Question

Alkaline-earth metals form dipositive ions, instead of unipositive ions, because

• A. the second ionization energy is almost double of the first ionization energy
• B. unipositive ions have stable configuration
• C. the hydration energy of dipositive ions compensates the higher value of second ionization energy
• D. unipositive ions are highly hydrated

Answer 1

Answer: (C)

Alkaline-earth metals form dispositive ions, instead of unipositive ions, because the hydration energy of dipositive ions compensates the higher value of second ionization energy.

Heat of hydration of an ion depends upon the charge/radius ratio. Compared to the heats of hydration of alkali metal ions of comparable size, the heats of hydration of alkaline-earth metal ions are approximately four times greater, e.g., $\Delta H_{hyd}N{a}^{+}$ (size 102 pm)

$=-397$ kJ/mol; $\Delta H_{hyd}C{a}^{2+}$ (size 100 pm)

$=-1650$ kJ/mol.