What a learner can do afterwards
- Finds the rotation axes, mirror planes, inversion centre and improper axes of a drawn molecule
- Assigns a point group by working through a decision sequence rather than by recognition
- Explains why a molecule with no improper axis of any kind can be chiral
- Predicts from symmetry alone whether a molecule can have a permanent dipole
1 · Read
Every molecule owns moves that leave it looking unchanged. Spins around an axis, reflections through a plane, inversion through a centre, and combined spin plus reflection moves all count. Train your eye on flat shapes first: fold the drawing in your mind and spot the axes and planes. Then carry the habit into three dimensions.
Work a decision sequence, asking about each element in turn, until the molecule earns its point group label. Water is bent and keeps only a twofold axis with two mirror planes, so it is C2v. Ammonia is a pyramid with a threefold axis, so it is C3v. Boron trifluoride is a flat triangle, so it is D3h. Never guess the label from memory when the sequence answers in seconds.
Chirality falls out of the same list. A molecule with no improper axis of any kind can be chiral, while any mirror plane or improper move forbids it. So a molecule with only rotations on its list is a chirality candidate. Check the improper entries before you declare any molecule achiral.
Polarity obeys the label too. Boron trifluoride is flat and symmetric, so its three bond dipoles cancel exactly and it stays nonpolar. Water is bent and ammonia is a pyramid, so their dipoles reinforce and both are polar. Symmetry alone tells you which molecules are even allowed a permanent dipole.
List the unchanged moves, work the sequence, and let the label predict chirality and polarity.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.