32 STRUCTURE AND BONDING
p orbital (which is less firmly held) whilst oxygen and sulphur have
one electron pair map orbital, the second electron being less firmly
held. The high values of the first ionisation energies of these upper
elements in Groups IV, V, VI and VII correctly imply that in-
sufficient energy is liberated in chemical reactions to enable these
elements to achieve noble gas configurations by electron loss.
TRANSITION ELEMENTS
The first ionisation energies of the first transition elements are
shown in Figure 2,3. The changes across these 10 elements contrast
>»o>
o
oto
Ni Cu
Cr
Atomic number
Figure 2.3, First ionisation energies oj the first series oj transition elements
sharply with the changes shown across a period of typical elements
and confirms that the d block elements need to be treated separately.
SUMMARY
- Ionisation energy decreases down a group of elements as the
atomic size increases. The elements in consequence become more
metallic down the group.
- With certain irregularities only, the ionisation energy increases
across a period. The elements therefore become less metallic across
a period.
