Transition Metals and Inner Transition Metals

Transition metals are the d-block elements in groups 3 to 12, characterized by partly filled d subshells, variable oxidation states, colored compounds and catalytic activity, and inner transition metals are the lanthanides and actinides, in which the f subshells are being filled. In brief: iron forms Fe²⁺ and Fe³⁺, copper compounds are often blue, and platinum catalyzes reactions in car exhaust systems; the lanthanides are used in magnets and lighting, and the actinides include uranium and plutonium.

The transition metals make up the central block of the periodic table and include most of the structural and industrial metals, such as iron, copper, nickel, titanium and zinc. The inner transition metals are placed in two rows below the table. This article describes their general properties, explains why they show different oxidation states and colors, lists key data in tables, works through some examples and gives the history of the lanthanides and actinides.

What are transition metals?

Transition metals are elements in the d-block. In the strictest definition (IUPAC), a transition element has an atom with a partially filled d subshell or can form cations with a partially filled d subshell. By this definition, zinc, cadmium and mercury (group 12, with a full d¹⁰ subshell in both the atom and the ion) are not true transition metals, though they are often included in the d-block for convenience. The first row, scandium to zinc, has the 3d subshell filling, and the second and third rows have 4d and 5d.

What are their typical properties?

• Hard, strong, dense metals with high melting and boiling points (mercury, a liquid, is an exception).
• Good conductors of heat and electricity.
• Malleable and ductile, with a shiny surface.
• Several oxidation states, for example Fe(II) and Fe(III), Mn from +2 to +7.
• Colored ions and compounds in solution and in the solid state.
• Good catalysts, for example iron in the Haber process, vanadium(V) oxide in the contact process and platinum in catalytic converters.
• Formation of complex ions, in which a central metal ion is surrounded by molecules or ions called ligands, for example [Cu(NH₃)₄]²⁺.
• Many are paramagnetic because of unpaired d electrons, and iron, cobalt and nickel are ferromagnetic.

Why do they have variable oxidation states?

The 4s and 3d electrons in the first row have similar energies, so after the 4s electrons are lost to form 2+ ions, one or more 3d electrons can also be lost without a large energy cost. The result is a series of oxidation states, such as +2 and +3 for iron and +2 to +7 for manganese. Higher states are more oxidizing, so potassium permanganate (Mn in +7) is a strong oxidizing agent.

Why are their compounds colored?

In a complex ion, the ligands split the five d orbitals into groups of different energy. Electrons can absorb visible light and jump between these levels, called d-d transitions. The ion appears with the complementary color of the light absorbed. Ions with no d electrons (Sc³⁺, Ti⁴⁺) or with a full d subshell (Zn²⁺, Cu⁺) have no d-d transitions and are colorless. The color depends on the metal, its oxidation state and the ligands: aqueous copper(II) ions are pale blue, while the ammonia complex is deep blue.

Data for the first-row transition metals

What are the inner transition metals?

The lanthanides (cerium to lutetium, atomic numbers 58 to 71, often with lanthanum) have the 4f subshell filling. They are silvery, reactive metals that are very similar to each other in chemistry, since their outer electrons are the same (usually forming 3+ ions). The similarity makes them hard to separate. The “lanthanide contraction”, a gradual decrease in the size of the ions across the series, affects the properties of the elements that follow. The actinides (thorium to lawrencium, 90 to 103, often with actinium) have the 5f subshell filling. All are radioactive, and those beyond uranium (atomic number 92) are mostly made artificially in reactors or accelerators.

Worked examples

1. Configuration of an ion. Fe³⁺: iron is [Ar] 3d⁶ 4s². Remove the two 4s electrons and one 3d electron: [Ar] 3d⁵, with 5 unpaired electrons.
2. Oxidation state in a compound. In KMnO₄, K is +1 and each O is −2, so Mn = +7 (since +1 + x + 4(−2) = 0, so x = +7).
3. Number of d electrons. Cr³⁺: chromium has 24 electrons, and the ion has 21: [Ar] 3d³. It has 3 unpaired electrons.
4. Why zinc ions are colorless. Zn²⁺ is [Ar] 3d¹⁰. The d subshell is full, so there is no d-d transition in the visible range.
5. Formula of a complex. The complex ion [Cu(NH₃)₄]²⁺ has Cu²⁺ (+2) and four neutral ammonia ligands, so the total charge is +2. The coordination number is 4.
6. Mass of iron from an ore. A sample of hematite (Fe₂O₃) has a mass of 100 g. The fraction of iron is 111.7 ÷ 159.7 = 0.699, so it contains 69.9 g of iron.

What are they used for?

• Iron and steel: construction and machinery. Steel is the most-used metal alloy.
• Copper: electrical wires and plumbing.
• Titanium: strong, light and corrosion-resistant, used in aircraft and medical implants.
• Platinum and palladium: catalytic converters and jewelry.
• Zinc: galvanizing steel to protect it from rust.
• Chromium and nickel: stainless steel and plating.
• Pigments: compounds of titanium (white), chromium (green and yellow) and cobalt (blue).

How were they discovered?

Copper, iron, gold and silver were known in antiquity. Many other transition metals were identified in the 18th and 19th centuries as chemistry became quantitative. The rare earth elements, which include the lanthanides, were first found in minerals from Ytterby in Sweden, beginning with yttrium in 1794. Their separation took more than a century. Neptunium, the first transuranium element, was synthesized in 1940 by Edwin McMillan and Philip Abelson. Glenn Seaborg’s group made plutonium in 1940 and several other actinides, and Seaborg shared the Nobel Prize in Chemistry in 1951 with McMillan.

Which mistakes are common?

Mistake 1: Removing 3d electrons before 4s when forming ions

The 4s electrons are removed first.

Mistake 2: Thinking that all transition metals form colored ions

Ions with empty or full d subshells, such as Sc³⁺ and Zn²⁺, are colorless.

Mistake 3: Saying that transition metals have a fixed charge

Most have several oxidation states. The name should include a Roman numeral, such as iron(III) chloride.

Mistake 4: Confusing the lanthanides with the actinides

Lanthanides fill the 4f subshell and are mostly stable, while actinides fill the 5f subshell and are all radioactive.