Atomic number and mass number
4 marks · NormalAn atom of lithium has atomic number 3 and mass number 7.
State what the atomic number tells you about the atom.
Explain how to calculate the number of neutrons in this atom and give the answer.
Lithium has isotopes. Define 'isotope'.
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(a) Atomic number tells you the number of protons in the nucleus of an atom.
(b) Subtract atomic number from mass number: 7 – 3 = 4 neutrons.
(c) Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Common mistakes: Forgetting definition of isotope; calculation error
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Marking points
B1 (a) · 1 mark · States atomic number is the number of protons in the nucleus.
M1 (b) · 1 mark · Explains that number of neutrons = mass number – atomic number.
A1 (b) · 1 mark · Gives correct answer: 4 neutrons.
B2 (c) · 1 mark · Defines isotope: same number of protons but different numbers of neutrons.
Worked solution
a) The atomic number tells you the number of protons in the nucleus of the atom.
b) To calculate number of neutrons: mass number – atomic number = 7 – 3 = 4.
c) Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.