40 STRUCTURE AND BONDING
that these two 'models' for the ethene structure are identical, so that
we may use whichever is the more convenient.
Double bonds also occur in other covalent compounds. By
considering each double bond to behave spatially as a single bond
we are able to use Table 2.8 to determine the spatial configurations
of such compounds.
Triple bonds are formed by the sharing of three pairs of electrons
to form a a and two n bonds. Spatially these three bonds behave
as a single bond. Consequently acetylene (ethyne) C 2 H 2 has the
linear configuration often represented as H — C^C — H.
In each of the examples given so far each element has 'achieved'
a noble gas configuration as a result of electron sharing. There are,
however, many examples of stable covalent compounds in which
noble gas configurations are not achieved, or are exceeded. In the
compounds of aluminium, phosphorus and sulphur, shown below,
the central atoms have 6, 10 and 12 electrons respectively involved
in bondin:
•rr
••aluminium chloride phosphorus sulphur hexafluonde
(vapour) pentafluoride(The spatial configurations of each of these compounds can be
deduced by reference to Table 2.8.)
These apparent anomalies are readily explained. Elements in
Group V, for example, have five electrons in their outer quantum
level but with the one exception of nitrogen, they all have unfilled
d orbitals. Thus, with the exception of nitrogen. Group V elements
are able to use all their five outer electrons to form five covalent
bonds. Similarly elements in Group VI, with the exception of
oxygen, are able to form six covalent bonds for example in SF 6. The
outer quantum level, however, is still incomplete, a situation found
for all covalent compounds formed by elements after Period 2. and
all have the ability to accept electron pairs from other molecules
although the stability of the compounds formed may be low*. This
- Phosphorus pentafluoride PF 5 will readily accept an electron pair from a fluoride
ion F~ to form the stable hexafluorophosphate (V) anion PF<~. This ion is isoelectronic
with SF 6 , and neither SF 6 nor PF^ show any notable tendency to accept further
electron pairs, though there is some evidence for the existence of an SF^ ion.