Atomic number and mass number
3 marks · EasyDefine the term 'isotope'.
Give the meaning of 'mass number'.
Define 'atomic number'.
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(a) Atoms of the same element with different numbers of neutrons · must show same element, different numbers of neutrons
(b) The total number of protons and neutrons in the nucleus · must show protons and neutrons, in the nucleus
(c) The number of protons in the nucleus of an atom · must show number of protons
Common mistakes: Mixing up atomic and mass number definitions
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Marking points
B1 (a) · 1 mark · Defines isotope as atoms of the same element with different numbers of neutrons
B2 (b) · 1 mark · States that mass number is the total number of protons and neutrons in the nucleus
B3 (c) · 1 mark · Defines atomic number as the number of protons in (the nucleus of) an atom
Worked solution
a) Isotopes are atoms of the same element with different numbers of neutrons, but the same number of protons.
b) Mass number is the total number of protons and neutrons in the nucleus of an atom.
c) Atomic number is the number of protons in the nucleus of an atom.