Variable Oxidation States in Transition Elements (HL) (DP IB Chemistry): Revision Note

Richard Boole

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Variable Oxidation States in Transition Elements

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)

Electronic configuration of the first d-series transition elements

  • Ti

    • 1s2 2s2 2p6 3s2 3p6 3d2 4s2

    • [Ar] 3d2 4s2

  • V

    • 1s2 2s2 2p6 3s2 3p6 3d3 4s2

    • [Ar] 3d3 4s2

  • Cr

    • 1s2 2s2 2p6 3s2 3p6 3d5 4s1

    • [Ar] 3d5 4s1

  • Mn

    • 1s2 2s2 2p6 3s2 3p6 3d5 4s2

    • [Ar] 3d5 4s2

  • Fe

    • 1s2 2s2 2p6 3s2 3p6 3d6 4s2

    • [Ar] 3d6 4s2

  • Co

    • 1s2 2s2 2p6 3s2 3p6 3d7 4s2

    • [Ar] 3d7 4s2

  • Ni

    • 1s2 2s2 2p6 3s2 3p6 3d8 4s2

    • [Ar] 3d8 4s2

  • Cu

    • 1s2 2s2 2p6 3s2 3p6 3d10 4s1

    • [Ar] 3d10 4s1

Exceptions

  • Two first-row d-block elements are exceptions to the Aufbau Principle:

    • Chromium and copper

  • In both cases, one 4s electron is promoted to the 3d subshell to create a more stable configuration:

    • Cr: [Ar] 3d⁵ 4s¹ (not [Ar] 3d⁴ 4s²)

    • Cu: [Ar] 3d¹⁰ 4s¹ (not [Ar] 3d⁹ 4s²)

  • These configurations are preferred because a half-filled (d⁵) or fully filled (d¹⁰) d-subshell is more stable

  • When transition metals form ions, they lose electrons from the 4s subshell first

  • This is because, once filled, the 4s orbital is pushed to a higher energy level than the 3d due to electron repulsion

  • The 4s becomes the outermost shell, so electrons are removed from it first

  • As a result, +2 is a common oxidation state, due to the loss of two 4s electrons

  • The ability of transition metals to form variable oxidation states is due to the similar energies of the 4s and 3d orbitals

Worked Example

Deducing the electronic configuration of transition element ions

State the full electronic configuration of:

  1. Cu

  2. Mn(III) ions 

  3. V4+

Answer 1 - Cu:

  • Cu atomic number = 29

  • 1s22s22p63s23p63d104s1 OR 1s22s22p63s23p64s13d10

  • Remember: Copper atoms prefer a complete d subshell 

Answer 2 - Mn(III):

  • Step 1: Write out the electron configuration of the atom first:

    • Mn atomic number = 25

    • 1s22s22p63s23p64s23d5

  • Step 2: Subtract the appropriate number of electrons starting from the 4s subshell

    • Mn(III) = 22 electrons

    • 1s22s22p63s23p63d4

Answer 3 - V4+:

  • Step 1: Write out the electron configuration of the atom first:

    • V atomic number = 23

    • 1s2 2s2 2p6 3s2 3p6 3d3 4s2

  • Step 2: Subtract the appropriate number of electrons starting from the 4s subshell

    • V4+ = 19 electrons

    • 1s22s22p63s23p63d1

The common oxidation states of transition elements

  • Ti: +3, +4

  • V: +3, +5

  • Cr: +3, +6

  • Mn: +2, +4, +7

  • Fe: +2, +3

  • Co: +2, +3

  • Ni: +2

  • Cu: +1, +2

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

Graph showing ionisation energy in kJ/mol for titanium and vanadium as successive electrons are removed. Titanium in blue, vanadium in orange.
Ionisation energies increase for the removal of successive electrons in titanium and vanadium
  • Ionisation energies increase with the number of electrons removed

  • The +2 and +3 oxidation states are common across all transition elements:

    • +3 is more stable in early transition metals (up to chromium)

    • +2 becomes more stable in later elements

  • Transition metal ions with oxidation states of +3 and higher are often polarising

    • They exhibit some covalent character in bonding

      Due to their high charge density, they attract bonding electrons strongly

  • The maximum oxidation state for a transition metal corresponds to the total number of electrons in the 4s and 3d orbitals

    • This maximum is reached at manganese (Mn), which can achieve +7

      • Example: the manganate(VII) ion, MnO4⁻, a strong oxidising agent

Examiner Tips and Tricks

  • You may sometimes see electronic configurations with:

    • 3d electrons written before 4s

    • 4s electrons written before 3d

  • Both ways are acceptable although putting the 3d electrons first is more conventional, even though 4s fills before 3d

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Richard Boole

Author: Richard Boole

Expertise: Chemistry Content Creator

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

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