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Property of the alkaline earth metals that increases with their atomic number is
A
Electronegativity
B
Solubility of their hydroxides in water
C
Solubility of their sulphates in water
D
Ionization energy
Detailed Solution
Down group 2 the atomic size increases, so ionisation energy and electronegativity both decrease.
Hydroxides: the lattice enthalpy decreases down the group more rapidly than the hydration enthalpy (the small $OH^-$ ion makes lattice enthalpy very sensitive to cation size), so the solubility increases: $Be(OH)_2 < Mg(OH)_2 < Ca(OH)_2 < Sr(OH)_2 < Ba(OH)_2$.
Sulphates: the lattice enthalpy stays nearly constant (large $SO_4^{2-}$ ion) while the hydration enthalpy falls, so the solubility decreases: $BeSO_4 > MgSO_4 > CaSO_4 > SrSO_4 > BaSO_4$.
Hence the property that increases with atomic number is the solubility of their hydroxides in water.
Hydroxides: the lattice enthalpy decreases down the group more rapidly than the hydration enthalpy (the small $OH^-$ ion makes lattice enthalpy very sensitive to cation size), so the solubility increases: $Be(OH)_2 < Mg(OH)_2 < Ca(OH)_2 < Sr(OH)_2 < Ba(OH)_2$.
Sulphates: the lattice enthalpy stays nearly constant (large $SO_4^{2-}$ ion) while the hydration enthalpy falls, so the solubility decreases: $BeSO_4 > MgSO_4 > CaSO_4 > SrSO_4 > BaSO_4$.
Hence the property that increases with atomic number is the solubility of their hydroxides in water.