Page 108 - Physical chemistry understanding our chemical world

CREATING FORMAL CHEMICAL BONDS 75

Why is silver iodide yellow?

Mixed bonding

Silver chloride is white; silver bromide is pale yellow; and silver iodide has a rich
yellow colour. We might first think that the change in colour was due to AgI incor-
porating the iodide anion, yet NaI or HI are both colourless, so the colour does not
come from the iodide ions on their own. We need to find a different explanation.
Silver iodide also has other anomalous properties: it is physically soft – it can
even be beaten into a sheet, unlike the overwhelming majority of ionic compounds.
More unusual still, it is slightly soluble in ethanol. Clearly, silver iodide is not a
straightforward ionic compound. In fact, its properties appear to overlap between
covalent (see Table 2.6) and ionic (see Table 2.7).
Silver iodide is neither wholly covalent nor wholly ionic; its bonding shows con-
tributions from both. In fact, most formal chemical bonds comprise a contribution
from both covalent and ionic forces. The only exceptions to this general rule are
homonuclear molecules such as hydrogen or chlorine, in which the bonding is 100
per cent covalent. The extent of covalency in compounds we prefer
to think of as ionic will usually be quite small: less than 0.1 per A ‘trimer’ is a species
cent in NaCl. For example, each C–H bond in methane is about 4% comprising three com-
ionic, but the bonding can be quite unusual in compounds compris- ponents (the Latin
ing elements from the p- and d-blocks of the periodic table. For tri means ‘three’).
The W 3 O 9 trimer has
example, aluminium chloride, AlCl 3 , has a high vapour pressure
a triangle structure,
(see p. 221); tungsten trioxide will sublime under reduced pres-
with a WO 3 unit at
sures to form covalent W 3 O 9 trimers; sulphur trioxide is a gas but
each vertex.
will dissolve in water. Each, therefore, demonstrates a mixture of
ionic and covalent bonding.
In other words, the valence bonds approach is suitable for com-
pounds showing purely ionic or purely covalent behaviour; we
require molecular orbitals for a more mature description of the We require ‘molec-
bonding in such materials. So the yellow colour of silver iodide ular orbitals’ for a
reflects the way the bonding is neither ionic nor covalent. We find, more mature descrip-
in fact, that the charge clouds of the silver and iodide ions overlap tion of the bonding in
to some extent, allowing change to transfer between them. We will such materials.
look at charge transfer in more detail on p. 459.

Oxidation numbers

Valency is the number of electrons lost, borrowed or shared in a
Numbers written as
chemical bond. Formal charges are indicated with Arabic numerals,
2+
so the formal charge on a copper cation is expressed as Cu , 1,2,3,...,etc.are called
‘Arabic
meaning each copper cation has a deficiency of two electrons. In
numerals’.
this system of thought, the charge on the central carbon of methane
is zero.