SPDF Electron Configuration of Transition Metals


This is part of preliminary HSC Chemistry course under the topic of Atomic Structure and Atomic Mass.

HSC Chemistry Syllabus

  • Model the atom's discrete energy levels, including electronic configuration and SPDF notation (ACSCH017, ACSCH018, ACSCH020, ACSCH021)
  • Investigate energy levels in atoms and ions through:
– Examining spectral evidence for the Bohr model and introducing the Schrödinger model 

      Schrödinger's Model & SPDF Notation

      Electron configuration for transition metals involves a more complex behaviour compared to the s- and p-block elements due to the involvement of the d orbitals. Transition metals are located in the d-block of the periodic table and include elements from groups 3 to 12. Their electron configurations can be somewhat unintuitive because of the energy levels of the 3d and 4s orbitals.

      General Rules for Transition Metals:

      `4s` Before `3d` in Filling

      When filling orbitals with electrons, the `4s` orbital is filled before the `3d` orbital. This is because, in terms of energy, the `4s` orbital is slightly lower than the `3d` orbital for atoms in their ground state when they are being filled. 

      For example, in the case of Scandium (Sc, atomic number 21), the electron configuration begins with filling the `4s` orbital before starting to fill the `3d` orbitals: `[Ar] 4s^2 3d^1`.

      `3d` Before `4s` in Ionisation

      When transition metals form ions, the electrons in the `4s` orbital are generally removed before those in the `3d` orbital, despite being filled earlier. This is because, once the `3d` orbitals begin to fill, the `4s` electrons are actually at a slightly higher energy level than the `3d` electrons.

      Transition metals often exhibit multiple oxidation states, which is a result of the relatively small energy difference between their `4s` and `3d` orbitals. When transitioning to positive ions, electrons are usually removed from the `4s` orbital first, despite it being filled before the `3d` orbital during the neutral atom's electron configuration process.


      Orbital diagram of iron atom and ions


      For example, iron (Fe, atomic number 26): The electron configuration is `[Ar] 4s^2 3d^6` in its neutral atomic state. However, in the Fe²⁺ ion, the configuration becomes , indicating the removal of two electrons from the `4s` orbital.

      Anomalies in Electron Configurations of Transitional Metals

      Some transition metals exhibit anomalies in their electron configurations due to the stability associated with half-filled or fully filled d sub-shells.


      • Chromium (Cr, atomic number 24) has an electron configuration of `[Ar] 4s^1 3d^5` instead of the expected `[Ar] 4s^2 3d^4`. This is because a half-filled d sub-shells (`3d^5`) offers extra stability.



      • Copper (Cu, atomic number 29): Exhibits another example of an anomaly where the electron configuration is `[Ar] 4s^1 3d^10` instead of `[Ar] 4s^2 3d^9`, to accommodate a fully filled `3d` sub-shell, which is energetically favourable.