General physical and chemical properties of the transition elements, titanium to copper

Chemistry A2 · Chemistry of transition elements. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A transition element is a d-block element that forms one or more stable ions with incomplete d orbitals. Scandium and zinc are in the d-block but do not fit this definition.

The 3d and 4s sub-shells are very close in energy. Different numbers of these electrons can be lost or shared for a similar energy cost, so the elements show variable oxidation states. When ions form, the 4s electrons are lost before the 3d electrons.

The other typical properties follow from the d orbitals. The elements act as catalysts, form complex ions with ligands and form coloured compounds.

Rules to remember

Transition element: a d-block element that forms at least one stable ion with incomplete d orbitals.
Zn²⁺ is [Ar] 3d¹⁰ (full) and Sc³⁺ is [Ar] 3d⁰ (empty), so zinc and scandium are not transition elements.
Atoms: Cr is [Ar] 3d⁵ 4s¹ and Cu is [Ar] 3d¹⁰ 4s¹. Ions lose 4s electrons first, e.g. Fe²⁺ is [Ar] 3d⁶.
3d_xy orbital: four lobes in the xy plane, pointing between the x and y axes.
3d_z² orbital: two lobes along the z-axis with a ring of electron density around the middle in the xy plane.
Catalysts because they have more than one stable oxidation state and vacant d orbitals that can accept lone pairs (dative bonds).
Complex ions form because vacant d orbitals of suitable energy accept lone pairs from ligands.

Common mistake

Students remove 3d electrons first when writing the configuration of an ion. The 4s electrons are always lost first: Fe is [Ar] 3d⁶ 4s², so Fe³⁺ is [Ar] 3d⁵, not [Ar] 3d³ 4s².

Worked example

Scandium forms only the ion Sc³⁺. Titanium forms Ti³⁺. Decide whether each is a transition element. (Sc is [Ar] 3d¹ 4s²; Ti is [Ar] 3d² 4s².)

Sc³⁺: remove the two 4s electrons, then the one 3d electron, giving [Ar] 3d⁰. The d orbitals are empty, not incomplete.
Ti³⁺: remove the two 4s electrons, then one 3d electron, giving [Ar] 3d¹.
3d¹ is an incomplete d sub-shell in a stable ion, so titanium fits the definition.

Answer: Titanium is a transition element; scandium is a d-block element but not a transition element.

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