Fill in the blanksQuestion 1 · Recall
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By the end you can state and explain the shapes and bond angles of , , , and using VSEPR theory.
A molecule's shape decides how it packs, how polar it is and how it fits into an enzyme. Your nose tells some molecules apart purely by shape.
Good news: you can predict shapes with one simple idea.
VSEPR theory (valence shell electron pair repulsion)
The electron pairs around a central atom repel each other and arrange themselves as far apart as possible, to minimise repulsion.
Count the regions of electron density around the central atom:
This lesson only has molecules with no lone pairs on the central atom.
Tie two balloons at their necks and they point in opposite directions. Three make a flat triangle. Four can't stay flat: they pop into a 3D tetrahedron.
Electron pairs do the same. Where the analogy breaks down: balloons push by touching, but electron pairs repel electrostatically, as negative charges.
| regions | shape | VSEPR-predicted bond angle | example |
|---|---|---|---|
| 2 | linear | 180° | |
| 3 | trigonal planar | 120° | |
| 4 | tetrahedral | 109.5° | |
| 5 | trigonal bipyramidal | 120° and 90° | |
| 6 | octahedral | 90° |
These are the standard VSEPR predictions used for these examples, not claims that every measured angle is exact.
Worked example: Boron trifluoride, BF₃
State and explain the shape of and its VSEPR-predicted bond angle.
Four pairs in a flat square would be 90° apart. In 3D they can do better: at the corners of a tetrahedron each pair is 109.5° from every other.
That's why is not flat.