General characteristic chemical properties of the first set of transition elements, titanium to copper — FAQ

No, this is a common misconception. A compound will be colourless if the central metal ion does not have a partially filled d-subshell. For example, compounds containing scandium(III) (Sc³⁺, 3d⁰ configuration), titanium(IV) (Ti⁴⁺, 3d⁰), copper(I) (Cu⁺, 3d¹⁰), and zinc(II) (Zn²⁺, 3d¹⁰) are typically white or colourless because d-d transitions are not possible.

For iron, the first two ionisation energies to remove the 4s electrons are relatively low, forming Fe²⁺. The third ionisation energy to remove a 3d electron is not prohibitively large because the 3d and 4s orbitals are close in energy, allowing stable Fe³⁺ to form. For calcium, after removing the two 4s electrons to form Ca²⁺, the third ionisation energy is extremely high as it would involve removing a core electron from the stable 3p sub-shell.

Not necessarily. While some substitutions do involve a change in coordination number (e.g., octahedral [Cu(H₂O)₆]²⁺ changing to tetrahedral [CuCl₄]²⁻), many do not. For example, when aqueous cobalt(II) ions react with excess ammonia, the six water ligands are replaced by six ammonia ligands, and the coordination number remains six: [Co(H₂O)₆]²⁺ + 6NH₃ → [Co(NH₃)₆]²⁺ + 6H₂O.

Zinc only forms the $Zn^{2+}$ ion, which has a full d-subshell ($3d^{10}$). Scandium only forms the $Sc^{3+}$ ion, which has an empty d-subshell ($3d^0$). Since neither forms stable ions with a partially filled d-subshell, they do not exhibit the characteristic properties like variable oxidation states or coloured compounds.

No, they are two different names for the same type of bond. A coordinate bond is a covalent bond where one atom provides both of the shared electrons. The term 'coordinate bond' is most commonly used in the context of transition metal complexes.

You are not expected to predict the exact colour from first principles at A-Level. However, you must learn the specific colours of common complexes, such as the blue of $[Cu(H_2O)_6]^{2+}$, the yellow-green of $[CuCl_4]^{2-}$, the pink of $[Co(H_2O)_6]^{2+}$, and the green of $Cr^{3+}(aq)$. The colour depends on the metal ion, its oxidation state, and the ligands attached.