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The
acetylene molecule is linear, with all four atoms in a straight
line. The C-H bond lengths are little different from those of other
molecules, but the three electron pairs in the triple bond pull
the carbon atoms together until they are only 1.21 Å apart,
compared with 1.34 Å for a double bond and 1.54 Å for
a single bond. If a nitrogen atom replaces one carbon, a triple
bond is still possible, but then the C-H with an electron pair bond
is replaced by N:, with its lone electron pair, and the result is
HCN (hydrogen cyanide), which is shown at the right. If the other
carbon atom also is replaced by N, the result is the triply bonded
N~ molecule. This is the end of the road for bonds between most
atoms. Quadruple bonds involving s and p orbitals are geometrically
impossible. An absolute requirement for bonding is that the AO's
from the two atoms overlap, and as the bond order increases from
single to double to triple, the atoms have to be pushed closer together
to achieve this overlap. No matter how the s and three p orbitals
are hybridized, the only way to make all four orbitals overlap with
the corresponding four from another atom is to push the atoms together
until their nuclear centers coincide-an impossible thing to do.
Hence, quadruply bonded C |
gas molecules, sharing all four bonding electrons on one C with
a single partner, are not found. This is one reason for the observed
dramatic difference in properties of the pure elements, between
solid diamond and N
