongly than bonding pairs, so in a water molecule the two O–H bonds are pushed together.

Answer the question above to continue.

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