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Thallium has two isotopes, thallium-203 and thallium-205. Its relative atomic mass is 204.4. (a) Explain why no thallium atom has a relative mass of 204.4. (b) Calculate the percentage of thallium atoms that are thallium-205.
explain: give reasons and connect them to evidence. calculate: work out the answer from the data.
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Scope and sources
Cambridge IGCSE Chemistry 0620 · 2026–2028 v2 · Original learning materials.
Relative masses (3.2 Core 1–3): relative atomic mass as the average mass of the isotopes of an element compared to 1/12 of the mass of an atom of carbon-12, relative molecular mass and relative formula mass as the sum of the relative atomic masses, and reacting masses in simple proportions, without the mole. The mole and molar mass, calculating relative atomic mass from isotope abundances, percentage yield and purity are left to other modules.
Apparatus, amounts and times in the experiment are illustrative. Real practical work follows a teacher-approved safety method.
Official Cambridge syllabus