Variable Oxidation States in Transition Elements (HL) (DP IB Chemistry): Revision Note
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:
Cu
Mn(III) ions
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

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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