World 1 · Structure & Bonding · 8 minsp³, sp² and sp
Shape follows from how many things are attached
Teaching this? Open projector mode
Count what is attached to a carbon — bonded atoms plus lone pairs. That count, the steric number, fixes the shape:
- 4 → sp³, tetrahedral, 109.5°
- 3 → sp², trigonal planar, 120°
- 2 → sp, linear, 180°
The electrons repel each other and end up as far apart as possible. That is the whole derivation.
A double bond counts as one thing for shape purposes, not two: the two carbons of ethene each have three attachments, so each is trigonal planar. The second bond of the pair (the π bond) sits above and below that plane rather than adding another direction.
This is why ethene is flat and cannot twist — twisting would break the π overlap — while ethane spins freely. That single difference is the subject of the whole conformation unit.
The carbon of a C=O group in acetone has how many attachments, and what shape?