STRUCTURE AND BONDING 29- How does the ease of ion formation change as we cross the
periodic table - What changes occur as we descend the groups of the table?
Consider first the formation of cations by electron loss. Here the
important energy quantity is the ionisation energy. As we have seen
(p. 15), the first ionisation energy is the energy required to remove
an electron from an atom, i.e. the energy for the process
M(g)-»M+(g)4- e~
(1 mole)the second, third and fourth ionisation energies being the additional
energies required to remove subsequent electrons from the in-
creasingly positively charged ion, the element and the ions formed
all being in the gaseous state. Ionisation energies can be obtained
from current-voltage plots for gaseous discharges or more con-
veniently and completely from spectroscopic measurements. For
convenience the transition and typical elements will be treated
separately.
IONISATION ENERGIES: TYPICAL ELEMENTS
Changes down the group
Table 2.1 gives data for Group I elements. The ionisation energies
are all positive, i.e. energy is absorbed on ionisation. Several con-
clusions can be drawn from this table:
- Energy must be supplied if these elements are to attain a noble
gas configuration. - Loss of one electron gives the noble gas configuration; the very
large difference between the first and second ionisation energies
implies that an outer electronic configuration of a noble gas is
indeed very stable. - Ionisation energy falls as the group is descended, i.e. as the
size of the atom increases and hence the distance between the
nucleus and the outer electron increases. - There is a marked contraction in size on the formation of an
ion, the percentage contraction decreasing as the percentage loss in
electrons decreases (for example Na -> Na^4 " involves loss of one of
eleven electrons, Cs -> Cs+ the loss of one of fifty-five electrons).
Some values for Group II and III elements are shown in Tables 2.2
and 2.3 respectively.