Page 92 - Physical chemistry understanding our chemical world

CREATING FORMAL CHEMICAL BONDS 59

2.3 Creating formal chemical bonds

Why is chlorine gas lethal yet sodium chloride is vital
for life?

The interaction requires electrons

Chlorine gas is very reactive, and causes horrific burns to the eyes
and throat; see p. 243. The two atoms are held together by means of The word ‘chlorine’
derives from the Greek
a single, non-polar covalent bond. Cl 2 has a yellow–green colour chloros, meaning
and, for a gas, is relatively dense at s.t.p. Conversely, table salt
‘green’.
−
(sodium chloride) is an ionic solid comprising Na and Cl ions,
+
held together in a three-dimensional array. What is the reason for
their differences in behaviour?
The outer shell of each ‘atom’ in Cl 2 possesses a full octet of electrons: seven
electrons of its own (which explains why it belongs to Group VII(b) of the periodic
table) and an extra electron from covalent ‘sharing’ with the other atom in the Cl 2
molecule. The only other simple interactions in molecular chlorine are the inevitable
induced dipolar forces, which are too weak at room temperature to allow for the
liquefying of Cl 2(g) .
Each chloride ion in NaCl also has eight electrons: again, seven electrons come
from the element prior to formation of a chloride ion, but the extra eighth electron
comes from ionizing the sodium counter ion. This extra electron resides entirely on
the chloride ion, so no electrons are shared. The interactions in solid NaCl are wholly
ionic in nature. Induced dipoles will also exist within each ion, but their magnitude
is utterly negligible when compared with the strength of the formal charges on the
−
Na and Cl ions. We are wise to treat them as absent.
+
So, in summary, the principal differences between Cl 2(g) and NaCl (s) lie in the
location and the interactions of electrons in the atoms’ outer shells. We say these
electrons reside in an atom’s frontier orbitals, meaning that we can ignore the inner
electrons, which are tightly bound to the nucleus.

Why does a bicycle tyre get hot when inflated?
Bonds and interactions involve energy changes

A bicycle tyre gets quite hot during its inflation. The work of
We look on p. 86 at the
inflating the tyre explains in part why the temperature increases, but
effect of performing
careful calculations (e.g. see pp. 86 and 89) show that additional
‘work’ while inflating a
factors are responsible for the rise in temperature.
bicycle tyre, and the
On a macroscopic level, we say we compress the gas into the
wayworkimpinges on
confined space within the tyre; on a microscopic level, interparti- the internal energy of
cle interactions form as soon as the gas particles come into close the gas.
proximity.