a. Element X has constant oxidation state. Y shows variable oxidation states.
b. X20 = 1s2 , 2s2 , 2p6 , 3s2 , 3p6 , 4s2
X26 = 1s2, 2s2 , 2p6 , 3s2 , 3p6 , 4s2 , 3d6
Y is a transitional element. In chemical reactions only two elections in ‘ s’ subshell or besides ‘s’ subshell electrons ‘d’ sub shell electrons also take part