Variable Oxidation States in Transition Elements (HL) (HL IB Chemistry)

Electron Configuration

• The full electronic configuration of the first-row transition metals is shown in the table below
• Following the Aufbau Principle electrons occupy the lowest energy subshells first
  • The 4s overlaps with the 3d subshell so the 4s is filled first
• Remember: You can abbreviate the first five subshells, 1s-3p, to [Ar] representing the configuration of argon (known as the argon core)

Table showing the electronic configuration of the first d-series transition elements

• There are two exceptions to the Aufbau Principle in the first row of the d-block:
  • Chromium
  • Copper
• In both cases, an electron is promoted from 4s to 3d to achieve a half-full and full d-subshell, respectively
• Chromium and copper have the following electron configurations, which are different to what you may expect:
  • Cr is [Ar] 3d⁵ 4s¹ not [Ar] 3d⁴ 4s²
  • Cu is [Ar] 3d¹⁰ 4s¹ not [Ar] 3d⁹ 4s²
• This is because the [Ar] 3d⁵ 4s¹ and [Ar] 3d¹⁰ 4s¹ configurations are energetically more stable and are preferred configurations
• When forming cations, remove the 4s electrons first

• When transition elements forms ions they lose electrons from the 4s subshell first
• This is because when the orbitals are occupied, the repulsion between electrons pushes the 4s into a higher energy state so that it now becomes slightly higher in energy than the 3d subshell
  • The 4s is now the outer shell and loses electrons first
• The loss of the 4s electrons means that +2 is a common oxidation state in transition metals
• The reason why the transition metals have variable oxidation states all comes down to energy

Table showing the common oxidation states of transition elements

Explaining variable oxidation states using successive ionisation energies

• Using titanium and vanadium as examples, the graph below shows that the first few ionisation energies are relatively small and relatively close together
  • This means that the energy difference associated with removing a small number of electrons enables transition metals to vary their oxidation state with ease

Graph of titanium and vanadium ionisation energies

Ionisation energies increase for the removal of successive electrons in titanium and vanadium

• The +2 and +3 oxidation states are shown by all the transition elements
  • The +3 state is more stable in the elements up to chromium
  • The +2 state is more stable in the later elements
• Transition metal ions with oxidation state +3 and above tend to be polarising and have a degree of covalent character in the bonds they form
  • The ions have a high charge density and pull electrons towards themselves
• The maximum oxidation state possible corresponds to the total number of electrons in the 4s and 3d which reaches a maximum at manganese
  • An example you may be familiar with is the manganate(VII) ion, MnO₄^– which is a powerful oxidising agent