AQA · Chemistry · 8462
GCSE Chemistry: Relative atomic mass
Atomic structure and the periodic table · AQA GCSE Chemistry (8462), specification point 4.1.1.6. Cards are generated from the exam board's own specification for "Relative atomic mass", and every card links back to the page it came from. Specification © AQA — used with attribution.
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Define the relative atomic mass of an element.
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The average mass of the element's atoms, taking account of the abundance of its isotopes.
The relative atomic mass of an element is an ? value that takes account of the abundance of its isotopes.
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average
Why is the relative atomic mass of chlorine (35.5) not a whole number?
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It is a weighted average of the masses of chlorine's isotopes (Cl-35 and Cl-37), weighted by their abundance.
What information is needed to calculate an element's relative atomic mass?
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The mass number and percentage abundance of each of its isotopes.
How is relative atomic mass calculated from isotope abundances?
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Ar = sum of (mass number × % abundance) for each isotope, divided by 100.
Chlorine is 75% Cl-35 and 25% Cl-37. What is its relative atomic mass?
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35.5 — (35 × 75 + 37 × 25) ÷ 100 = 35.5.
What symbol is used for relative atomic mass?
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Ar (A with subscript r).
What is the relative atomic mass of an element whose isotopes have the same abundance? (e.g. 50% X-10, 50% X-12)
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The simple mean of the mass numbers — here 11.
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