IB Chemistry SL 1.2.2 Isotopes

Practise comparing isotopes of the same element and calculating relative atomic mass from isotopic composition. Distinguish the shared proton number from differing neutron…

Exam points

contrast two isotopes by identifying the same proton count and different neutron numbers
calculate relative atomic mass from isotope masses and percentage or relative abundances
judge whether an isotopic substitution changes mass-dependent or electronic properties

[Maximum number: 4]

Chlorine occurs in Group 7, the halogens.

(a)

Two stable isotopes of chlorine are ${ }^{35} \mathrm{Cl}$ and ${ }^{37} \mathrm{Cl}$ with mass numbers 35 and 37 respectively.

[ 4 ]

(i)

Define the term isotopes of an element.

[ 2 ]

(ii)

Using the mass numbers of the two isotopes and the relative atomic mass of chlorine from Table 5 of the Data Booklet, determine the percentage abundance of each isotope.

Percentage abundance ${ }^{35} \mathrm{Cl}$ :

Percentage abundance ${ }^{37} \mathrm{Cl}$ :

[ 2 ]

$\left(\right.$ for $\left.{ }^{35} \mathrm{Cl}: x \%\right) 35 x+3700-37 x=3545$ ;

Marking guidance:

Allow other alternative mathematical arrangements.
${ }^{35} \mathrm{Cl}=77.5 \%$ and ${ }^{37} \mathrm{Cl}=22.5 \%$ ;

Award [1 max] for correct percentages if no correct working is shown.

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1.2.2—Isotopes question 1

Question (a)

Question (i)

Question (ii)