Chemical periodicity of other elements — common mistakes

Examiners frequently test the definition of a transition element. You must be precise: 'a d-block element that forms one or more stable ions with a partially filled d sub-shell'. Also, be ready to explain why Sc and Zn do not fit this definition. When asked for examples of catalysts, always state the specific element or compound (e.g., 'Iron' or '$V_2O_5$') and the process it is used in (Haber or Contact process).

Although zinc is in the d-block, it does not meet the definition of a transition element. Its only stable ion is $Zn^{2+}$, which has an electronic configuration of $[Ar] 3d^{10}$. Since this ion has a completely full d sub-shell, and not a partially filled one, zinc is not classified as a transition element.

A transition metal compound will be white or colourless if the central metal ion does not have a partially filled d sub-shell. For example, in Scandium(III) oxide ($Sc_2O_3$), the ion is $Sc^{3+}$ ($[Ar] 3d^0$), and in Zinc sulfate ($ZnSO_4$), the ion is $Zn^{2+}$ ($[Ar] 3d^{10}$). In both cases, a d-d electron transition cannot occur, so they do not absorb visible light.

They provide an alternative reaction pathway with a lower activation energy. For heterogeneous catalysts like Iron in the Haber process, they provide a reactive surface for reactant molecules to adsorb and react. For homogeneous catalysts, their ability to switch between different oxidation states allows them to accept and donate electrons at different stages of a reaction cycle, effectively helping to shuttle electrons between reactants.

Transition metals have high melting and boiling points due to the strong metallic bonding. This involves the delocalisation of electrons from both the 4s and 3d orbitals into a 'sea' of electrons. The involvement of d-electrons results in more delocalised electrons and a stronger attraction between the positive metal ions and the electron sea, requiring more energy to overcome.