5.6.1 Transition Elements

• Transition metals are elements with an incomplete d-subshell that can form at least one stable ion with an incomplete d-subshell
• This definition distinguishes them from d-block elements, because scandium and zinc do not fit the definition
  • Scandium only forms the ion Sc³⁺, configuration [Ar] 3d⁰
  • Zinc only forms the ion Zn²⁺, configuration [Ar] 3d¹⁰

• The elements of the first transition series are therefore titanium to copper

Electron Configuration

• The full electronic configuration of the first d-series transition metals is shown in the table below
• Following the Aufbau Principle electrons occupy the lowest energy subshells first
• The 4s overlaps with the 3d subshell so the 4s is filled first
• Remember that you can abbreviate the first five subshells, 1s-3p, as [Ar] representing the configuration of argon( known as the argon core)

Table showing the electronic configuration of the first d-series transition elements

• From AS Chemistry you should recall two exceptions to the Aufbau Principle, chromium and copper
• In both cases an electron is promoted from the 4s to the 3d to achieve a half full and full d-subshell, respectively
• Chromium and copper have the following electron configurations, which are different to what you may expect:
  • Cr is [Ar] 3d⁵ 4s¹ not [Ar] 3d⁴ 4s²
  • Cu is [Ar] 3d¹⁰ 4s¹ not [Ar] 3d⁹ 4s²

• This is because the [Ar] 3d⁵ 4s¹ and [Ar] 3d¹⁰ 4s¹ configurations are energetically more stable

• The electronic configurations of an iron atom and its common ions, Fe²⁺ and Fe³⁺, are shown below
  • Fe atom 1s²2s²2p⁶3s²3p⁶3d⁶4s²
  • Fe²⁺ ion 1s²2s²2p⁶3s²3p⁶3d⁶
  • Fe³⁺ ion 1s²2s²2p⁶3s²3p⁶3d⁵

General properties

• Although the transition elements are metals, they have some properties unlike those of other metals on the periodic table, such as:
  • Variable oxidation states
  • Form complex ions
  • Form coloured compounds
  • Behave as catalysts

Variable Oxidation States

• Like other metals on the periodic table, the transition elements will lose electrons to form positively charged ions
• However, unlike other metals, transition elements can form more than one positive ion
  • They are said to have variable oxidation states

• Because of this, Roman numerals are used to indicate the oxidation state on the metal ion
  • For example, the metal sodium (Na) will only form Na⁺ ions (no Roman numerals are needed, as the ion formed by Na will always have an oxidation state of +1)
  • The transition metal iron (Fe) can form Fe²⁺ (Fe(II)) and Fe³⁺ (Fe(III)) ions

Forming Complex ions

• Another property of transition elements caused by their ability to form variable oxidation states, is their ability to form complex ions
• A complex ion is a molecule or ion, consisting of a central metal atom or ion, with a number of molecules or ions surrounding it
• A molecule or ion surrounding the central metal atom or ion is called a ligand
• Due to the different oxidation states of the central metal ions, a different number and wide variety of ligands can form bonds with the transition element
  • For example, the chromium(III) ion can form [Cr(NH₃)₆]³⁺, [Cr(OH)₆]^3- and [Cr(H₂O)₆]³⁺ complex ions

Forming coloured compounds

• Another characteristic property of transition elements is that their compounds are often coloured
  • For example, the colour of the [Cr(OH)₆]^3- complex (where oxidation state of Cr is +3) is dark green
  • Whereas the colour of the [Cr(NH₃)₆]³⁺ complex (oxidation state of Cr is still +3) is purple

Transition elements as catalysts

• Since transition elements can have variable oxidation states, they make excellent catalysts
• During catalysis, the transition element can change to various oxidation states by gaining electrons or donating electrons from reagents within the reaction
• Substances can also be adsorbed onto their surface and activated in the process

• 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
• The decomposition of hydrogen peroxide is a common reaction in the study of chemical kinetics and uses manganese(IV) oxide as the catalyst

2H₂O₂ (g) →  2H₂O (aq) + O₂ (g)