Mass Number, Atomic Number and Isotopes
Get to grips with Mass Number, Atomic Number and Isotopes for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. The atomic number is the number of protons, the mass number counts protons and neutrons, and isotopes differ only in their number of neutrons.
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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 atom is the number of protons in its nucleus. It defines which element the atom is, so all atoms of a given element have the same atomic number.
The mass number is the total number of protons and neutrons. The number of neutrons is therefore the mass number minus the atomic number.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers. Because chemical behaviour depends on electrons, isotopes of an element are chemically identical, but their nuclear stability can differ, which is why some are radioactive.
Revision notes
Atomic and mass number
Atomic number = number of protons. It defines the element.
Mass number = protons + neutrons. Neutrons = mass number − atomic number. In a neutral atom the number of electrons equals the number of protons.
Isotopes
Isotopes have the same number of protons but different numbers of neutrons.
They have the same atomic number but different mass numbers. Carbon-12 and carbon-14 are the standard example, both with 6 protons but with 6 and 8 neutrons respectively.
Nuclear notation
An atom is written with the mass number above and the atomic number below the element symbol.
So carbon-14 is written with 14 above and 6 below the C. Reading these two numbers correctly is essential for nuclear equations.
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 are chemically identical.
Worked examples
Example 1
An atom has a mass number of 23 and an atomic number of 11. State its numbers of protons, neutrons and electrons. [3 marks]
Model answer
Protons = 11 (1 mark). Neutrons = 23 − 11 = 12 (1 mark). Electrons = 11, because the atom is neutral (1 mark).
Example 2
Define an isotope. [2 marks]
Model answer
Isotopes are atoms of the same element with the same number of protons (1 mark) but different numbers of neutrons (1 mark).
Example 3
Explain why isotopes of an element behave identically in chemical reactions. [2 marks]
Model answer
Isotopes have the same number of protons and therefore the same number of electrons (1 mark), and chemical behaviour depends on the electrons rather than the neutrons (1 mark).
Common mistakes
Subtracting in the wrong order for neutrons.
Neutrons = mass number − atomic number, so larger minus smaller.
Saying isotopes have different atomic numbers.
They have the same atomic number and different mass numbers.
Confusing the two numbers in nuclear notation.
Mass number goes on top, atomic number below.
Saying isotopes behave differently chemically.
They are chemically identical; only their nuclear stability differs.
Exam tips
- Write protons, neutrons and electrons out separately.
- Remember the mass number is always the larger figure.
- Explain chemical identity through equal electron numbers.
- Learn the nuclear notation layout for the next topic.
Key terms
- Atomic number
- The number of protons in a nucleus.
- Mass number
- The total number of protons and neutrons.
- Isotope
- An atom with the same protons but different neutrons.
- Nuclear notation
- Writing mass number above and atomic number below the symbol.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.