AQA GCSE Chemistry Foundation (8462), Foundation tier · Atomic structure and the periodic table › Atoms, isotopes and electronic structure
Practise Relative atomic mass. 8 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.
Relative atomic mass (Ar) is an average value for the atoms of an element that takes account of the abundance of each of its isotopes. You must be able to calculate it from percentage abundances: a common 2 to 3 mark calculation.
Key facts
Ar = average value for an element's atoms, taking account of the abundance of its isotopes
Ar = sum of (mass number × % abundance) ÷ 100
Ar has no units
Ar is nearer to the mass number of the most abundant isotope
Chlorine: 75% chlorine-35 + 25% chlorine-37 gives Ar = 35.5
Notes
What relative atomic mass means
Many elements exist as a mixture of isotopes, which have different masses.
The relative atomic mass (symbol Ar) of an element is an average value that takes account of the abundance (the percentage of atoms) of each of its isotopes.
Ar is the number shown for each element in the periodic table on the data sheet, e.g. chlorine 35.5. It has no units.
Ar is often not a whole number, because it is an average. It is closer to the mass number of the more abundant isotope.
Calculating Ar
diagram
Ar = (mass number × % abundance of the first isotope + mass number × % abundance of the second isotope + …) ÷ 100
Worked example: chlorine is 75% chlorine-35 and 25% chlorine-37. (35 × 75) + (37 × 25) = 2625 + 925 = 3550 3550 ÷ 100 = 35.5
Ar of chlorine is 35.5: nearer to 35, the more abundant isotope.
Sense check: the answer must lie between the smallest and largest mass numbers, and nearer to the mass number of the most abundant isotope.
If you are given the number of atoms of each isotope instead of percentages, divide by the total number of atoms instead of by 100.
How to answer each type of question
Calculate relative atomic mass from two isotopes
2 marksGrade 5
Multiply each mass number by its percentage abundance.
Add the results.
Divide by 100 and round as the question asks.
Example. Copper has two isotopes: copper-63 (69%) and copper-65 (31%). Calculate the relative atomic mass of copper. Give your answer to 1 decimal place.
Dividing by the number of isotopes (e.g. by 2) instead of by 100.
Adding the mass numbers and halving, which ignores the abundances.
Rounding part-way through the calculation, or giving the wrong number of decimal places.
Giving Ar units such as grams.
More tips
Memory tricks
Sense check: Ar must lie between the lightest and heaviest mass numbers.
If two isotopes are 50% each, Ar is exactly halfway between their mass numbers.
Type it in one go with brackets to avoid rounding errors: (35 × 75 + 37 × 25) ÷ 100.
For a real element, compare your answer with the Ar in the periodic table: it should be very close.
Exam technique
Write out the whole expression before you calculate, so you earn the method mark even if you slip on the arithmetic.
Give the answer to the number of decimal places the question asks for (often 1 decimal place).
In 'explain' questions about Ar, use the key words: average, isotopes, abundance.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
Explain what relative atomic mass meansAn average value for the atoms of an element that takes account of the abundance of each isotope.
Grade 5
Calculate Ar for two isotopesMultiply each mass number by its percentage abundance, add the results, then divide by 100.
Grade 5
Explain why Ar is rarely wholeIt is an average of isotopes with different mass numbers, weighted by how common each one is.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
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