Page 97 - Physical chemistry understanding our chemical world

64 INTRODUCING INTERACTIONS AND BONDS

thunderstorm easily overcomes the N≡N bond energy, fixing the nitrogen without
recourse to a catalyst.

Aside

In the high mountains of Pashawa in Pakistan, near the border with Afghanistan, thun-
derstorms are so common that the soil is saturated with nitrates deriving from the
nitric acid formed by lightning. The soil is naturally rich in potassium compounds.
Ion-exchange processes occur between the nitric acid and potassium ions to form large
amounts of potassium nitrate, KNO 3 , which forms a thick crust of white crystals on the
ground, sometimes lending the appearance of fresh snow.
High concentrations of KNO 3 are relatively toxic to plant growth because the ratio
+
of K to Na is too high, and so the soil is not fertile.
+

Why does a satellite need an inert coating?

Covalency and bond formation

A satellite, e.g. for radio or TV communication, needs to be robust to withstand its
environment in space. In particular, it needs to be protected from the tremendous
gravitational forces exerted during take off, from the deep vacuum of space, and from
atoms in space.
Being a deep vacuum, there is a negligible ‘atmosphere’ sur-
The hydrogen atoms in rounding a satellite as it orbits in space. All matter will exist solely
space form a ‘hydride’ as unattached atoms (most of them are hydrogen). These atoms
with the materials on impinge on the satellite’s outer surface as it orbits. On Earth, hydro-
the surface of the satel- gen atoms always seek to form a single bond. The hydrogen atoms
lite. in space interact similarly, but with the satellite’s tough outer skin.
Such interactions are much stronger than the permanent hydrogen
bonds or the weaker, temporary induced dipoles we met in Section 2.1. They form a
stronger interaction, which we call a covalent bond.
The great American scientist G. N. Lewis coined the word covalent, early in the
20th century. He wanted to express the way that a bond formed by means of electron
sharing. Each covalent bond comprises a pair of electrons. This pairing is permanent,
so we sometimes say a covalent bond is a formal bond, to distinguish it from weak
and temporary interactions such as induced dipoles.
The extreme strength of the covalent bond derives from the way electrons accumu-
late between the two atoms. The space occupied by the electrons as they accumulate
is not random; rather, the two electrons occupy a molecular orbital that is orientated
spatially in such a way that the highest probability of finding the electronic charges
is directly between the two atomic nuclei.
As we learn about the distribution of electrons within a covalent bond, we start
with a popular representation known as a Lewis structure. Figure 2.11 depicts the