Lesson 3: Molecules in 3D

Lone pairs bend molecules

Section 4 of 4

Carbon dioxide and water both have a middle atom with two partners. Carbon dioxide is straight, but water is bent. The difference is oxygen's two lone pairs.

Lone pairs are electrons too, so they push on bonds just like other bonds do. Water's oxygen has 2 bonds and 2 lone pairs: 4 directions, arranged like methane's. We only see where the atoms are, so the molecule looks bent.

Lone pairs push a little harder than bonds, so they squeeze the bond angle a bit below 109.5°. In water the angle is 104.5°.

Water is bent because of oxygen's two lone pairs An oxygen atom with two hydrogen atoms below it, the bonds 104.5 degrees apart, a little less than 109.5. Above the oxygen, two separate lone pairs, each drawn as two dots, point up to the left and up to the right. 104.5° lone pair lone pair
Water is bent: oxygen's two lone pairs and two bonds make 4 directions, and the bonds end up 104.5° apart.

Ammonia, NH₃, works the same way. Nitrogen has 3 bonds and 1 lone pair, 4 directions again. Seeing only the atoms, we get a pyramid, with the nitrogen at the top and the three hydrogen atoms as the base. Its bonds are about 107° apart.

Ammonia is a pyramid A nitrogen atom at the top of a pyramid with three hydrogen atoms forming the triangular base, drawn with dashed edges. The bonds are about 107 degrees apart. Above the nitrogen sits one lone pair, drawn as two dots. lone pair a pyramid: bonds about 107° apart
Ammonia is a pyramid: nitrogen's lone pair sits on top and pushes the three bonds down, about 107° apart.
Water, H₂OOne oxygen atom joined by single bonds to two hydrogen atoms, in a bent shape.OHH
Turn water around in 3D to see its bent shape. The model shows atoms, not lone pairs.

Quick check

Why is a water molecule bent instead of straight?

That's the end of lesson 3

In this lesson you found out:

  • Ball-and-stick and space-filling models
  • Why bonds spread out as far apart as they can
  • The tetrahedron, flat triangle and straight line shapes
  • How lone pairs bend water and ammonia
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