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 $[A r] 3 d 6 [Ar] 3d^6$ , 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.