(i) Ionization enthalpy:
The amount of energy needed to remove or add an electron to gain inert gas configuration in a neutral gaseous atom is known as ionization energy or ionization enthalpy.
Alkali metals have one electron on its valence shell and alkaline earth metal contains two valence electrons (completely filled subshell).
Therefore, removing one electron is easier than removing two of the electrons to attain complete octet. Hence, alkali metals need less energy than alkaline earth metals.
Ionization energy of alkaline earth metals is greater than alkali metals.
(ii) Basicity of oxides:
Both alkali metals and alkaline earth metals form basic oxides.
Due to the lower ionization enthalpy of alkali metals its oxides can be easily ionized and hence basicity is more when compared with alkaline earth metals.
(iii) Solubility of hydroxides:
As we move along the period the size and charge will increase.
Therefore, alkaline earth metals have higher lattice energy as they have smaller size. As lattice energy increases, solubility of hydroxides also increases.
Hydroxides of alkaline earth metals are more soluble than alkali metals.
Final answer:
(i) ionization enthalpy : alkaline earth metals > alkali metals
(ii) basicity of oxides: alkali metals > alkaline earth metals
(iii) solubility of hydroxides: alkaline earth metals > alkali metals