How it works
Ten small molecules, glycine first. Each central atom is asked in turn: the shape around it, from tetrahedral to linear, then whether it is sp³, sp² or sp hybridized.
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Count the electron groups
Count the electron groups around the atom. Each atom bonded to it is one group, whatever the bond, and each lone pair is one. Four groups mean sp³: tetrahedral with no lone pair, trigonal pyramidal with one, bent with two. Three groups mean sp² and trigonal planar. Two groups mean sp and linear. That one count is how you label each carbon atom with the appropriate geometry, and label each carbon atom with the appropriate hybridization, in any molecule.
- A double or a triple bond counts as one electron group.
- Lone pairs count toward the hybridization, but the name of the shape describes only where the atoms are.
Questions and answers
How do you label the molecular shape of the amino acid glycine?
Atom by atom. The nitrogen has three bonds and a lone pair: trigonal pyramidal. The CH₂ carbon has four bonds: tetrahedral. The carboxyl carbon has three electron groups: trigonal planar. The OH oxygen has two bonds and two lone pairs: bent.
What is the hybridization of each carbon in acetic acid?
The CH₃ carbon is sp³: it has four electron groups. The carboxyl carbon is sp²: it has three, because its double bond to oxygen counts as one.
Is a carbon with a triple bond sp hybridized?
Yes, when it has two electron groups, as each carbon of ethyne does: one triple bond and one single bond. The two groups point in opposite directions, so the shape around it is linear.
What is the difference between electron-pair geometry and molecular shape?
Electron-pair geometry counts every electron group, lone pairs included. The molecular shape names only where the atoms are. The nitrogen of an amine has a tetrahedral arrangement of four groups but a trigonal pyramidal shape.