The chemical properties of elements is determined by the electron shell configurations of each element. Different electron shell configuration = different chemical properties. And the number of electrons that an element has is determined by the number of protons the element has in it's nucleus. Iron has 26 protons, so a neutral atom of iron will have 26 electrons, while nickle has 28 protons, therefore when neutral will have 28 electrons. And since they have differing numbers of electrons, their electron shells differ and therefore their chemical properties differ.