28 STRUCTURE AND BONDING
THE ELECTRONIC THEORY OF VALENCYAfter Dalton, in 1807, had put forward the theory that chemical
combination consisted of a union between atoms, chemists began
their search for the cause and mechanism of the unions. Many ideas
were put forward during the following years but, following the
discoveries about the structure of the atom, it was realised that the
nuclei of atoms were unaffected by chemical combination and that
union of atoms must result from interaction between the extra-
nuclear electrons. Kossel and Lewis, working independently in 1916,
recognised that the atoms of the different noble gases, with the one
exception of helium, each had an outer quantum level containing
eight electrons; they therefore suggested that this arrangement must
be connected with stability and inactivity, and that reactions
occurred between atoms such that each element attained a noble
gas configuration. The rearrangement of electrons into stable octets
could occur in two ways: (a) by giving or receiving electrons or (b)
by sharing electrons.
Since 1916 it has been discovered that some noble gases (originally
called the inert gases) do form compounds and also there are many
reactions known in which elements do not achieve a noble gas
configuration. Nevertheless, the theory was a considerable advance
towards modem ideas and provides a good basis for discussion.ELECTRON TRANSFER BONDING—ELECTROVALENCY
The electronic configuration of any element can quickly be deduced
from the periodic table. Consider the reaction, for example, between
sodium Is^2 2s^2 2p^6 3s^1 (2,8,1) and chlorine Is^2 2s^2 2p^6 3s^2 3p^5 (2.8.7).
The theory tells us that combination will occur by electron transfer
from the sodium to the chlorine to produce the noble gas con-
figurations 2,8 (Ne) and 2,8,8 (Ar) respectively. Sodium, atomic
number 11, becomes the sodium cation Na+, and chlorine the
chloride anion Cl~. Electrostatic attraction between these two ions
then holds the compound together. This kind of bonding is found
in 'giant ionic lattice' compounds and is an example of electro-
valency, the bond being said to be ionic. A full discussion of the
chemical energetics of such processes will be found in Chapter 3
but at this point it is desirable to consider the energy changes
involved in the electron transfer process. The questions to be
answered are briefly:
- What energy changes occur when an element achieves a noble
gas configuration?