Modern inorganic chemistry

THE TRANSITION ELEMENTS 361
Chapter 6). We note, however, that there is not a smooth increase in
the magnitude of these properties as the atomic number increases;
the metals seem to divide into two sets, Sc-Mn and Mn-Zn with
hpeaks' at Ti-V and Co-Ni, and this is well illustrated by a graph
of boiling point against atomic number (Figure 13.1).
This division of the first transition series into two "sets' is clearly
related to the filling of the d orbitals—at the dividing element,
manganese, the 3d level is half-filled (one electron in each d orbital),
thereafter the singly-occupied d orbitals become double-filled until
filling is complete at copper and zinc. The fact that the configurations
3d^5 (half-full) and 3d?^10 (full) are obtained at chromium and copper
respectively, in each case (see Table 13.1) (at the cost of removing an
electron from the 4s level) suggests that these configurations 3d^5
and 3d^10 are particularly "stable'; we shall see confirmation of this
idea when the chemical properties are examined later. In the dis-
cussion of the metallic bond in Chapter 2 we have already seen
that the notable physical properties of the transition metals (greater
density, hardness, etc) are attributed to the greater number of
valency electrons per atom available for bonding in the metal, and
this number clearly depends on the number of d electrons.

Table 13.3

FURTHER PHYSICAL PROPERTIES OF THE FIRST TRANSITION SERIES ELEMENTS

Element Sc Ti V Cr Mn Fe Co Ni Cu Zn

Atomic-

radius (nm) 0,161 0.145 0.132 0.137 0.137 0.124 0.125 0.125 0.128 0.133

Radius of

M2+(nm) — 0.090 0.088 0.088 0.080 0.076 0.074 0.072 0.069 0.074

1st ionisation

energy

(kJmor^1 )631 656 650 653 717 762 758 737 745 906

When we look at other physical properties of these transition
elements {Table 13.3), the regularities which we have previously
observed in the groups are not so clear across the series. The atomic
radius (in the metal) falls from scandium to vanadium, rises again
in chromium and manganese, falls at iron and thereafter rises slowly
until zinc is reached. The radius of the M^24 " ion falls irregularly to
copper and rises again at zinc; the first ionisation energy rises
irregularly, then sharply at zinc.