Transition elements are found in the d-block

Most of the elements found in the middle section of the periodic table are classified as transition elements or transition metals. This section is known as the d-block. The highest energy electrons in transition elements are found in the d sub-shell.

The main transition elements we will focus on are the metals in the first row of the d-block, from titanium (Ti) to copper (Cu).

Transition elements have a partially filled d sub-shell

Transition elements are defined as d-block elements that form one or more stable ions with partially filled d orbitals. This means transition elements have electron configurations with between 1 and 9 electrons in their d sub-shell.

Scandium and zinc are found in the d-block but do not meet the definition of a transition element:

• Scandium only forms Sc^3+^ ions with an electron configuration of [Ar], meaning an empty d sub-shell.
• Zinc only forms Zn^2+^ ions with an electron configuration of [Ar]3d^10^, meaning a full d sub-shell.

Electron configurations of transition elements

The electron configurations of the period 4 transition metal atoms are:

Chromium has the configuration [Ar]3d⁵4s¹ rather than [Ar]3d⁴4s² because a half-filled d sub-shell (d⁵) provides greater stability.

Similarly, copper has the configuration [Ar]3d¹⁰4s¹ rather than [Ar]3d⁹4s² because a full d sub-shell (d¹⁰) provides greater stability.

s electrons are removed first when ions are formed

When transition metal atoms form positive ions, the 4s electrons are removed first, followed by the 3d electrons.

For example, iron can form both Fe²⁺ and Fe³⁺ ions:

• Fe = [Ar]3d⁶4s²
• Fe²⁺ = [Ar]3d⁶
• Fe³⁺ = [Ar]3d⁵

Useful chemical properties

Transition elements have several useful chemical properties, including:

• Existing in variable oxidation states (e.g. manganese exhibits oxidation states from +7 to +2).
• Acting as catalysts (e.g. iron is used as a catalyst in the Haber process).
• Forming coloured ions (e.g. chromium forms orange Cr₂O₇^2- ions and green Cr³⁺ ions).
• Forming complex ions.

Transition elements show variable oxidation states because the 4s and 3d sub-shells have very similar energies, so only small amounts of energy are needed to remove differing numbers of electrons to form stable ions in different oxidation states.

The common oxidation states and colours of the ions formed by first row transition elements are summarised below:

*Cr^3+^ is technically pale purple, but in practice aqueous Cr^3+^ solutions usually look green.