1-8
Hybrid Orbitals: Bonding in Complex Molecules
Mixing of atomic orbitals from the same atom results in new atomic orbitals of
different energy and directionality.
sp Hybrids produce linear structures.
An incorrect structure for BeH2 is predicted if 2s and 2p orbitals of Be are
overlapped with the 1s orbitals of H:
Mixing the 2s orbital with one of the 2p orbitals of Be results in two new hybrid
sp orbitals, each made up of 50% s and 50% p character. The resulting bond
angle is 180o which corresponds with the observed bond angle in the BeH2
molecule.
Hybridization does not change the number of orbitals on the atom. In this case
two atomic orbitals are replaced by two new hybrid orbitals. The two unhybridized p orbitals are still available to hold electrons.
sp2 Hybrids create trigonal structures.
Hybridization of a 2s and two 2p orbitals results in three new hybrid orbitals that
point to the corners of an equilateral triangle.
The remaining p orbital points up and down, perpendicular to each of the three
hybrid orbitals.
Bond angles in molecules using sp2 hybridization are approximately 120o
The molecule, BH3 is isoelectric with the methyl cation, CH3+. Both involve sp2
hybridization about the central atom.
sp3 Hybridizaton explains the shape of tetrahedral carbon compounds.
When the 2s and all three 2p orbitals are hybridized, four hybrid orbitals called
sp3 orbitals are formed. These orbitals point to the corners of a regular
tetrahedron.
Bond angles in molecules using sp3 hybridization are approximately 109.5o
Hybrid orbitals may contain lone electron pairs: ammonia and water.
Not all hybrid orbitals participate in bond formation. Some may contain lone pairs
of electrons.
The bond angles in ammonia are 107.3o and that in water is 104.5o, both close
to 109.5o. The slightly smaller bond angles in ammonia and water are due to
the slightly larger volume requirements for lone pair electrons, which forces the
remaining bonding pair electrons closer together.
Pi bonds are present in ethene (ethylene) and ethyne (acetylene).
Molecules containing double or triple bonds contain unhybridized p orbitals that
overlap lengthwise rather than end on.