Electronic Structure of d-Block Elements: MDCAT Chemistry notes

Electronic Structure of d-Block Elements for MDCAT: 3d configurations, Cr and Cu anomalies, ion configurations, colour, oxidation states and complexes.

Unit: Transition Elements · Updated 25 September 2026

Electronic configurations of the 3d series

Transition elements fill the $(n-1)d$ subshell. They are the outer transition elements; the f-block elements are the inner transition elements.

Element	Configuration	Element	Configuration
Sc (21)	$[\mathrm{Ar}]3d^1 4s^2$	Fe (26)	$[\mathrm{Ar}]3d^6 4s^2$
Ti (22)	$[\mathrm{Ar}]3d^2 4s^2$	Co (27)	$[\mathrm{Ar}]3d^7 4s^2$
V (23)	$[\mathrm{Ar}]3d^3 4s^2$	Ni (28)	$[\mathrm{Ar}]3d^8 4s^2$
Cr (24)	$[\mathrm{Ar}]3d^5 4s^1$	Cu (29)	$[\mathrm{Ar}]3d^{10} 4s^1$
Mn (25)	$[\mathrm{Ar}]3d^5 4s^2$	Zn (30)	$[\mathrm{Ar}]3d^{10} 4s^2$

Cr and Cu are anomalous because half-filled ($d^5$) and completely filled ($d^{10}$) subshells are extra stable. One 4s electron moves into 3d. So Cr has five unpaired d electrons, while Zn has all its d orbitals fully paired.

Configurations of ions

When a transition metal forms an ion, it loses its 4s electrons first, and only then 3d electrons.

$\mathrm{Mn^{2+}}$: $3d^5$, with 5 unpaired electrons (the same as $\mathrm{Fe^{3+}}$)
$\mathrm{Fe^{2+}}$: $3d^6$, with 4 unpaired electrons
$\mathrm{Co^{2+}}$: $3d^7$, with 3 unpaired electrons
$\mathrm{Cr^{3+}}$: $3d^3$, with 3 unpaired electrons
$\mathrm{Cu^{+}}$: $3d^{10}$, so its d subshell is full
$\mathrm{Cu^{2+}}$: $3d^9$, so its d subshell is partly filled
$\mathrm{Zn^{2+}}$: $3d^{10}$, so its d subshell is full

Paramagnetism depends on the number of unpaired electrons, so it is greatest for $d^5$ ions.

Characteristic properties

Variable oxidation states, because 4s and 3d electrons are close in energy. Mn reaches the maximum, +7 (as in $\mathrm{KMnO_4}$). Zn shows only +2 because its d subshell is full; Sc normally shows only +3.
Colour comes from d–d transitions in a partly filled d subshell that has been split by ligands. $\mathrm{Cu^{2+}}$ is blue. $\mathrm{Zn^{2+}}$ ($d^{10}$) and $\mathrm{Sc^{3+}}$ ($d^0$) compounds are colourless or white.
High melting points, hardness and good conductivity, due to strong metallic bonding involving the unpaired d electrons. Binding energy rises to a maximum around group VIB (Cr, Mo, W) and then falls.
Interstitial compounds form when small atoms such as H, B, C and N fit into the holes in the metal lattice.
Alloys include steel (Fe with C), brass (Cu–Zn) and bronze (Cu–Sn). Graphite is an element, not an alloy.
Coinage metals are Cu, Ag and Au, in group IB.

Complexes

A ligand donates a lone pair to the metal ion, forming a coordinate covalent bond. Ligands are classified by the number of donor atoms: monodentate has one (e.g. $\mathrm{NH_3}$, $\mathrm{CN^-}$), bidentate has two (e.g. ethylenediamine, oxalate) and polydentate has many.

$\mathrm{K_4[Fe(CN)_6]}$ (potassium ferrocyanide) is a complex salt. It has 6 ligands, coordination number 6, and $\mathrm{[Fe(CN)_6]^{4-}}$ is a complex anion.
Coordination number 6 gives an octahedral shape, e.g. $\mathrm{[Co(NH_3)_6]^{3+}}$.
Coordination number 4 gives a tetrahedral shape (e.g. the copper(I) complex $\mathrm{[Cu(CN)_4]^{3-}}$) or a square planar shape (e.g. $\mathrm{[Pt(NH_3)_4]^{2+}}$, $\mathrm{[AuCl_4]^-}$). $\mathrm{[Fe(CO)_5]}$ is trigonal bipyramidal.

Common MDCAT traps

Cr is $3d^5 4s^1$, not $3d^4 4s^2$.
Remove 4s electrons before 3d electrons when forming ions, so $\mathrm{Mn^{2+}}$ is $3d^5$.
Cu⁺ is $d^{10}$ (full). Cu²⁺ is the ion with a partly filled d subshell.
A double salt dissociates completely into simple ions in water. A complex salt like $\mathrm{K_4[Fe(CN)_6]}$ does not release free $\mathrm{Fe^{2+}}$.

Quick revision

Cr and Cu configurations are stabilized by $d^5$ and $d^{10}$.
Mn shows the highest oxidation state (+7) in the 3d series.
Zn has a fixed +2 oxidation state and colourless compounds.
$\mathrm{Co^{2+}}$ has 3 unpaired electrons.
Colour arises from d–d transitions.