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Account for the following : (i) `Be(OH)_(2)` is insoluble but `Ba(OH)_(2)` is fairly soluble in water .
(ii) `Be(OH)_(2)` is amphoteric while `Mg(OH)_(2)` is basic .

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(i) Be has small size , high ionization enthalpy and comparatively high electronegativity . Therefore , `Be(OH)_(2)` is essentially covalent and hence is insoluble in water . However , as we move down the group from Be to Ba , the atomic size increases , ionization enthalpy and electronegativity decrease . therefore , ionic characters of their hydroxides increases down the group . Further , it has been found that with increase in size the lattice down the group from Be to Ba . In other words , `Be(OH)_(2)` is insoluble but `Ba(OH)_(2)` is fairly soluble in water .
(ii) Because of higher sum of `Delta_(i)H_(1) + Delta_(i) H_(2)` of Be than that of Mg , the M-OH bond can break less easily in `Be(OH)_(2)` than in `Mg(OH)_(2)` . In other words , `Be(OH)_(2)` acts as a weak base even weaker than `Mg(OH)_(2)` .
Further because of smaller size and higher electronegativity than that of Mg , `Be(OH)_(2)` can accept `OH^(-)` ions from alkalies to form beryllate ion but Mg`(OH)_(2)` does not .
`Be(OH)_(2) + 2OH^(-) to Be(OH)_(4)^(2-) to underset("Beryllate ion")(BeO_(2)^(2-) ) + 2H_(2)O`
`Mg(OH)_(2) + 2OH^(-) cancelto Mg(OH)_(4)^(2-)`
Thus , `Be(OH)_(2)` acts as a weak acid but `Mg(OH)_(2)` does not . Combining both the facts , we conclude that the `Be(OH)_(2)` is amphoteric but `Mg(OH)_(2)` is basic .

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