Characteristic Properties of Transition Elements (HL) (DP IB Chemistry): Revision Note

Characteristic properties of transition elements

• Although transition elements are metals, they have distinctive properties compared to other metals, due to their incomplete d sublevels

• These include:

  • Variable oxidation states

  • High melting points

  • Magnetic properties (due to unpaired d electrons)

  • Catalytic behaviour

    • Used in catalytic converters and biological processes

  • Formation of coloured compounds

  • Ability to form complex ions with ligands

• These properties arise from the presence of partially filled d orbitals, which allow for diverse bonding and electronic transitions

For more information about the electrical conductivity and melting points of transition metals, see our revision note on the Physical Properties of Transition Elements

Variable oxidation states

• Like other metals, transition elements lose electrons to form positive ions

• However, unlike most metals, transition elements can form more than one type of positive ion

  • This property is known as having variable oxidation states

• Roman numerals are used to indicate the oxidation state of a metal in a compound:

  • For example, sodium (Na) only forms Na⁺, so no Roman numeral is needed

  • In contrast, the transition metal iron (Fe) can form Fe²⁺ (iron(II)) and Fe³⁺ (iron(III)) ions

Magnetic properties

• Transition metal magnetism arises from unpaired electrons in the d orbitals

• Each spinning electron generates a tiny magnetic dipole

• In most materials, paired electrons spin in opposite directions, cancelling out any magnetic effect

• Therefore, substances with only paired electrons are non-magnetic

• Some transition metals have unpaired electrons, which can become aligned in an external magnetic field, producing magnetism

• Iron, cobalt, and nickel exhibit strong magnetic properties due to their unpaired d electrons

  • Alloys like steel are also magnetic because they contain iron

Arrangement of electrons in orbitals for iron, cobalt and nickel

• When iron, cobalt, or nickel are heated and cooled in a magnetic field, the magnetic dipoles align and the material retains its magnetism

• Aligned magnetic regions within the metal are called domains

• Heating or striking a permanent magnet can disrupt domain alignment, reducing its magnetism

Transition elements as catalysts

• Transition metals are often used as catalysts in the elemental form or as compounds

• The ability of transition metals to form more than one stable oxidation state means that they can accept and lose electrons easily

• This enables them to catalyse certain redox reactions

  • This makes them effective in catalysing redox reactions, as they can be oxidised and reduced repeatedly

  • This catalytic behaviour is a direct result of their variable oxidation states

• There are two types of catalyst:

  • A heterogeneous catalyst is in a different physical state (phase) from the reactants

    • The reaction occurs at active sites on the surface of the catalyst

    • An example is the use of iron, Fe, in the Haber process for making ammonia

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

• A homogeneous catalyst is in the same physical state (phase) as the reactants

Further examples of transition metal catalysts

• Nickel is used in the hydrogenation of alkenes

  • Also applied in hydrogenating vegetable oils

CH₂=CH₂ (g) + H₂ (g) → CH₃CH₃ (g)

• Manganese(IV) oxide acts as a catalyst in the decomposition of hydrogen peroxide:

2H₂O₂ (g) →  2H₂O (aq) + O₂ (g)

Catalytic converters

• Used in car exhaust systems to reduce air pollution

• Typically contain platinum and rhodium catalysts

• These are finely divided and supported on a ceramic honeycomb base to maximise surface area and efficiency

Catalytic converter diagram

• Carbon monoxide, nitrogen dioxide and unburnt hydrocarbons are sources of pollution in car exhaust

• The transition metal catalysts facilitate the conversion of these pollutants into less harmful products:

2NO (g) + 2CO (g) → N₂ (g) + 2CO₂ (g)

CH₃CH₂CH₃ (g) + 5O₂ (g) → 3CO₂ (g)  + 4H₂O (g)  

• Some transition metals are precious metals so they can be very expensive

  • In order to minimise the cost and maximise the efficiency of the catalyst the following measures can be taken:

    • Increasing the surface area of the catalyst

    • Coating an inert surface medium with the catalyst to avoid using large amounts of the catalyst

  • This is achieved by spreading the catalyst over a hollow matrix such as a honeycomb-like structure

Biological catalysts

• Many of the enzyme catalysed reactions in the body make use of homogeneous transition metal catalysts

• An example of this is haemoglobin, abbreviated to Hb, which transports oxygen around the blood:

Haemoglobin structure diagram

The structure of haem

• The iron(II) ion is in the centre of a large heterocyclic ring called a porphyrin

• It is bonded to four nitrogen atoms within the porphyrin, giving it a coordination number of four

• These bonds form a square planar arrangement around the iron(II) ion

• The Hb molecule contains four porphyrin rings so each Hb can transport four oxygen molecules

Forming coloured compounds

• One distinctive property of transition elements is that their compounds are often coloured

• For example,

  • [Cr(OH)₆]^3- is dark green

  • [Cr(NH₃)₆]³⁺ is purple

• Both complexes contain chromium in the +3 oxidation state, but different ligands cause different colours

For more information about transition metals as coloured compounds, see our revision note on Colour in Transition Metal Complexes

Forming complex ions

• Another key property of transition metals is their ability to form complex ions, enabled by their variable oxidation states

  • A complex ion consists of a central metal ion surrounded by ligands (molecules or ions that donate a lone pair of electrons)

  • The type and number of ligands can vary depending on the metal’s oxidation state

  • Example: Chromium(III) can form several complex ions, including:

    • [Cr(NH₃)₆]³⁺

    • [Cr(OH)₆]^3-

    • [Cr(H₂O)₆]³⁺

For more information about complex ions and transition metals, see our revision note on Coordinate Bonds