mass number
Pronunciation
Noun
B2 Upper-intermediate
Quick answer
The total number of protons and neutrons in the nucleus of an atom.
Usage notes
Mass number is often confused with atomic number, which counts only the protons, not the neutrons as well.
Test yourself
Which word fits the gap? "Carbon-14 has a _____ of fourteen, six protons plus eight neutrons."
Answer: mass number
In your own words, what does "mass number" mean?
The total number of protons and neutrons in the nucleus of an atom.
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