atom has four valence
orbitals: three mutually perpendicular p orbitals, each with a single nodal plane, and one
spherically symmetric s orbital.
A carbon atom in
a typical molecule
can be regarded as bonding with four orbitals
consisting of weighted sums (termed hybrids) of these s and p orbitals. One common pattern has four equivalent orbitals, each formed by combining the three p orbitals with the s orbital;
this is sp3 hybridization. An sp3 carbon atom
forms four sigma bonds,
usually in a roughly tetrahedral arrangement. Another common pattern has
three equivalent orbitals
formed by combining two p orbitals
with the s orbital;
this is termed sp2 hybridization. An sp2 carbon atom
forms three roughly coplanar sigma bonds,
usually separated by ~120 , and one pi bond (or
several fractional pi bonds).
If a single p orbital
is combined with the s orbital,
the result is sp hybridization, forming two sigma bonds and two pi bonds
(usually in a straight line). Atoms of
other kinds (e.g., N and O) can hybridize in an analogous manner.