Why Transition Metals Show So Many Oxidation States

Iron forms green iron(II) salts and yellow-brown iron(III) salts, copper sulphate crystals are blue, and iron and nickel speed up industrial reactions without being used up. All of this comes from the partly filled d orbitals of the transition elements.

This lesson covers the electronic configuration and general trends of the 3d series — variable oxidation states, catalytic behaviour, colour and magnetism — and how to calculate the spin-only magnetic moment.

**The 3d transition elements, scandium to zinc, have the general configuration $[\mathrm{Ar}]3d^{1-10}4s^{1-2}$, and their partly filled d orbitals give them variable oxidation states, catalytic power, coloured compounds and paramagnetism.

Electronic configuration:**

- Electrons enter the 3d orbitals across the series, from scandium, $3d^{1}4s^2$, to zinc, $3d^{10}4s^2$
- Exceptions: chromium is $3d^{5}4s^1$ and copper is $3d^{10}4s^1$, because half-filled and completely filled d subshells are extra stable
- A transition element has a partly filled d subshell in the atom or a common ion, so zinc, with $3d^{10}$ in both, is not a typical transition element

Physical trends. High melting points and hardness come from metallic bonding that involves d electrons; atomic radii shrink at first across the series and then level off as added d electrons shield poorly but repel one another.

Variable oxidation states:

- The 4s and 3d energies are close, so different numbers of electrons can be lost
- Manganese shows every state from +2 to +7, reaching +7 in $\mathrm{KMn}{\mathrm{O}}_4$
- Scandium shows only +3 and zinc only +2, while ions such as $\mathrm{M}{\mathrm{n}}^{2+}$ ($3d^5$) are especially stable

Catalytic behaviour:

- Changing oxidation state lets these metals form intermediates that give an easier reaction path
- Their surfaces adsorb reactants and weaken bonds
- Examples: iron in the Haber process, vanadium(V) oxide in the contact process, nickel in hydrogenation

Colour:

- In a complex or hydrated ion, the five d orbitals split into sets of different energy
- An electron absorbs visible light to jump between these levels, and the complementary colour is seen
- $\mathrm{T}{\mathrm{i}}^{3+}$ ($3d^1$) is purple and $\mathrm{C}{\mathrm{u}}^{2+}$ ($3d^9$) is blue, while $\mathrm{S}{\mathrm{c}}^{3+}$ ($3d^0$) and $\mathrm{Z}{\mathrm{n}}^{2+}$ ($3d^{10}$) are colourless because no d-d transition is possible

Magnetic properties. Ions with unpaired d electrons are paramagnetic and are drawn into a magnetic field, more strongly the more unpaired electrons they have; ions with all electrons paired, such as $\mathrm{Z}{\mathrm{n}}^{2+}$, are diamagnetic.

An everyday example. Nickel catalysts in vanaspati factories adsorb hydrogen and oil molecules on their surface, weakening bonds so that hydrogenation happens at a practical rate.

The substance. The 4s electrons are lost before the 3d electrons when ions form — so $\mathrm{F}{\mathrm{e}}^{2+}$ is $[\mathrm{Ar}]3d^6$, not $[\mathrm{Ar}]3d^{4}4s^2$, even though 4s fills first.