The rule for determining the mass number of an atom or ion is; Mass number = atomic number + number of neutrons
The atomic number of an atom refers to the number of protons in the nucleus of the atom. It is the same as the number of electrons in the neutral atom.
The neutrons are found in the nucleus. The number of protons and neutrons in an atom is called the atomic number of that atom.
Hence, the rule for determining the mass number of an atom or ion is; Mass number = atomic number + number of neutrons
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