STRUCTURE AND BONDING 31Group II elements can be seen to follow a pattern very like that
found in Group I. Note, however, that the energy required to
attain a noble gas configuration is considerably higher indicating
that the elements will be less 'metallic' or electropositive in their
chemistry (Chapter 6).
The elements in Group III show several irregularities which are
of interest. The apparent irregularity in the first ionisation energy of
gallium, relative to aluminium, can be attributed to the filling of the
inner d orbitals of the first transition series (atomic numbers 21-31)
which causes a contraction in atomic size (see Table 2.3.) Similarly
the filling of inner orbitals in the lanthanide series results in the
apparently irregular value given for thallium. Similar tables for
elements in other groups can be constructed to show irregularities
similar to those of the Group III elements.
Changes in ionisation energy across the periods
The number of electrons in the outermost quantum level of an atom
increases as we cross a period of typical elements. Figure 2.2 shows
plots of the first ionisation energy for Periods 2 and 3.
The discontinuities observed correspond to changes in electronic
configuration. Boron and aluminium both have one electron in a
AlAtomic number
Figure 2.2, First ionisation energies of elements in Periods 2 and 3