The transition metals are the largest group of elements on the periodic table. They got their name because English chemist Charles Bury described a transition series of elements in 1921. Bury examined the transition from an inner electron layer with 8 electrons to a layer with 18 electrons and from a layer of 18 electrons to one with 32. Today, most people think of these elements as transitioning from one side of the periodic table to the other. Moving from left to right across the periodic table, an electron is added to the d orbital of each atom, transitioning from group 2 to group 13.
Here’s a look at the different ways of defining the transition metals, a list of which elements are included, and a summary of their common properties.
Key Takeaways: Transition Metals
- The IUPAC defines transition metals as elements that have a partially filled d subshell or that form cations with an incomplete d subshell.
- Most textbook lists use the broader “d-block elements” definition, covering groups 3 to 12.
- The classification of group 3 and group 12 varies, depending on whether electron configuration or periodic table layout is emphasized.
- Transition metals commonly have variable oxidation states, colorful compounds, and strong tendencies to form coordination complexes.
- These elements serve important roles in catalysis, metallurgy, electronics, pigments, and biological systems.
- Zinc, cadmium, and mercury usually behave as post-transition metals because they maintain the d¹⁰ configuration in their ions, but many authors still include them.
- There is a long-standing debate about which elements belong in group 3, and this affects which elements are counted as transition metals.
Transition Metal Definition
There are multiple ways of defining a transition metal:
IUPAC Definition
The most common definition of a transition metal is the one accepted by the IUPAC. A transition metal is an element with a partially-filled d subshell or the capacity to produce cations with an incomplete d subshell.
- Partially filled d-subshell in element or its cations.
- Excludes Zn, Cd, Hg, and likely Cn.
- Includes Sc, Y, La? (if following d electron rule) but this is disputed.
d-block Definition (Common Textbook Definition)
- Groups 3–12.
- Includes group 12.
Lanthanides/Actinides
Some sources call the lanthanide and actinide series the inner transition metals because they fill the 4f and 5f orbitals. However, this name is historical rather than technical. Under the IUPAC definition, an element requires e a partially filled d subshell in the element or its cations to qualify as a transition metal. Lanthanides and actinides do not meet this requirement because f electrons dominate their chemistry rather than d electrons. For this reason, they are f block elements and are not transition metals under the strict definition, although many textbooks still group them with transition metals in a broader sense.
Group 3 Controversy
Group 3 is either Sc, Y, La, and Ac or Sc, Y, Lu, and Lr. Using La and Ac places the f block beneath them and aligns with long-standing textbook layouts. Using Lu and Lr produces a more consistent pattern of electron configurations and aligns better with IUPAC’s current discussions. The choice affects whether La and Ac are d-block elements or whether Lu and Lr hold those positions. Because transition metal definitions often depend on d-electron behavior, this debate influences which elements belong to the transition series.
List of Transition Metal Elements
Using the IUPAC definition, there are 40 transition metals. They are:
- Atomic numbers 21 (scandium) to 30 (zinc)
- Atomic numbers 39 (yttrium) to 48 (cadmium)
- Atomic numbers 71 (lutetium) to 80 (mercury)
- Atomic numbers 103 (lawrencium) to 112 (copernicium)
The full list is:
- Scandium
- Titanium
- Vanadium
- Chromium
- Manganese
- Iron
- Cobalt
- Nickel
- Copper
- Zinc
- Yttrium
- Zirconium
- Niobium
- Molybdenum
- Technetium
- Ruthenium
- Rhodium
- Palladium
- Silver
- Cadmium
- Lutetium
- Hafnium
- Tantalum
- Tungsten
- Rhenium
- Osmium
- Iridium
- Platinum
- Gold
- Mercury
- Lawrencium
- Rutherfordium
- Dubnium
- Seaborgium
- Bohrium
- Hassium
- Meitnerium
- Darmstadtium
- Roentgenium
- Copernicium
Technically, the elements zinc, cadmium, and mercury (group 12) should be considered post-transition rather than transition metals because they have a full d10 configuration and normally produce ions that retain this configuration. Experimental evidence of mercury behaving as a transition metal was obtained in 2007. Copernicium should probably be excluded on the same basis, although its oxidation properties have not been verified experimentally. However, most people include these elements in the transition metal list.
