Atomic Number, Mass Number and Isotopes
Recap Atomic Number, Mass Number and Isotopes for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Isotopes have the same number of protons but different numbers of neutrons; practise calculating relative atomic mass from isotope abundances.
Free downloads
These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
The atomic number of an element is the number of protons in its atoms, and it defines which element the atom is. Every atom of a given element has the same atomic number.
The mass number is the total number of protons and neutrons. To find the number of neutrons, subtract the atomic number from the mass number. In the periodic table, the atomic number is the smaller value and the mass number the larger.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have identical chemical properties because chemical behaviour depends on electron arrangement, which is unchanged.
Revision notes
Atomic and mass number
Atomic number = number of protons. It defines the element.
Mass number = number of protons + number of neutrons. Neutrons = mass number − atomic number. In a neutral atom, electrons = protons.
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
They therefore have the same atomic number but different mass numbers. Chlorine-35 and chlorine-37 are the standard example.
Relative atomic mass
The relative atomic mass is an average value that accounts for the abundance of each isotope.
It is calculated as the sum of (isotope abundance × isotope mass number) divided by 100. This is why relative atomic masses are often not whole numbers.
Key points
- Atomic number is the number of protons.
- Atomic number defines the element.
- Mass number is protons plus neutrons.
- Neutrons = mass number − atomic number.
- Isotopes have the same protons but different neutrons.
- Isotopes have identical chemical properties.
Worked examples
Example 1
An atom has a mass number of 39 and an atomic number of 19. State its number of protons, neutrons and electrons. [3 marks]
Model answer
Protons = 19 (1 mark). Neutrons = 39 − 19 = 20 (1 mark). Electrons = 19, because the atom is neutral (1 mark).
Example 2
Explain why isotopes of an element have identical chemical properties. [2 marks]
Model answer
Isotopes have the same number of protons and therefore the same number of electrons (1 mark). Chemical properties depend on the arrangement of electrons, which is the same for all isotopes of an element (1 mark).
Example 3
Chlorine has two isotopes: 75 per cent chlorine-35 and 25 per cent chlorine-37. Calculate its relative atomic mass. [3 marks]
Model answer
Multiply each mass number by its abundance: (75 × 35) + (25 × 37) = 2625 + 925 = 3550 (2 marks). Divide by 100: 3550 ÷ 100 = 35.5 (1 mark).
Common mistakes
Subtracting the wrong way for neutrons.
Neutrons = mass number − atomic number, so the larger minus the smaller.
Saying isotopes have different chemical properties.
Their electron arrangement is identical, so their chemistry is the same.
Confusing atomic number with mass number.
The atomic number is the smaller value and equals the proton count.
Forgetting to divide by 100 in the relative atomic mass calculation.
The abundances are percentages, so the total must be divided by 100.
Exam tips
- Write out protons, neutrons and electrons separately.
- Remember chemistry depends on electrons, not neutrons.
- Show both stages of the relative atomic mass calculation.
- Check the atomic number is always the smaller of the two values.
Key terms
- Atomic number
- The number of protons in an atom.
- Mass number
- The total number of protons and neutrons.
- Isotope
- An atom with the same protons but different neutrons.
- Relative atomic mass
- An average mass accounting for isotope abundance.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.