sp^3 -hybridized orbitals have 25% s character and 75% p character. They form tetrahedral
geometry with 109.5° bond angles. Carbons with all single bonds are sp^3 -hybridized.
sp^2 -hybridized orbitals have 33% s character and 67% p character. They form trigonal planar
geometry with 120° bond angles. Carbons with one double bond are sp^2 -hybridized.
sp-hybridized orbitals have 50% s character and 50% p character. They form linear geometry with
180° bond angles. Carbons with a triple bond, or with two double bonds, are sp-hybridized.
Resonance describes the delocalization of electrons in molecules that have conjugated bonds.
Conjugation occurs when single and multiple bonds alternate, creating a system of
unhybridized p-orbitals down the backbone of the molecule through which π electrons can
delocalize.
Resonance increases the stability of a molecule.
The various resonance forms all contribute to the true electron density of the molecule; the
more stable the resonance form, the more it contributes. Resonance forms are favored if they
lack formal charge, form full octets on electronegative atoms, or stabilize charges through
induction and aromaticity.