Some people exclude lutetium and lawrencium from the list. But, lutetium and lawrencium are technically group 3 elements that fit in the “space” in the periodic table. There are also scientists and educators who include the full lanthanide and actinide series as transition metals.
Transition Metal Properties
The transition metals display several characteristic properties:
- Transition metal atoms have metallic character. In other words, atoms readily lose electrons.
- They often form colored compounds. The colors are due to d-d electronic transitions.
- They readily form complexes.
- They display multiple positive oxidation states. This is because of the low energy gap between states.
- They are good catalysts.
- They are silver metals at room temperature. The exceptions are copper and gold.
- They are solids at room temperature. The exception is mercury.
- They are paramagnetic (attracted to a magnetic field). Generally, paramagnetism results from unpaired d-electrons. Three important elements regarding magnetism are iron, cobalt, and nickel. All three elements produce a magnetic field.
- They exhibit metallic luster.
- They have low ionization energies.
- They are hard.
- The metals have high melting and boiling points (except mercury).
- They are good electrical and thermal conductors.
- They form alloys.
Uses and Applications of Transition Metals
Transition metals are essential across modern industry, chemistry, engineering, and biology because they combine strength, conductivity, catalytic behavior, and variable oxidation states.
Industrial and Structural Applications
- Iron, manganese, chromium, and nickel are key ingredients in steel and stainless steel.
- Titanium and its alloys are used in aerospace, medical implants, and high-strength, low-density tools.
- Tungsten is used for cutting tools, lamp filaments, and high-temperature materials.
Catalysis
- Platinum, palladium, and rhodium find use in catalytic converters.
- Iron catalyzes ammonia synthesis in the Haber process.
- Copper and silver serve as catalysts in organic and redox reactions.
Electronics and Conductive Applications
- Copper is common in electrical wiring, motors, and circuits.
- Silver provides high conductivity for specialized electronics and photovoltaic applications.
- Gold resists corrosion and forms stable electrical contacts.
Pigments and Colors
- Chromium compounds produce green pigments.
- Cobalt compounds produce blues.
- Manganese and iron oxides appear in pottery glazes and paints.
Magnetic and Optical Applications
- Iron, cobalt, and nickel are ferromagnetic.
- Rare transition metals such as osmium and iridium are important in precision instruments and high-density alloys.
Electron Configurations and Why They Are Irregular
Electron configurations in the transition series often deviate from expected patterns because the energies of the ns and (n minus 1)d orbitals are very close. Electrons may shift to achieve lower energy through half-filled or fully filled subshells.
Key Points
- Chromium and copper show well-known exceptions, adopting 3d⁵4s¹ and 3d¹⁰4s¹ rather than the expected 3d⁴4s² and 3d⁹4s².
- Similar anomalous configurations occur throughout the 4d and 5d series.
- Relativistic effects influence the heavier elements, causing contraction of the 6s orbital and stabilizing or destabilizing certain electron arrangements.
- These irregularities explain many transition metal behaviors, such as multiple oxidation states and complex formation.
Colors and Complexes
Transition metals commonly form colored compounds due to electronic transitions within the d orbitals.
Why Transition Metals Form Complexes
- The partially filled d orbitals allow bonding with ligands that donate electron pairs.
- Coordination compounds form because metal ions attract surrounding molecules or ions such as water, ammonia, cyanide, or chloride.
Why Complexes Are Colored
- When ligands bind to a metal, they split the d orbitals into different energy levels.
- Electrons absorb specific wavelengths of light to move between these levels.
- The transmitted or reflected light appears colored.
- Metals with empty or full d subshells (such as Zn²⁺, Cd²⁺, and Hg²⁺) often form colorless compounds because d to d transitions cannot occur.
Biological Roles of Transition Metals
Many transition metals are essential for life because they participate in enzyme function, electron transfer, oxygen transport, and redox chemistry.
Examples
- Iron is central to hemoglobin and cytochromes.
- Cobalt is part of vitamin B12.
