Application of the Uniqueness Principle and Diagonal Effect

Both lithium and magnesium salts are usually highly hydrated, while sodium salts are not. 

What is the reason for these similarities Primarily it is a size effect. The Hthium and magnesium ions (ionic radii = 0.73 A and 0.86 A, respectively) are of appropriate size to fit into a lattice formed by oxide ions (ionic radius = 1.26 A) such that the attraction of cations and anions is maximized and the repulsions between the larger anions is minimized. (See Chapter 8 for more details on solid-state energetics.) The carbonate and nitrate anions (radii = 1.78 A and 1.79 A, respectively) are just large enough that they get in one another s way, and their mutual repulsions become more of a dominant factor. Accordingly, the carbonates and nitrates decompose to the more energeticaUy favorable oxides. 

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