Cambridge O Level Chemistry · Syllabus 5070 · The Periodic Table
Transition Element
What is Transition Element?
A metal from the central block of the Periodic Table, between Group II and Group III. Transition elements have high densities and high melting points, show variable oxidation numbers, form coloured compounds, and are often useful as catalysts, either as the element itself or as one of its compounds.
This definition is part of the The Periodic Table chapter in Cambridge O Level Chemistry.
Transition Element in context
Down Group I the alkali metals become more reactive and lower-melting as the single outer electron is lost more easily, while down Group VII the halogens become less reactive as it becomes harder to gain the extra electron needed to fill the outer shell. A more reactive halogen displaces the halide ion of a less reactive one from solution. Transition elements, between Groups II and III, show variable oxidation numbers rather than one fixed charge, and Group VIII noble gases are unreactive because their outer shell is already full.
Common mistakes with Transition Element
- 3 · “Group number = ion charge” applied everywhere Defect Deducing a \(+4\) ion for carbon, or a \(+7\) ion for chlorine, by reading the group number as the charge. Fix The relationship holds for Groups I, II and III (charge \(=\) group number, positive) and for Groups V, VI and VII (charge \(=\) group number \(-\,8\), negative). Group IV and the transition elements do not follow it, and Group VIII normally forms no ions at all.
- 8 · “All transition metal compounds are coloured” Defect Turning a typical property into a universal law, and then using it to reject a correct identification. Fix Transition elements form coloured compounds and are often catalysts. Neither statement is “always”. Titanium(IV) oxide, the white pigment in paint, and scandium oxide are both white solids. One property alone never classifies an element — look for the cluster.
- Error 4 · “Carbon is in Group IV, so it forms \(\mathrm{C^{4+}}\) ions.” Defect The group-to-charge rule applied outside its range. Chemistry Four outer electrons is equally far from losing all of them and from gaining four. Group IV elements normally share electrons instead, forming covalent bonds as in \(\mathrm{CO_2}\) and \(\mathrm{CH_4}\). Repair The rule covers Groups I–III and V–VII. Group IV is an exception, and so are the transition elements. Transfer What ion does silicon \((2,8,4)\) form in its simple compounds? None — it shares.
- Error 5 · “Iron is in the eighth column, so its ion is \(\mathrm{Fe^{8+}}\).” Defect Main-group reasoning applied to a transition element. Chemistry Transition elements show variable oxidation numbers, so no single charge follows from their position. Iron forms \(\mathrm{Fe^{2+}}\) and \(\mathrm{Fe^{3+}}\). Repair For a transition element, read the charge from the Roman numeral in the name or from the formula — never from a column count. Transfer What is the charge on the copper ion in copper(II) sulfate? \(+2\).
- Error 17 · “All transition metal compounds are coloured, so a white compound rules it out.” Defect A typical property hardened into a universal law, then used backwards. Chemistry Transition elements form coloured compounds and are often catalysts. Neither is “always”: titanium(IV) oxide is the white pigment in paint. And a property that is typical of a class cannot be used to exclude a member on one counter-example. Repair Classify on the cluster: high melting point, high density, variable oxidation number, coloured compounds, catalysis. Transfer A metal melts at \(1668\,{}^\circ\mathrm{C}\), has density \(4.51\;\mathrm{g/cm^3}\), and forms a white oxide. Transition element? Probably yes — two strong signals, and one white compound does not rule it out.
Examiner tips on Transition Element
- How the mark scheme reads your answer. For “give three properties that show \(\mathrm{Y}\) is a transition element”, the marks go to three distinct properties each tied to the data you were given — not to three ways of saying “it is dense”. Scan the five and pick the ones the question has actually supplied evidence for.
Questions students ask about Transition Element
Why can't you predict a transition element's ion charge from its group position?
Transition elements show variable oxidation numbers, so no single charge follows from their position between Group II and Group III. Iron forms both \(\mathrm{Fe^{2+}}\) and \(\mathrm{Fe^{3+}}\), so the charge must be read from the Roman numeral in the compound's name or from its formula, never from a column count.