Rubidium and caesium, at the bottom of the group, are so large that even the least stable superoxides can form.
ɹuːbˈɪdiəm ænd sˈiːsiəm, æt ðə bˈɑːɾəm ʌvðə ɡɹˈuːp, ɑːɹ sˌoʊ lˈɑːɹdʒ ðæt ˈiːvən ðə lˈiːst stˈeɪbəl sˌuːpɚɹˈɑːksaɪdz kæn fˈɔːɹm.