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

A transition element is a d-block element that forms at least one stable ion with a partially filled d sub-shell.

Across the first row, the 3d sub-shell fills: Ti [Ar]3d²4s², V [Ar]3d³4s², … Fe [Ar]3d⁶4s², … with two exceptions: $\text{Cr} = [\text{Ar}]3d^5 4s^1 \qquad \text{Cu} = [\text{Ar}]3d^{10}4s^1$ because a half-filled (3d⁵) or fully-filled (3d¹⁰) sub-shell is extra stable.

When a transition metal forms an ion, the 4s electrons are removed before the 3d: $\text{Fe}^{2+} = [\text{Ar}]3d^6 \qquad \text{Fe}^{3+} = [\text{Ar}]3d^5 \qquad \text{Cu}^{2+} = [\text{Ar}]3d^9$

Scandium and zinc sit in the d block but are not transition elements, because the definition requires an ion with a partially filled d sub-shell:

• Sc forms only Sc³⁺, which is [Ar]3d⁰ — an empty d sub-shell.
• Zn forms only Zn²⁺, which is [Ar]3d¹⁰ — a full d sub-shell.

Because neither has a partly filled d sub-shell in its ion, they lack the characteristic transition-metal properties: their compounds are typically colourless and they show only one oxidation state.

Physical properties. Compared with the s-block metals, transition metals have higher melting points, higher densities and are harder. This is because more electrons are delocalised into the metallic bonding (both 4s and some 3d electrons) and the ions are smaller, giving stronger metallic bonding.

Characteristic chemical properties — all arising from the partly filled 3d sub-shell:

• Variable oxidation states (4s and 3d are close in energy, so different numbers of electrons can be lost).
• Form complex ions (vacant d orbitals accept lone pairs from ligands).
• Coloured compounds (d–d transitions absorb visible light).
• Catalytic activity (variable oxidation states for homogeneous catalysis; surface adsorption for heterogeneous).

These four properties are the backbone of the whole transition-elements topic.