Transition Elements (OCR A Level Chemistry)

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Electron Configuration of a Transition Element

  • 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 copper6.2.1 Transition elements and d-block elements, downloadable AS & A Level Chemistry revision notes

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

Element Electronic configuration
Ti 1s2 2s2 2p6 3s2 3p6 3d2 4s2
V 1s2 2s2 2p6 3s2 3p6 3d3 4s2
Cr 1s2 2s2 2p6 3s2 3p6 3d5 4s1
Mn 1s2 2s2 2p6 3s2 3p6 3d5 4s2
Fe 1s2 2s2 2p6 3s2 3p6 3d6 4s2
Co 1s2 2s2 2p6 3s2 3p6 3d7 4s2
Ni 1s2 2s2 2p6 3s2 3p6 3d8 4s2
Cu 1s2 2s2 2p6 3s2 3p6 3d10 4s1

  • 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 4snot [Ar] 3d9 4s2

  • This is because the [Ar] 3d5 4s1 and [Ar] 3d10 4sconfigurations 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

Coloured Ions & Catalytic Behaviour

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)

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Sonny

Author: Sonny

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Sonny graduated from Imperial College London with a first-class degree in Biomedical Engineering. Turning from engineering to education, he has now been a science tutor working in the UK for several years. Sonny enjoys sharing his passion for science and producing engaging educational materials that help students reach their goals.