The number of alkali metal(s), from Li, K, Cs, Rb having ionization...

The number of alkali metal(s), from

Li, K, Cs, Rb

having ionization enthalpy greater than

400kJ{mol}^{- 1}

and forming stable super oxide is

Correct answer

Step-by-step solution

The super oxide ${O}_{2}^{-}$ ion is stable only in the presence of large cations $K, Rb, Cs$ . $K, Rb$ have ${IE}_{1} > 400kJ / mol$ from the table below. Ionisation enthalpy decreases down the group as metallic character increases.

Metal	$Li$	$Na$	$K$	$Rb$	$Cs$
$IE ( KJ / mol )$	$520$	$496$	$419$	$403$	$376$

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