Isomorphism (crystallography)

In crystallography, crystals are isomorphous if they have identical symmetry and if the atomic positions can be described with a set of parameters (unit cell dimensions and fractional coordinates) whose numerical values differ only slightly.

Double sulfates, such as Tutton's salt, with the generic formula MI2MII(SO4)2.6H2O, where MI is an alkali metal and MII is a divalent ion of Mg, Mn, Fe, Co, Ni, Cu or Zn, form a series of isomorphous compounds which were important in the nineteenth century in establishing the correct atomic weights of the transition elements.

Alums, such as KAl(SO4)2.12H2O, are another series of isomorphous compounds, though there are three series of alums with similar external structures, but slightly different internal structures.

These requirements ensure that the forces within and between molecules and ions are approximately similar and result in crystals that have the same internal structure.

[5] According to Ferenc Szabadváry, one of the clues that helped Berzelius determine the atomic weights of the elements was "the discovery of Mitscherlich that compounds which contain the same number of atoms and have similar structures, exhibit similar crystal forms (isomorphism).