What is isomorphism

Q. $C{r}_2{O}_3$, $F{e}_2{O}_3$ and $A{l}_2{O}_3$ are isomorphous but $NaN{O}_3$ and $KN{O}_3$ are not isomorphous.

Q. Isomorphous salts are:

Q. White vitriol is not isomorphous with :

Q. Isomorphic alternation of generations is found in