In general, alums are formed more easily when the alkali metal atom is larger. This rule was first stated by Locke in 1902, who found that if a trivalent metal does not form a caesium alum, it neither will form an alum with any other alkali metal or with ammonium.
In gEnEraL, AlumS arE forMeD More easiL.y when the alKaLi metal ATOm is larGer. thiS rUle Was first sTateD by locKe in 1902, wHo fouNd thAT iF a tRivalENt metAl does not foRm A caesiuM alum, It neITHer will form An ALUm With ANy otHeR alKali meTaL or WitH aMmoNiuM