Unit 5
Molecular shape
Why is water bent while carbon dioxide is straight? Shape decides whether a molecule is polar and how molecules attract one another — and through that, boiling points, solubility and life itself. This unit starts from electron-pair repulsion, passes through symmetry, hybrid and molecular orbitals and the colours of conjugated molecules, and ends with the forces between molecules.
- 1 Valence-shell electron-pair repulsion (VSEPR) Electron pairs repel and spread as far apart as they can — that single idea predicts the shapes of most small molecules, and explains why the bond angles of CH₄, NH₃ and H₂O shrink one after another. 30 min
- 2 Molecular polarity Every bond can be polar while the molecule is not. Add bond dipoles as vectors, then see how lone pairs send NH₃ and NF₃ in opposite directions. 25 min
- 3 Symmetry and point groups Turned by 120°, an ammonia molecule looks exactly as before. Find all of a molecule’s invisible moves — its point group — and you can tell at once whether it can have a dipole moment and whether it is chiral. 35 min
- 4 Hybrid orbitals s and p orbitals blend into hybrids that point in definite directions — explaining 180°, 120° and 109.5°, and the π bonds of ethene and ethyne. 30 min
- 5 Molecular-orbital theory Combine atomic orbitals into orbitals that belong to the whole molecule and fill them one electron at a time: bond order, bond length, bond energy — and why oxygen sticks to a magnet. 35 min
- 6 Conjugation and colour: π electrons in a box Three cyanine dyes that differ only by –CH=CH– units run from magenta through blue to green. Shut their π electrons in a one-dimensional box and you can calculate the wavelengths they absorb — then test the model against nine measured absorption maxima. 35 min
- 7 Intermolecular forces Dispersion, dipole forces and hydrogen bonds decide the temperature at which a substance boils — from the anomalous hydride boiling points to the shallow well of the Lennard-Jones potential. 30 min