A-LevelChemistryAtoms, molecules and stoichiometry9701/22 · May/June 2025 · Q210 Marks

A sample of iron contains three different isotopes and has a relative atomic mass, Ar, of 55.8.

(a)(i)Define relative atomic mass.[2]
(a)(ii)Table 2.1 shows the abundances of two of the isotopes present in the sample of iron. Table 2.1 isotope | relative isotopic mass | abundance/% 54Fe | 53.9 | 6.0 56Fe | 55.9 | 91.9 Use Table 2.1 to calculate the relative isotopic mass of the third isotope of iron in the sample. Show your working.[2]
(b)Deduce the number of pairs of electrons in the 3d sub-shell in an iron(II) ion.[1]
(c)Sketch the shape of the lowest energy orbital in the shell with principal quantum number n = 2.[1]
(d)Complete Table 2.2 to show information about particles in one ion of 56Fe3+. Table 2.2 particle | number of particles present in one ion of 56Fe3+ electrons | neutrons | 30[2]
(e)The atomic radius of iron is 1.26 × 10–10m. Suggest the change to the radius, if any, after an iron atom reacts to produce an Fe3+ ion. Explain your answer.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

2(a)(i) M1 average (weighted) mass of (an) atom(s) (of an element) OR average (weighted) mass of the isotope(s) (of an element)
2(a)(i) M2 compared to the mass of the unified atomic mass unit OR on a scale in which a carbon-12 atom / isotope has a mass of exactly 12 (units) OR on a scale in which the mass of a carbon-12 atom/isotope is exactly 12 (units) OR compared to 1/12 mass of one atom of C-12
2(a)(i) route 2 M1 average mass of one mol of isotopes / atoms (of an element)
2(a)(i) route 2 M2 relative / compared to 1/12 of the mass of one mol of C-12 atoms/isotopes OR when the mass of one mol C-12 atoms / isotopes is exactly 12 (000) g
2(a)(ii) M1 relative abundance of unknown isotope = (100 – 91.9 – 6.0) = 2.1%
2(a)(ii) M2 55.8 = (53.9 × 6.0 + 55.9 × 91.9 + ? × M1) / 100 = 56.9
2(b) 1 (pair)
2(c) [diagram of a spherical orbital]
2(d) M1 23
2(d) M2 neutrons 30
2(e) M1 smaller/decreases
2(e) M2 loss of outer / valence shell (of electrons) OR loss of electrons so increase in (force of) attraction from nucleus OR loss of electrons so increase nuclear attraction

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