- Copper is involved in electron transport and pigment formation.
- Manganese is part of the photosynthetic water-splitting complex.
- Zinc acts as a cofactor for hundreds of enzymes, even though it is not a transition metal under the strict IUPAC definition.
- Molybdenum participates in nitrogen metabolism.
Toxicity
Some transition metals such as chromium(VI), cadmium, and mercury pose significant biological risks and require careful environmental management.
Periodic Trends of Transition Metals
Transition metals show characteristic trends across each period and down each group. These trends differ from those of main group elements because the d and f subshells affect size, charge, and bonding.
Atomic Size
- Atomic radii decrease from left to right across a row, but the decrease is smaller in the 4d and 5d series because of the lanthanide contraction.
- Atomic radii increase down a group.
Ionization Energies
- Increase across a row due to increasing nuclear charge.
- Remain relatively similar down a group because the added f electrons reduce shielding efficiency.
Oxidation States
- Low oxidation states appear at the left, higher oxidation states in the middle, and lower oxidation states again near the right.
- The highest oxidation states occur in manganese, technetium, and rhenium.
Magnetism
- Unpaired d electrons produce paramagnetic behavior.
- Maximum magnetism occurs near the center of each series.
Density and Melting Point
- Middle transition metals have the highest densities and melting points.
Comparison With Post-Transition Metals
Post-transition metals such as aluminum, gallium, indium, tin, thallium, lead, bismuth, and polonium differ from transition metals in electronic structure and chemical behavior.
Key Differences
- Post-transition metals have p-block electron configurations rather than d-block.
- Their ions usually have stable oxidation states and do not show the variable oxidation behavior typical of d-block metals.
- They tend to be softer, have lower melting points, and oxidize more readily.
- Group 12 metals, although often listed with transition metals, generally behave like post-transition metals because they retain the d¹⁰ configuration in their ions.
Frequently Asked Questions (FAQs)
Why are transition metals called transition metals?
Because they represent a transition from elements with typical s block behavior to elements with p block behavior, and because their d electrons transition between energy levels in characteristic ways.
Why do transition metals form colored compounds?
Color results from d to d electronic transitions in coordination complexes.
Why do transition metals have multiple oxidation states?
The energies of the ns and (n minus 1)d orbitals are similar, allowing electrons to be removed in different combinations.
Is zinc a transition metal?
Under the IUPAC definition, zinc is not a transition metal because Zn²⁺ has a full d¹⁰ configuration. It is commonly included in broader textbook definitions.
Are lanthanides and actinides transition metals?
They are often called inner transition metals, but this term is not used by IUPAC. They are f block elements and not considered transition metals under the strict definition.
Why is mercury a liquid?
Relativistic effects contract the 6s orbital in mercury, weakening metallic bonding and lowering the melting point.
Which transition metals are magnetic?
Iron, cobalt, and nickel are ferromagnetic. Many others are paramagnetic due to unpaired d electrons.
Glossary
- Coordination complex: A structure that forms when a central metal ion binds ligands.
- Coordination number: The number of ligand atoms attached to a metal center.
- Crystal field splitting: The separation of d orbital energies caused by ligand interactions.
- d-block: The center block of the periodic table where d orbitals are filled.
- d electrons: Electrons in the (n minus 1)d orbitals responsible for transition metal behavior.
- Electron configuration: The arrangement of electrons around an atom.
- Ligand: A molecule or ion that donates electron pairs to a metal ion.
- Oxidation state: The hypothetical charge an atom has in a compound.
- Paramagnetic: A substance attracted to a magnetic field due to unpaired electrons.
- Relativistic effect: Changes in orbital energies in heavy elements caused by electrons moving at relativistic speeds.
- Transition metal: An element with a partially filled d subshell or that forms ions with incomplete d subshells.
- Variable oxidation states: The ability of an element to form ions with different charges.
References
- Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
- IUPAC (1997). Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”).
- Jensen, William B. (2003). “The Place of Zinc, Cadmium, and Mercury in the Periodic Table”. Journal of Chemical Education. 80 (8): 952–961. doi:10.1021/ed080p952