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 .

Account for the following : (i) Be(OH)_(2) is insoluble but Ba(OH)_(2)

1.	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 .
Answer» Solution :(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 .