Page 99 - Physical chemistry understanding our chemical world

66 INTRODUCING INTERACTIONS AND BONDS

Why does water have the formula H 2 O?

Covalent bonds and valence

The water molecule always has a composition in which two hydrogen atoms combine
with one oxygen. Why?
The Lewis structure in Figure 2.11 represents water, H 2 O. Oxygen is element num-
ber eight in the periodic table, and each oxygen atom possesses six electrons in its
outer shell. Being a member of the second row of the periodic table, each oxygen
atom seeks to have an outer shell of eight electrons – we call this trend the ‘octet
rule’. Each oxygen atom, therefore, has a deficiency of two electrons. As we saw
immediately above, an atom of hydrogen has a single electron and, being a row 1
element, requires just one more to complete its outer shell.
The Lewis structure in Figure 2.12 shows the simplest way in
The concept of the full which nature satisfies the valence requirements of each element:
outer shell is crucial if each hydrogen shares its single electron with the oxygen, meaning
we wish to understand that the oxygen atom now has eight electrons (six of its own – the
covalent bonds. crosses in the figure) and two from the hydrogen atoms. Looking
now at each hydrogen atom (the two are identical), we see how
each now has two electrons: its own original electron (the dot in the diagram) together
with one extra electron each from the oxygen (depicted as crosses).
We have not increased the number of electrons at all. All we have done is shared
them between the two elements, thereby enabling each atom to have a full outer shell.
This approach is known as the electron-pair theory.

Valence bond theory

The valence bond theory was developed by Linus Pauling (1901–1992) and others in
the 1930s to amalgamate the existing electron-pair bonding theory of G. N. Lewis and
new data concerning molecular geometry. Pauling wanted a single, unifying theory.
He produced a conceptual framework to explain molecular bonding, but in practice
it could not explain the shapes of many molecules.

Atomic nucleus
Electrons
Figure 2.12 Lewis structure of the covalent water molecule. The inner shell of the oxygen atom
has been omitted for clarity