General properties of transition metalsAQA A-Level Chemistry: Revision notes
Section 1
What makes a metal a transition metal?
A transition metal is a d-block element that forms at least one ion with an incomplete d sub-level. The characteristic properties of the elements titanium to copper arise from this incomplete d sub-level in their atoms or ions.
The definition is about ions, not just atoms. Zinc atoms and Zn²⁺ ions are [Ar]3d¹⁰ and scandium forms only Sc³⁺, which is [Ar] with no d electrons, so neither is a transition metal. Copper atoms are [Ar]3d¹⁰4s¹, but Cu²⁺ is [Ar]3d⁹, so copper is.
Do not say copper is a transition metal because its atom has a partially filled d sub-level: Cu atoms are 3d¹⁰. It is the Cu²⁺ ion, 3d⁹, that qualifies.
Section 2
Electron configurations of d-block elements and ions
Across the first d-block row the 4s sub-level fills before 3d, but the 4s electrons are lost first when an atom forms a positive ion. Two elements break the pattern: chromium is [Ar]3d⁵4s¹ and copper is [Ar]3d¹⁰4s¹, because half-full and full d sub-levels are especially stable.
Worked example: iron is [Ar]3d⁶4s². Fe²⁺ loses both 4s electrons, giving [Ar]3d⁶. Fe³⁺ loses one 3d electron as well, giving [Ar]3d⁵. Both ions have an incomplete d sub-level.
Nickel: Ni is [Ar]3d⁸4s², so Ni²⁺ is [Ar]3d⁸.
Write the atom's configuration first, then remove 4s electrons before 3d. Never leave a 4s electron in an ion of a first-row transition metal.
Section 3
Characteristic properties
Four properties are characteristic of transition metals and their compounds:
- Complex formation, e.g. [Cu(H₂O)₆]²⁺
- Coloured ions, e.g. blue Cu²⁺(aq), pale green Fe²⁺(aq)
- Variable oxidation state, e.g. Fe²⁺ and Fe³⁺, or Cu⁺ and Cu²⁺
- Catalytic activity, e.g. iron in the Haber process
Variable oxidation states arise because the 4s and 3d sub-levels are close in energy, so electrons can be lost from both without a large increase in the energy needed.
Section 4
Ligands and co-ordinate bonding
A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons. Common ligands are H₂O, NH₃ and Cl⁻, each of which has a lone pair. Both electrons in a co-ordinate (dative covalent) bond come from the ligand, and the metal ion is the electron-pair acceptor.
An ion such as NH₄⁺ cannot be a ligand because it has no lone pair.
The ligand donates the electron pair to the metal. Never write that the metal donates a pair to the ligand.
Section 5
Complexes and co-ordination number
A complex is a central metal atom or ion surrounded by ligands. The co-ordination number is the number of co-ordinate bonds to the central metal atom or ion.
- [Cu(H₂O)₆]²⁺: six H₂O ligands, co-ordination number 6
- [CuCl₄]²⁻: four Cl⁻ ligands, co-ordination number 4
- [Ag(NH₃)₂]⁺: two NH₃ ligands, co-ordination number 2
- [Cu(NH₃)₄(H₂O)₂]²⁺: four NH₃ and two H₂O, co-ordination number 6
The charge on the complex is the sum of the charge on the metal ion and the charges on the ligands. Water and ammonia are neutral, so [Co(NH₃)₆]²⁺ has the charge of Co²⁺. In [CoCl₄]²⁻ the charge is +2 + 4(−1) = 2−.
Count bonds, not ligands, when finding co-ordination number. With only monodentate ligands the two are the same.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on General properties of transition metals
- A student is comparing the electronic structures of some d-block elements. Iron has the configuration [Ar]3d⁶4s², scandium [Ar]3d¹4s², zinc [Ar]3d¹⁰4s² and copper [Ar]3d¹⁰4s¹.Explain why zinc is not classified as a transition metal even though it is a d-block element.2 marks
- Cobalt(II) chloride dissolves in water to give the pink complex ion [Co(H₂O)₆]²⁺. Adding excess ammonia solution forms [Co(NH₃)₆]²⁺, and adding concentrated hydrochloric acid forms the blue complex ion [CoCl₄]²⁻.Ammonia, NH₃, can act as a ligand but the ammonium ion, NH₄⁺, cannot. Explain why.2 marks
- Solid iron(II) sulfate dissolves in water to give a pale green solution containing [Fe(H₂O)₆]²⁺. On standing in air the solution slowly turns yellow-brown as iron(III) ions form. Finely divided iron is used as the catalyst in the Haber process.Identify three characteristic properties of transition metals that are shown by the information, quoting the evidence for each.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).