Oxides, oxoacids, oxoanions and hydroxides

Thallium(I) halides, TlX, are stable compounds which in some ways resemble Ag(I) halides. Thallium(I) fluoride is very soluble in water, but TlCl, TlBr and Til are sparingly soluble the trend in solubilities can be traced to increased covalent contributions in the ionic lattices for the larger halides, a situation that parallels the trend for the Ag(I) halides (see Section 5.15). In the solid state, TIF has a distorted NaCl lattice, while TlCl and TlBr adopt CsCl structures. Thallium(I) iodide is dimorphic below 443 K, the yellow form adopts a lattice derived from an NaCl structure in which neighbouring layers are slipped with respect to each other and, above 443 K, the red form crystallizes with a CsCl lattice. Under high pressures, TlCl, TlBr and TU become metallic in character. 

The structure of BPO4 is derived from that of Si02 by replaciug alteruate Si atoms by B or P atoms. Explaiu how this description relates to the isoelectronic principle. 

It is a general observation that, within the / -block, basic character increases down a group. Thus  

Thallium(I) oxide is soluble in water and the resulting hydroxide is as strong a base as KOH. 

B2O3 with P4O10) catalyses the hydration of alkenes and dehydration of amides to nitriles. The structure of BPO4 can be considered in terms of Si02 (see Section 14.9) in which alternate Si atoms have been replaced by B or P atoms. 

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