Writing the electron configuration for a transition metal

The transition metals are d-block elements (Groups 3-12 of the Periodic Table) whose atoms have a partially filled d-orbital or that form cations with partially filled d-orbitals. This means that when we are considering the transition metals of Period 4, scandium and zinc are not classified as transition metals although they are d-block elements.

When writing the electron configuration of a d-block element we need to remember that the 4s orbital is of lower energy than the 3d orbital so it is usually filled first.

element	electron configuration
Sc	1s2 2s2 2p6 3s2 3p6 3d1 4s2	[Ar] 3d1 4s2
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
Zn	1s2 2s2 2p6 3s2 3p6 3d10 4s2	[Ar] 3d10 4s2

Note that chromium and copper are exceptions to this rule …

The energy needed to promote a 4s electron is repaid in terms of increased stability for the atom because there is less repulsion between parallel spin electrons, each occupying its own 3d sub-orbital, than there would be between two spin-paired electrons in the 4s orbital.

A similar argument can be made for copper.

When d-block elements form cations they lose the 4s electrons before the 3d electrons.

e.g. Cu²⁺ 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁹

Fe³⁺ 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵

Why does this happen?

• the 4s orbital is more affected by the charge on the metal ion and by the presence of anions or ligands close by because of the diffuse spherical shape of the orbital so ….
• in ions or compounds the order of the 4s and 3d orbitals is reversed with the 4s orbital now becoming higher in energy than the 3d orbital so the 4s electrons are always lost first.

Practice questions

1. A student stated that scandium and zinc are d-block elements and so they must also be transition metals. Discuss this statement using appropriate electron configurations to support your answer.
2. Which of these ions has the electron configuration [Ar] 3d⁶ ?

A. Cr³⁺ B. Fe²⁺ C. Mn³⁺ D. Co³⁺ E. Mn²⁺

3. Deduce whether the following ions have unpaired d-orbital electrons:

(a) Cu⁺ (b) V³⁺ (c) Mn²⁺

Answers

1. Scandium and zinc are both d-block elements because in each case the last added electron / highest energy electron in the atom goes into a d-orbital.

Sc 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹ 4s² and Zn 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s²

However they are not transition metals because neither forms an ion with an incomplete d-orbital. Scandium forms a Sc³⁺ ion with the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ (d-orbitals are empty) and zinc forms Zn²⁺ ion with the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ (d-orbitals are full).

2. B and D

3. V³⁺ and Mn²⁺