Assertain (A): K, Rb and Cs form superoxides.
Reason (R) : The stability of the superoxides increases from 'K' to 'Cs' due to decrease in lattice energy.
The correct answer is

Assertion: K, Rb and Cs form superoxides. Reason: The stability of the superoxides increases from 'K' to 'Cs' due to decrease in lattice energy.

K, Rb and Cs form mainly superoxides.

Assertion : K, Rb and Cs can also form superoxides. Reason : Their ionic radii increase in the order Cs^(+) lt Rb^(+) lt K^(+) .

The following are some statments about oxides of alkali metals i) The basis nature and solubility of these oxides increases from Li to Cs ii) The stability of superoxide of IA group elements increases down the group due to increase in lattice energy iii) KO_2 is organe red coloured and paramagnetic

Alkali metals on combustion in excess of air form different type of oxides depending on the size of metal cations e.g . lithium forms mainly the oxide Li_2O , sodium forms peroxide and some super oxides NaO_2 and potassium, Rubidium and caesium form the superoxides, MO_2 . The stability of peroxide and superoxide increases, as the size of the metal cation increases because of the stabilisation of larger anion by larger cations through lattice energy effect. These oxides are easily hydrolysed by water. The oxides and the peroxides are colourless when pure, but the superoxides are yellow or orange in colour. Peroxide and superoxide of sodium and potassium are widely used as oxidising agent and as air purifier in space capsules, submarines and breathing mask. When sodium reacts with excess of oxygen at 350^@ C , the oxidation number of oxygen per atom changes from: