All subjects · Chemistry · WCH15 (Unit 5)

Transition metals: electron configurations and oxidation states

Transition metals share a set of properties: variable oxidation states, coloured ions, complex ions and catalytic activity. All of them come from their partly filled d sub-shells.

Definition (spec 17.1)

A transition metal is a d-block element that forms one or more stable ions with incompletely filled d orbitals. In Period 4, titanium to copper are transition metals. Scandium (only Sc3+, 3d0) and zinc (only Zn2+, 3d10) are in the d block but are not transition metals.

Configurations (spec 17.2)

Atoms fill 4s before 3d: Fe [Ar] 3d6 4s2; exceptions Cr [Ar] 3d5 4s1 and Cu [Ar] 3d10 4s1.

When forming ions, 4s electrons are lost first, then 3d:

Variable oxidation states (spec 17.3)

The 3d and 4s sub-shells are close in energy, so different numbers of electrons can be lost (or used in bonding) for similar energy costs. Typical states: V +2 to +5; Cr +3, +6; Mn +2, +4, +7; Fe +2, +3; Cu +1, +2. High oxidation states usually occur in oxo-ions, such as MnO4- (+7) and Cr2O72- (+6).

Worked example

Write the electron configuration of V3+ and explain why it is a transition metal ion.

  1. V is [Ar] 3d3 4s2 (23 electrons).
  2. Lose the two 4s electrons, then one 3d electron.

Answer: V3+ is [Ar] 3d2, an incompletely filled d sub-shell

Key idea: Transition metal: forms a stable ion with a partly filled d sub-shell (so not Sc or Zn). Lose 4s before 3d. Variable oxidation states because 3d and 4s energies are similar.

Check yourself

  1. What is the electron configuration of Fe3+?
    • [Ar] 3d5
    • [Ar] 3d3 4s2
    • [Ar] 3d6
    • [Ar] 3d4 4s1
    Show answer

    [Ar] 3d5. Fe is [Ar] 3d6 4s2; remove 2 electrons from 4s and 1 from 3d.

  2. Why is zinc not classed as a transition metal?
    • It is not in the d block
    • It has no electrons in a d sub-shell
    • Its only ion, Zn2+, has a full 3d sub-shell
    • It forms no ions
    Show answer

    Its only ion, Zn2+, has a full 3d sub-shell. Zn2+ is [Ar] 3d10.

  3. Why is scandium not classed as a transition metal?
    • Its only ion, Sc3+, has no d electrons
    • Scandium has a full d sub-shell
    • Scandium does not form ions
    • Scandium is in the s block
    Show answer

    Its only ion, Sc3+, has no d electrons. Sc3+ is [Ar] 3d0, so its d sub-shell is empty, not partly filled.

  4. What is the electron configuration of Cu+?
    • [Ar] 3d9 4s1
    • [Ar] 3d8 4s2
    • [Ar] 3d9
    • [Ar] 3d10
    Show answer

    [Ar] 3d10. Cu is [Ar] 3d10 4s1; removing the 4s electron gives 3d10.

  5. What is the oxidation number of manganese in MnO4-?
    Show answer

    +7. x + 4(-2) = -1, so x = +7.

  6. What is the oxidation number of chromium in Cr2O72-?
    Show answer

    +6. 2x + 7(-2) = -2, so 2x = 12 and x = +6.

  7. Why do transition metals show variable oxidation states?
    • They have full d sub-shells
    • The 3d and 4s sub-shells are close in energy
    • They are in the s block
    • They have very high ionisation energies
    Show answer

    The 3d and 4s sub-shells are close in energy. Similar energies mean different numbers of electrons can be removed.

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Last updated 7 November 2026. Spotted a mistake? Tell us.