A-LevelChemistryAtoms, molecules and stoichiometry9701/21 · Oct/Nov 2022 · Q110 Marks

Atoms with nuclei containing an odd number of protons tend to have fewer isotopes than those with an even number of protons.

(a)(i)Complete Table 1.1 to show the numbers of protons, neutrons and electrons in the two stable isotopes of gallium. Table 1.1 isotope number of protons number of neutrons number of electrons 69Ga 71Ga[2]
(a)(ii)Define relative atomic mass.[2]
(a)(iii)The relative atomic mass of gallium, Ar, is 69.723. The relative isotopic masses of 69Ga and 71Ga are: 69Ga, 68.926; 71Ga, 70.925. Use this information to calculate the percentage abundance of 69Ga in elemental gallium. Show your working. Assume that the element contains only the 69Ga and 71Ga isotopes. Give your answer to four significant figures. percentage abundance of 69Ga = ................................. %[2]
(b)(i)Explain why both isotopes of potassium have the same chemical properties.[1]
(b)(ii)State the full electronic configuration of an atom of potassium.[1]
(b)(iii)The first, second and third ionisation energies of potassium are 418, 3070 and 4600 kJ mol–1, respectively. Use this information to explain why potassium is in Group 1.[2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

[1] columns 1 & 3 identical
[1] 69Ga: 31 protons, 38 neutrons, 31 electrons; 71Ga: 31 protons, 40 neutrons, 31 electrons.
[1] M1 (weighted) average / mean mass of the isotopes / average mass of the atom(s) (of an element)
[1] M2 compared to (the mass of) the unified atomic mass unit
[1] 69.723 = 68.926x + 70.925(1−x) :: x = 0.6013 / 69.723 = (68.926x + 70.925(100 – x)) / 100
[1] 60.13%
[1] they have the same electron arrangement / electronic configuration
[1] 1s2 2s2 2p6 3s2 3p6 4s1
[1] M1 big increase in IE between first and second
[1] M2 second (and third) electron(s) is removed from inner shell OR second (and third) electron(s) is removed from a shell closer to the nucleus OR second (and third) electron(s) has a stronger nuclear attraction ora

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