Rubidium can form Rb6O and Rb9O2 (copper-coloured) upon oxidation in air, while caesium forms an immense variety of oxides, such as the ozonide CsO3 and several brightly coloured suboxides, such as Cs7O (bronze), Cs4O (red-violet), Cs11O3 (violet), Cs3O (dark green), CsO, Cs3O2, as well as Cs7O2.
ɹuːbˈɪdiəm kæn fˈɔːɹm ˌɑːɹbˈiː sˈɪks ˈoʊ ænd ˌɑːɹbˈiː nˈaɪn ˈoʊ tˈuː kˈɑːpɚkˈʌlɚd əpˌɑːn ˌɑːksɪdˈeɪʃən ɪn ˈɛɹ, wˌaɪl sˈiːsiəm fˈɔːɹmz ɐn ɪmˈɛns vɚɹˈaɪəɾi ʌv ˈɑːksaɪdz, sˈʌtʃ ɐz ðɪ ˈoʊzənˌaɪd sˌiːˈɛs ˈoʊ θɹˈiː ænd sˈɛvɹəl bɹˈaɪtli kˈʌlɚd sʌbˈɑːksaɪdz, sˈʌtʃ ɐz sˌiːˈɛs sˈɛvən ˈoʊ bɹˈɑːnz, sˌiːˈɛs fˈoːɹ ˈoʊ ɹˈɛdvˈaɪəlɪt, sˌiːˈɛs ɪlˈɛvən ˈoʊ θɹˈiː vˈaɪəlɪt, sˌiːˈɛs θɹˈiː ˈoʊ dˈɑːɹk ɡɹˈiːn, sˌiːˈɛs ˈoʊ, sˌiːˈɛs θɹˈiː ˈoʊ tˈuː, æz wˈɛl æz sˌiːˈɛs sˈɛvən ˈoʊ tˈuː.