Which of the following statement(s) is/are correct?

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Lattice energies of alkali metal super oxides are less than those of the peroxides

Order of ionic radii $O^{2 -} < O_{2}^{-} < O_{2}^{2 -}$

Thermal stability of super oxides increases with increase in the size of alkali metal ion

Thermal stability of alkali metal oxides decreases down the group

answer is A, B, C, D.

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Detailed Solution

The bond order in $O_{2}^{-}$ is 1.5 while that of $O_{2}^{2 -}$ is 1.0. So bond length in $O_{2}^{-}$ is less than in $O_{2}^{2 -} .$ Thus $O_{2}^{2 -}$ is larger than $O_{2}^{-} . O_{}^{2 -}$ is smaller than both $O_{2}^{-} & O_{2}^{2 -} (O^{2 -} = 126 p m, O_{2}^{2 -} = 180 p m and O_{2}^{-} = 163 p m)$

A) $L . E α \frac{Z_{+} \times Z_{-}}{r_{c} + r_{a}} .$ As the size of cation $(r_{c})$ increases down the group L.E decreases thus stability decreases

B) Since $Z_{-}$ is 1 in $O_{2}^{-}$ (super oxide ion ) 2 in $O_{2}^{2 -}$ (peroxide ion) the LE of super oxides will be less than peroxides

D) Thermal stability of super oxides is a combined effect of lattice energies of oxides, super oxide and decomposition energy of super oxide. Stability of super oxides increases down the group.

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