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GCSE Chemistry · key term
The total number of protons and neutrons in the nucleus of one atom.
Used by AQA, Edexcel and OCR
Specification 4.1.1.5, 4.1.1.6 · Paper 1
Revise: Atomic Size and MassCarbon-12 has 6 neutrons, whereas carbon-14 has 8. Their different neutron numbers give them different mass numbers without changing which element they are.
Specification 1.7, 1.9, 1.10, 1.11, 1.12, 1.23 · Paper 1, Paper 2
The total number of protons and neutrons in the nucleus of a particular atom.
Revise: Mass NumberThe mass number therefore counts the protons and neutrons only. It does not include electrons.
The total number of protons and neutrons in the nucleus of an atom.
The total number of protons and neutrons in the nucleus of an atom or ion.
Specification C1.2e · Paper 1 (Foundation), Paper 3 (Higher)
The total number of protons and neutrons in an atom's nucleus.
Revise: Subatomic CalculationsA mass number belongs to one particular isotope and is a whole number. It should not be confused with the relative atomic mass commonly displayed on a periodic table, which can be a decimal because it takes account of the element's isotopes.
Paper 1 (Foundation), Paper 2 (Foundation), Paper 3 (Higher), Paper 4 (Higher)
Revise: Calculating particle numbersNotice that electrons are not included in the mass number.
The total number of protons and neutrons in the nucleus of a particular atom or isotope.
Read nuclear symbols carefully: the mass number is the upper number and the atomic number is the lower number.
Do not confuse mass number with relative atomic mass. A mass number counts particles in one particular atom and is a whole number.
Do not find an ordinary mean of the isotope mass numbers unless the isotopes are equally abundant.
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