Modern inorganic chemistry

STRUCTURE AND BONDING 39

The basic tetrahedral shape is even more distorted producing an

/°\

H H bond angle of 105°:

SX-'

If the spatial arrangement of atoms is required this can be deduced

from the basic structure by neglecting the positions occupied by

lone pairs of electrons. Water, for example, can be described as a

V shape whilst ammonia is a trigonal pyramid.

MULTIPLE COVALENT BONDS

Double and triple covalent bonds can be formed between elements

by the sharing of two or three electron pairs respectively. Consider

the formation of ethene (ethylene), C 2 H 4 :

X

H. x H H ^H

+ _>. x C x C or C f~^ X /-* x /^ /-** = C

4H- »X X "H H' "H

The two kinds of covalent bond are not identical, one being a simple
covalent bond, a sigma (a) bond, the other being a stronger (but
more reactive) bond called a n bond (p. 56). As in the formation
of methane both elements attain noble gas configurations. We can
consider the formation of ethene as the linking of two tetrahedral
carbon atoms to form the molecule C 2 H 4 represented as:

H

this approach implying repulsion between the two bonding pairs.

Careful consideration of this model correctly indicates that all the

atoms lie in one plane. Spatially the double bond is found to behave

as a single electron pair and reference to Table 2.8 then (correctly)

suggests that each carbon has a trigonal planar arrangement.

The modern quantum-mechanical approach to bonding indicates