The formulae and structures of ionic compounds

In order to understand the valence of ions it is necessary to consider the electronic configuration in more detail. The gain or loss of electrons is most 

Atoms in at the lower part of Groups 13, 14 and 15 are able to take two ionic states. For example, tin (Sn) has an outer electron configuration [Kr] 4d 5s 5p. Loss of the two p electrons will not leave the ion either with a noble gas configuration or with a d ° configuration but it will still possess a series of closed shells that is moderately stable. This is the Sn state, with a configuration of [Kr] 5s 6d. However, loss of the two s electrons will produce the stable configuration [Kr] 6d of Sn . The atoms that behave in this way are characterised by two valence states, separated by a charge difference of - -2. The examples are indium [In (1+, 3+)], thallium [T1 (l-b, 3-b)], tin [Sn (2-f, 4-H)], lead [Pb (2+, 4+)], antimony [Sb (3-b, 5-b)] 

The transition metal ions generally have a number of d electrons in their outer shell, and because the energy difference between the various configurations is small, the arrangement adopted will depend upon a variety of external factors, such as the geometry of the crystal structure (see also Chapter 12 and Section S4.5). The lanthanides have an incomplete 4f shell of electrons, and the actinides an incomplete 5f shell. In these elements, the f orbitals are shielded from the effects of the surrounding crystal structure. The d and f electrons control many of the important optical and magnetic properties of solids. 

The concept of allocating a fixed size to each ion is an attractive one and has been extensively utilised. Ionic radii are generally derived from X-ray crystal- 

Although the majority of the ions of elements can be considered to be spherical, the lone-pair ions are definitely not so. These ions - In, Tl, Sn, Pb , Sb and Bi - tend to be surrounded by an irregular coordination polyhedron of anions. This is often a distorted trigonal bipyramid, and it is hard to assign a unique radius to such ions. 

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