Transition metals, configurations and oxidation statesEdexcel International A Level Chemistry: Revision notes

The d-block is the block of the periodic table in which the highest-energy electrons are in a d sub-shell. In Period 4 it runs from scandium (Sc) to zinc (Zn).

A transition metal is a d-block element that forms one or more stable ions with incompletely-filled d orbitals.

This definition is about the ions, not the atoms. It excludes two d-block elements:

• Scandium forms only Sc³⁺, which is [Ar] with no d electrons.
• Zinc forms only Zn²⁺, which is [Ar] 3d¹⁰, a full d sub-shell.

Copper is a transition metal because it forms Cu²⁺ ([Ar] 3d⁹), even though Cu⁺ is 3d¹⁰.

The 4s sub-shell is filled before 3d, so a scandium atom is [Ar] 3d¹ 4s². Here [Ar] means 1s² 2s² 2p⁶ 3s² 3p⁶.

• Sc [Ar] 3d¹ 4s²
• Ti [Ar] 3d² 4s²
• V [Ar] 3d³ 4s²
• Cr [Ar] 3d⁵ 4s¹
• Mn [Ar] 3d⁵ 4s²
• Fe [Ar] 3d⁶ 4s²
• Co [Ar] 3d⁷ 4s²
• Ni [Ar] 3d⁸ 4s²
• Cu [Ar] 3d¹⁰ 4s¹
• Zn [Ar] 3d¹⁰ 4s²

Chromium and copper are the two exceptions. One 4s electron is promoted so that the 3d sub-shell is half-full (Cr) or full (Cu), which is a lower-energy arrangement.

You must be able to write the full configuration, starting 1s² 2s² 2p⁶ 3s² 3p⁶, from the atomic number.

When a Period 4 d-block atom forms a positive ion, the 4s electrons are lost first, then the 3d electrons.

• Fe → Fe²⁺: [Ar] 3d⁶ 4s² → [Ar] 3d⁶
• Fe²⁺ → Fe³⁺: [Ar] 3d⁶ → [Ar] 3d⁵
• Cu → Cu⁺ [Ar] 3d¹⁰ and Cu²⁺ [Ar] 3d⁹
• Cr³⁺ is [Ar] 3d³ and Mn²⁺ is [Ar] 3d⁵
• Sc³⁺ is [Ar] and Zn²⁺ is [Ar] 3d¹⁰

Once the atom has become an ion, the 3d sub-shell is lower in energy than 4s, which is why the 4s electrons leave first even though 4s filled first.

The 4s and 3d sub-shells are close in energy, so electrons from both can be lost or used in bonding. The successive ionisation energies rise gradually rather than jumping, and the energy needed is paid back by the extra bonding or lattice enthalpy.

So a transition metal forms ions of more than one oxidation number, e.g. Fe²⁺ and Fe³⁺, Cu⁺ and Cu²⁺, and vanadium +2 to +5.

Contrast calcium, [Ar] 4s². After the two 4s electrons the next would come from the 3p sub-shell, much closer to the nucleus, so the third ionisation energy is huge and calcium only forms Ca²⁺.

The highest oxidation number of the early elements equals the total number of 4s and 3d electrons: manganese reaches +7 in MnO₄⁻.

Use the rule that the oxidation numbers in a compound add up to its overall charge, with oxygen usually −2.

Worked example. Find the oxidation number of manganese in MnO₄²⁻.

x + 4(−2) = −2, so x = +6.

Other common examples: Cr in Cr₂O₇²⁻ is +6; V in VO₂⁺ is +5; Fe in Fe₂O₃ is +3; Cu in Cu₂O is +1.

When the oxidation number rises the metal has been oxidised; when it falls it has been reduced.