bei48482_FM

Many-Electron Atoms 245

atoms accordingly form negative ions of charge ereadily. Reasoning of this kind

accounts for the similarities of the members of the various groups of the periodic

table.

Size

Although, strictly speaking, an atom of a certain kind cannot be said to have a defi-

nite size, from a practical point of view a fairly definite size can usually be attributed

to it on the basis of the observed interatomic spacings in closely packed crystal lattices.

Figure 7.11 shows how the resulting radii vary with atomic number. The periodicity

here is as conspicuous as in the case of ionization energy and has a similar origin in

the partial shielding by inner electrons of the full nuclear charge. The greater the shield-

ing, the lower the binding energy of an outer electron and the farther it is on the average

from the nucleus.

The relatively small range of atomic radii is not surprising in view of the binding-

energy curves of Fig. 7.8. There we see that in contrast to the enormous increase in

the binding energies of the unshielded 1selectrons with Z, the binding energies of

the outermost electrons (whose probability-density distributions are what determine

atomic size) vary through a narrow range. The heaviest atoms, with over 90 elec-

trons, have radii only about 3 times that of the hydrogen atom, and even the cesium

atom, the largest in size, has a radius only 4.4 times that of the hydrogen atom.

0 10 20 30 40 50 60 70 80 90

0.1

0.2

0.3

Li

Na

K

Rb

Cs

Atomic radius, nm

Atomic number

Figure 7.11Atomic radii of the elements.

bei48482_Ch07.qxd 1/23/02 9:02 AM Page 245