sp3

Meaning: In sp³ hybridization, one s orbital and three p orbitals on the same atom blend into four equivalent hybrid orbitals. They point toward the corners of a tetrahedron, so bond angles are near 109.5°. Each hybrid orbital has 25% s character and 75% p character. A carbon with four single bonds (as in alkanes) is sp³. Nitrogen and oxygen with four electron domains (bonds plus lone pairs), as in ammonia, water, amines, alcohols and ethers, are also treated as sp³. Their bond angles are slightly compressed by lone-pair repulsion. An sp³ atom makes only sigma bonds, and its single bonds can rotate.

Equation: 1 s + 3 p → 4 sp³ orbitals, with 25% s character each. Here s and p are atomic orbitals (counted, so dimensionless), and the tetrahedral bond angle θ ≈ 109.5° ≈ 1.91 rad. Steric number = bonded atoms + lone pairs = 4.

On the MCAT: You may be asked to assign hybridization from a structure, predict geometry, or rank bonds. More s character gives a shorter, stronger bond and a more acidic C–H, so sp C–H is more acidic than sp², which is more acidic than sp³. A stereocenter is usually an sp³ carbon with four different groups.

Common confusion: Students equate sp³ with having four bonds. Count lone pairs too, so water's oxygen is sp³ with a bent shape of about 104.5°. Some heteroatom lone pairs, such as the nitrogen in an amide, sit in a p orbital for resonance, so that atom is treated as sp².