Titanium to copper sit in the middle of period 4, filling the 3d subshell. A partly filled d subshell in a stable ion is what makes an element a transition element, and it explains their family likeness: variable oxidation states, coloured compounds, complexes and catalysis.
A transition element is a d-block element that forms one or more stable ions with an incomplete d subshell. Scandium (only Sc3+, 3d0) and zinc (only Zn2+, 3d10) are d-block elements but not transition elements. Across Ti to Cu, electrons enter the inner 3d subshell, so the extra nuclear charge is largely shielded: atomic radii and first ionisation energies stay very similar. When ions form, the 4s electrons are lost first. Chromium (3d54s1) and copper (3d104s1) are exceptions to the filling order. Transition elements show variable oxidation states, act as catalysts, form coloured compounds and form complex ions.
Write the configuration of the atom first, then remove the 4s electrons before any 3d electrons when the ion forms.
| Element | Atom | Common ion | Ion configuration | Transition element? |
|---|---|---|---|---|
| Sc | [Ar]3d14s2 | Sc3+ | [Ar] | no: d subshell empty |
| Ti | [Ar]3d24s2 | Ti3+ | [Ar]3d1 | yes |
| Cr | [Ar]3d54s1 | Cr3+ | [Ar]3d3 | yes |
| Mn | [Ar]3d54s2 | Mn2+ | [Ar]3d5 | yes |
| Fe | [Ar]3d64s2 | Fe2+, Fe3+ | [Ar]3d6, [Ar]3d5 | yes |
| Cu | [Ar]3d104s1 | Cu2+ | [Ar]3d9 | yes (Cu2+ has an incomplete d subshell) |
| Zn | [Ar]3d104s2 | Zn2+ | [Ar]3d10 | no: d subshell full |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which statement defines a transition element?
What is the electronic configuration of Fe3+?
What is the electronic configuration of a chromium atom?
Why is zinc not classed as a transition element?
Compare the transition metals with calcium, the s-block metal just before them, and notice how little their radii and ionisation energies change from Ti to Cu.
| Property | Ca (s-block) | Ti | Fe | Cu |
|---|---|---|---|---|
| atomic (metallic) radius / nm | 0.197 | 0.147 | 0.126 | 0.128 |
| first ionisation energy / kJ mol−1 | 590 | 658 | 759 | 745 |
| density / g cm−3 | 1.55 | 4.51 | 7.87 | 8.96 |
| melting point / °C | 842 | 1668 | 1538 | 1085 |
When forming ions, remove the 4s electrons first: Fe2+ is [Ar]3d6, not [Ar]3d44s2. Chromium is 3d54s1 and copper 3d104s1. Zinc is in the d block but is not a transition element, because Zn2+ has a full 3d subshell; scandium is excluded because Sc3+ has no d electrons.
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Why do the atomic radii of the elements Ti to Cu change very little across the series?
How many unpaired electrons are there in a Mn2+ ion?
Which ion has the configuration [Ar]3d9?
Which property is NOT characteristic of transition elements compared with s-block metals?
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