AQA GCSE Chemistry (8462), Higher tier · Atomic structure and the periodic table › Atoms, isotopes and electronic structure
Practise Relative atomic mass. 14 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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Explain why the relative atomic mass of an element is usually not a whole number.
It is a weighted mean of the masses of isotopes with different mass numbers.
Bromine has two isotopes, bromine-79 and bromine-81. The two isotopes have almost equal abundances. The atomic number of bromine is 35. Suggest the approximate relative atomic mass of bromine.
80
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy2 marks
Relative atomic mass takes into account the different isotopes of an element.
(a) What is the relative atomic mass of an element? Tick (✓) one box.[1]
The mass of the most common isotope only
An average mass of the isotopes, taking into account their abundance
The total mass of all the isotopes added together
The mass of the heaviest isotope only
(b) An element has two isotopes, with mass numbers 63 and 65. The two isotopes have equal abundances. What is the relative atomic mass of the element? Tick (✓) one box.[1]
63
64
65
128
Show the answer and mark scheme
(a)Answer: An average mass of the isotopes, taking into account their abundance
(b)Answer: 64
Question 2Medium2 marks
Chlorine has two isotopes, chlorine-35 and chlorine-37. The relative atomic mass of chlorine is 35.5.
A student says: 'Because chlorine has isotopes with mass numbers 35 and 37, its relative atomic mass must be exactly 36.' Evaluate this claim.[2]
Show the answer and mark scheme
Answer: The student is wrong: relative atomic mass is a weighted average that depends on the abundance of each isotope, not simply the midpoint; since the actual value (35.5) is closer to 35, chlorine-35 must be the more abundant isotope.
relative atomic mass is a weighted average that depends on the (percentage) abundance of each isotope, not just the midpoint of the mass numbers
the actual value (35.5) is closer to 35, so chlorine-35 must be more abundant than chlorine-37
Question 3Hard7 marks
Boron has two isotopes, 10B and 11B. The relative atomic mass of boron is 10.8.
(a) Calculate the percentage abundance of 10B.[3]
(b) A student said: 'The relative atomic mass of boron should be 10.5 because boron has two isotopes.' Explain why the student is wrong.[2]
(c) Boron trifluoride, BF3, contains boron. Fluorine has only one isotope, 19F. Explain how many different relative formula masses BF3 molecules can have. Give the values.[2]
Show the answer and mark scheme
(a)Answer: 20%
abundance of 10B = x: 10x + 11(100 − x) = 1080
1100 − x = 1080
x = 20 (%)
(b)Answer: 10.5 would only be correct if the two isotopes were equally abundant; there is more 11B (80%), so the average is closer to 11.
10.5 would only be correct if both isotopes had equal abundance / 50% each
there is more 11B / 11B is 80%, so the mean is closer to 11
(c)Answer: Two, because boron has two isotopes: 67 and 68.
two, because boron has two isotopes (and fluorine has one)