Chemistry · Rung 6 of 24

The shape of molecules

A molecule's shape is decided by the electron pairs on the central atom, which spread out as far as they can because they repel. That is VSEPR theory, and it gives five shapes worth knowing. Two pairs: linear, 180°, like CO₂. Three: trigonal planar, 120°. Four pairs and four bonds: tetrahedral, 109.5°, like methane. Four pairs with one lone: trigonal pyramidal, 107°, like ammonia. Four pairs with two lone: bent, 104.5°, like water. Lone pairs push harder than bonding pairs, which is why the angle closes from 109.5 to 107 to 104.5. And this is not decoration: water boils at 100 °C instead of −80 because it is bent and therefore polar. Were it linear, there would be no oceans.

Where this sits

Comes after
Bonds
Opens up
Carbon chemistry

Exercises from this rung

Generated by the app, from easiest to hardest

  1. CO₂ has two double bonds and no lone pairs on the carbon. What shape does it take?

    • Linear
    • Tetrahedral
    • Bent
    • Trigonal planar

    Why What counts in VSEPR is groups of electrons, and a double bond counts as one group: there are two, and two groups get as far apart as possible at 180°. That gives linear. Compare water, which also has two bonds but two lone pairs as well: four groups, which is why it comes out bent.

    easier

  2. What does VSEPR theory predict?

    • The shape of the molecule in space
    • How many electrons each atom has
    • Which bonds are strongest
    • The boiling point

    Why VSEPR stands for “valence shell electron pair repulsion”, and the whole theory fits in one sentence: pairs repel and settle as far from each other as they can. With that alone, and by counting bonding and lone pairs, you predict the five classroom shapes without solving a single equation. It has the best ratio of how little it asks to how much it gets right.

    in between

  3. Ammonia has three bonds and one lone pair. What shape does it take?

    • Trigonal pyramidal
    • Bent
    • Trigonal planar
    • Tetrahedral

    Why Four electron groups arrange as a tetrahedron, but only three are atoms: what is left is a pyramid on a triangular base with the nitrogen at the apex and the lone pair pointing up. It is not trigonal planar, which is what three groups with no lone pair give, as in BF₃. That lone pair is also what makes ammonia a base: it is what grabs the proton.

    harder