IB Chemistry HLTopic 3 — Classifying the ElementsPaper 1 & 2Core idea~12 min read
Characteristic Properties of Transition Elements (HL)
Transition metals are the interesting metals. They come in several oxidation states, their compounds are coloured, they catalyse reactions, some of them are magnetic, and they build complexes with molecules arranged around them. Five apparently unrelated properties — and every one of them traces back to a partly filled d sublevel.
📘 What you need to know
A transition element is a d-block metal that forms at least one stable ion with a partially filled d sublevel.
That definition excludes scandium (Sc3+ is d0) and zinc (Zn2+ is d10), even though both are in the d-block.
Characteristic properties: variable oxidation states, coloured compounds, catalytic activity, magnetic behaviour and the formation of complex ions.
A ligand is a molecule or ion that donates a lone pair to a metal ion, forming a coordinate (dative) bond.
A complex ion is a central metal ion surrounded by ligands. The number of coordinate bonds is the coordination number.
Catalysis works because the metal can change oxidation state easily, accepting and releasing electrons during the reaction.
Heterogeneous catalysts are in a different phase from the reactants; homogeneous catalysts are in the same phase.
The definition, and why it excludes two elements
Being in the d-block is not enough. The IB definition requires the element to form an ion with an incomplete d sublevel, and two of the ten first-row d-block elements fail that test.
Scandium is [Ar]3d14s2. Its only common ion is Sc3+, which is [Ar] — the d sublevel is completely empty.
Zinc is [Ar]3d104s2. Its only common ion is Zn2+, which is [Ar]3d10 — the d sublevel is completely full.
Neither has a partly filled d sublevel in its ion, so neither shows the characteristic behaviour. Zinc compounds are white, zinc has only one oxidation state, and zinc is not magnetic. That is not a coincidence — it is the definition doing its job.
This is the neatest exam question in the whole topic, and it comes up regularly: “Explain why zinc is not considered a transition element.” The answer is one line about Zn2+ being 3d10, but you have to know it cold.
Where the properties come from
Every property here needs d orbitals that are partly occupied — either so electrons can move between them, or so unpaired electrons exist, or so electrons can be gained and lost without much cost.
Variable oxidation states
Most metals have one oxidation state and stick to it. Sodium is always +1; magnesium is always +2. Transition metals routinely offer several, because the 4s and 3d subshells are so close in energy that removing a different number of electrons costs almost the same.
Iron gives Fe2+ and Fe3+; copper gives Cu+ and Cu2+; manganese runs from +2 to +7. This is exactly why Roman numerals exist in chemical names — iron(II) chloride and iron(III) chloride are genuinely different compounds with different colours and different chemistry.
Complex ions and ligands
A transition metal ion is small and highly charged, which makes it very attractive to anything carrying a lone pair. Molecules and ions with lone pairs — water, ammonia, chloride, hydroxide, cyanide — donate those pairs into empty orbitals on the metal, forming coordinate bonds.
The charge on the complex is the metal ion’s charge plus the charges on the ligands. Water is neutral, so [Cu(H2O)6]2+ keeps the 2+; with six chloride ligands it would be [CuCl6]4−.
Catalysis
Transition metals are outstanding catalysts, and the reason is the variable oxidation states. A catalyst has to accept electrons at one stage and give them back at another, and a metal that can flip between +2 and +3 at almost no energy cost is ideally suited to that.
Catalyst
Type
Reaction it catalyses
Iron
Heterogeneous
N2(g) + 3H2(g) ⇌ 2NH3(g) in the Haber process
Nickel
Heterogeneous
Hydrogenation of alkenes, including hardening vegetable oils
Manganese(IV) oxide
Heterogeneous
2H2O2(aq) → 2H2O(l) + O2(g)
Platinum and rhodium
Heterogeneous
Converting CO and NO in a catalytic converter
Iron in haemoglobin
Homogeneous
Binding and releasing oxygen in the blood
Heterogeneous catalysts work at active sites on their surface, which is why they are used as fine powders or thin coatings on a honeycomb support — maximum surface area for the least amount of an expensive metal.
Magnetism
An electron spinning generates a tiny magnetic field. When electrons are paired, their spins are opposite and the two fields cancel. Only unpaired electrons produce a net magnetic effect — and a partly filled d sublevel is precisely where unpaired electrons live.
Iron, cobalt and nickel are the strongly magnetic ones. In these metals the magnetic regions, called domains, can be permanently aligned, which is why they make magnets and why steel does too.
You are not asked about the different categories of magnetism. The current syllabus only expects you to link magnetic behaviour to the presence of unpaired d electrons.
Worked examples
WORKED EXAMPLE
Explain why zinc is in the d-block but is not classed as a transition element. [2]
Mark 1: give the configurationsZn: [Ar] 3d¹⁰ 4s² Zn²⁺: [Ar] 3d¹⁰Mark 2: apply the definition
A transition element must form at least one ion with a partially filled d sublevel. Zinc’s only common ion has a completely full 3d.
No incomplete d sublevel, so zinc is not a transition elementback it up with evidence: zinc compounds are white and zinc has only one oxidation state
WORKED EXAMPLE
For the complex ion [Fe(CN)6]3−, state the ligand, the coordination number, and the oxidation state of iron.
Step 1: identify the ligand
The species attached to the metal is CN−, the cyanide ion.
Step 2: count the coordinate bonds
There are six cyanide ligands, each donating one lone pair.
coordination number = 6Step 3: balance the chargesFe + 6(−1) = −3, so Fe = +3Cyanide ligand, coordination number 6, iron(III)a coordination number of 6 means an octahedral shape
WORKED EXAMPLE
Explain why transition metals make good catalysts, and say what a catalytic converter’s honeycomb structure is for.
Step 1: the electronic reason
The 4s and 3d subshells are close in energy, so the metal can gain and lose electrons easily and move between oxidation states.
Step 2: what that allows
It can accept electrons at one stage of the reaction and release them at another, providing an alternative pathway of lower activation energy.
Step 3: the honeycomb
It is an inert support that spreads a thin layer of an expensive metal over a very large surface area.
Variable oxidation states make the chemistry work; the honeycomb makes it affordablemore active sites per gram of platinum is the entire commercial argument
💡 Exam tip
Learn the definition word for word: forms at least one ion with a partially filled d sublevel.
Scandium and zinc are the two standard exceptions. Be ready to justify both with configurations.
When naming ligands and coordination numbers, count the bonds, not the ligands, if any ligand attaches twice.
For complex ion charges, add the metal’s oxidation state to the total ligand charge.
Link catalysis to variable oxidation states, not just to “being a metal”.
Link magnetism to unpaired electrons, and say that paired spins cancel.
⚠ Common mix-up
Saying every d-block element is a transition element. Scandium and zinc are the exceptions.
Applying the definition to the atom instead of the ion. Zinc atoms and zinc ions both have a full 3d, but it is the ion that the definition tests.
Confusing a ligand with a counter ion. Only species bonded directly to the metal are ligands.
Forgetting that the ligands contribute charge. Six neutral waters change nothing; six chlorides change a lot.
Saying transition metals are catalysts because they have a large surface area. Surface area helps a heterogeneous catalyst work better, but it is not why they are catalytic.
Claiming all transition metal compounds are magnetic. Only those with unpaired electrons show the effect.
Up next: Variable Oxidation States in Transition Elements (HL) — we look at the electron configurations behind those multiple oxidation states, including the two elements that refuse to follow the filling rules.
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