An element, X, forms an ion with 10 electrons and forms the following types of compounds: with group I metals or alkali metals: M 2 X and with group III metals M 2 X 3 . Element X is most likely: A. Cl B.N C. O D. S The answer is oxygen but i dont know why. Please explain why and why not the other elements. Solution Given: element forms ion with 10 electrons, verifying it for all Cl - =18 electrons N 3- =10 electrons O 2- =10 electrons S 2- =18 electrons By this, oxygen and nitrogen can be the possibilities Given: element forms M 2 X ( M=+1 and X=-2) with alkali metals and M 2 X 3 (M=+3 and X=-2) with group 3 ,from this ion produced has -2 charge which is only oxygen. Hence, proved. .