Modern inorganic chemistry

STRUCTURE AND BONDING 57

This representation of the double bond applies to other double

bonds also, for example C==O, S=O, P=O, and so on.

The element before carbon in Period 2, boron, has one electron

less than carbon, and forms many covalent compounds of type

BX 3 where X is a monovalent atom or group. In these, the boron

'uses' three sp^2 hybrid orbitals to form three trigonal planar bonds,

like carbon in ethene, but the unhybridised 2p orbital is vacant,

i.e. it contains no electrons. In the nitrogen atom (one more electron

than carbon) one orbital must contain two electrons—the lone pair;

hence sp^3 hybridisation will give four tetrahedral orbitals, one

containing this lone pair. Oxygen similarly hybridised will have two

orbitals occupied by lone pairs, and fluorine, three. Hence the

hydrides of the elements from carbon to fluorine have the structures

s"\ / A

^H H- \\H H^ \ ^'\

H H H H

with the line-pair orbitals indicated by broken lines. The co-ordinate
link is formed by overlap of a doubly-occupied (lone pair) orbital
with an unoccupied orbital. The projecting charge-clouds of
molecules like water or ammonia also impart other properties.
The concentration of negative charge on one side of the molecule
makes the molecule electrically polar, i.e. one end is positive, the
other (lone pair) end is negative; the molecule is then a dipole and
the magnitude of the polarity is expressed as the dipole moment.* In
molecules such as NH 3 , H 2 O, the positive end of the dipole is
'concentrated' at the small hydrogen atoms and there is conse-
quently a strong electrostatic attraction between these and the
negative charge-clouds of neighbouring molecules; this particularly
strong attraction is the origin of hydrogen bonding. The projecting
charge-clouds can also be attracted by ions so that positive ions, for
example, become hydrated (or "ammoniated') by attraction of the
lone pair charge-clouds to the ion, as, for example, the hydrated
A13+ ion (p. 45),
The elements of Period 2 (Li—F) cannot have a covalency greater
than 4, because not more than four orbitals are available for bonding.
In Period 3 (Na—Cl) similar behaviour would be expected, and
indeed the molecule SiH 4 is tetrahedral like that of CH 4 , and
PH 3 is like NH 3 with a lone pair occupying one tetrahedral position.

* Note that this kind of polarity is not the same as bond polarity (p. 51).