What a learner can do afterwards
- Defines a transition element by its d sub-shell and explains why scandium and zinc do not qualify
- Gives the common oxidation states of a named transition metal with a compound of each
- Explains colour as a d sub-shell split by ligands, with part of the visible spectrum absorbed
- Names a heterogeneous and a homogeneous catalyst and explains the part the oxidation states play
1 · Read
A transition element forms at least one stable ion with a partly filled d sub-shell. Scandium forms only Sc3+, which has no d electrons. Zinc forms only Zn2+, whose d sub-shell is full with 10 electrons. Neither has an ion with a partly filled d sub-shell, so neither counts as a transition element, even though both sit in the d block and the scandium atom itself is 3d1. When these atoms form ions, the 4s electrons are lost before the 3d ones.
Iron shows two common oxidation states: +2 in iron(II) chloride, FeCl2, and +3 in iron(III) chloride, FeCl3. The 4s and 3d electrons are close in energy, so iron can lose different numbers of them. Each state keeps a partly filled d sub-shell: Fe2+ is 3d6 and Fe3+ is 3d5.
In a complex, ligands (molecules or ions bonded to the metal by a lone pair) split the d sub-shell into two sets of levels. A d electron can absorb visible light and jump to the upper set, and we see the colours left over. Cu2+ is 3d9, so the jump is possible and copper(II) compounds are coloured. In Zn2+ every d orbital is full, so no electron can move up, and zinc compounds are white. Different ligands give a different split, so the colour changes when the ligands change.
Many transition metals catalyse reactions because they can change oxidation state and change back. A heterogeneous catalyst is in a different phase from the reactants, like solid iron with nitrogen and hydrogen gases in the Haber process. In the Contact process, solid vanadium(V) oxide goes from +5 to +4 and back. A homogeneous catalyst is in the same phase: Fe2+ ions catalyse peroxodisulfate reacting with iodide ions in solution, going to Fe3+ and back to Fe2+.
A partly filled d sub-shell gives several oxidation states, split d levels give colour, and changing oxidation state and back lets these metals catalyse.
Original chemistry lesson and assessment text written for os-taxonomy.
The reading on what counts as a transition element is original writing for os-taxonomy. The rewritten readings were written for this dataset. Other readings are adapted from the Khan Academy article on redox reactions. The video is a linked Khan Academy video. License terms for the Khan Academy sources were not confirmed.
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