Iron, copper, nickel and their neighbours sit in the central block of the table. They are the metals of bridges, wires and catalysts: dense, strong, high-melting, and able to form brightly coloured compounds.
The transition elements are metals with high densities and high melting points. They form coloured compounds, and the elements and their compounds often act as catalysts. Extended: transition elements have ions with variable oxidation numbers, such as iron(II), Fe2+, and iron(III), Fe3+.
Fig. 1: characteristic properties of transition elements
Section 01
Transition metals versus Group I.
Compare a typical transition metal, iron, with a Group I metal, sodium.
Property
Transition metal (iron)
Group I metal (sodium)
melting point
high, 1538 °C
low, 98 °C
density
high, 7.9 g/cm3
low, 0.97 g/cm3
hardness and strength
hard and strong
soft
colour of compounds
often coloured
white; colourless solutions
reaction with water
slow (rusting)
vigorous
catalytic activity
often catalysts
not used as catalysts
Stage 1 · Learn
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Which property is typical of transition elements?
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Which transition metal is the catalyst in the Haber process?
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What colour is copper(II) sulfate solution?
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Compared with sodium, iron has:
Section 02
Colours, catalysts and oxidation numbers.
These properties explain many uses of the transition elements.
Coloured compounds: copper(II) sulfate solution is blue; iron(II) compounds are pale green; iron(III) compounds are orange-brown. These colours are used to identify the ions.
Catalysts: iron in the Haber process; vanadium(V) oxide in the Contact process; nickel in the hydrogenation of vegetable oils.
Extended, variable oxidation numbers: iron forms FeCl2 and FeCl3; copper forms Cu2O and CuO. The Roman numeral in the name gives the oxidation number: iron(II), iron(III), copper(I), copper(II).
Uses: iron and steel for construction, copper for wiring, titanium for aircraft.
Examiner trap
"Coloured compounds" is about the compounds, not the metals: copper metal is pink-brown, but the property that marks a transition element is that its compounds and solutions are coloured. And a catalyst is not used up, so "iron reacts in the Haber process" is wrong.
Stage 2 · Exam
Exam-style questions
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(Extended) Iron forms two chlorides, FeCl2 and FeCl3. What does this show about transition elements?
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An element X has a high melting point, forms a green chloride and a brown chloride, and speeds up a reaction without being used up. X is most likely:
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Which is a use of a transition element as a catalyst?
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(Extended) What is the oxidation number of copper in copper(I) oxide, Cu2O?
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