Page 107 - Physical chemistry understanding our chemical world

74 INTRODUCING INTERACTIONS AND BONDS

Why is argon gas inert?

First electron affinity and reactivity

Gases such as helium, neon and argon are so unreactive that we
Krypton, xenon and call them the inert gases. They form no chemical compounds, and
radon will form a their only interactions are of the London dispersion force type.
very limited number They cannot form hydrogen bonds, since they are not able to bond
of compounds, e.g. with hydrogen and are not electronegative.
with fluorine, but only The outer shell of the helium atom is full and complete: the
under quite excep- shell can only accept two electrons and, indeed, is occupied by
tional conditions.
two electrons. Similarly, argon has a complete octet of electrons
in its outer shell. Further reaction would increase the number of
electrons if argon were to undergo a covalent bond or become an anion, or would
decrease the number of electrons below the ‘perfect’ eight if a cation were to form.
There is no impetus for reaction because the monatomic argon is already at its position
of lowest energy, and we recall that bonds form in order to decrease the energy.
+
Sodium atoms always seek to lose a single electron to form the Na monocation,
because the outer valence shell contains only one electron – that is why we assign
sodium to Group I(a) of the periodic table. This single electron helps us explain why
+
it is so favourable, energetically, to form the Na cation: loss of the electron empties
the outer shell, to reveal a complete inner shell, much like removing the partial skin
of an onion to expose a perfectly formed inner layer. So, again, removal of sodium’s
single outer electron occurs in order to generate a full shell of electrons.
But if we look at an element like magnesium, there are several ionization pro-
cesses possible:

+
(1) Formation of a monocation: Mg (g) → Mg (g) + e (g) .
−
+
(2) Formation of a dication: Mg (g) → Mg 2+ (g) + e (g) .
−
The energy change in reaction (1) is called the first ionization
Care:do not con- energy and the energy associated with reaction (2) is the second
fuse the symbols for ionization energy. We symbolize the two processes as I (1) and I (2)
molecular iodine I 2 and respectively.
the second ionization The second ionization energy is always larger than the first,
energy I (2) . Hint: note because we are removing a negative electron from a positively
carefully the use of charged cation, so we need to overcome the attractive force bet-
italic type.
ween them. The value of I (1) for a magnesium atom is 734 kJ
−1
+
mol , but I (2) for removing an electron from the Mg monoca-
−1
tion is 1451 kJ mol . Both ionization energies are huge, but I (2) is clearly much the
larger. Table 2.8 contains many other ionization energies for elements 1–10.
It is clear from Table 2.8 that each ionization energy is larger than the one before.
Also note that the last two ionization energies of an element are always larger than the
others. The sudden rise follows because the last two energies represent the removal
of the two 1 s electrons: removal of electrons from the 2s and 2p orbitals is easier.