text: In chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings:\n\nIt commonly refers to any element in the d-block of the periodic table, including zinc, cadmium and mercury. This corresponds to groups 3 to 12 on the periodic table. \nMore strictly, IUPAC defines the transition metals as any element with an incomplete d subshell or that may form stable ions only with an incomplete d subshell (IUPAC definition: "An element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell."). By this definition, zinc, cadmium, and mercury are excluded from the transition metals, as they have a d10 configuration. Only a few transient species of these elements that leave ions with a partly filled d subshell have been formed, and mercury(I) only occurs as Hg22+, which does not strictly form a lone ion with a partly filled subshell, and hence these three elements are inconsistent with the latter definition.[1] They do form ions with a 2+ oxidation state, but these retain the 4d10 configuration. Element 112 may also be excluded although its oxidation properties are unlikely to be observed due to its radioactive nature. This definition includes groups 3 to 11 on the periodic table. \nThe first definition is simple and has been traditionally used. However, many interesting properties of the transition elements as a group are the result of their partly filled subshells from d orbitals, a property which all members of the d-block except zinc, cadmium and mercury share\nTransition elements tend to have high tensile strength, density and melting and boiling points. As with many properties of transition metals, this is due to d orbital electrons' ability to delocalise within the metal lattice. In metallic substances, the more electrons shared between nuclei, the stronger the metal.\nTransition metals form good homogeneous or heterogeneous catalysts, for example iron is the catalyst for the Haber process. Nickel or platinum is used in the hydrogenation of alkenes.