The mass number of a nucleus is

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sometimes more than and sometimes equal to its atomic number Mass number of a nucleus is defined as the sum of the number of neutron and protons present in the nucleus, i.e. the number of nucleons in the nucleus, whereas atomic number is equal to the number of protons present. Therefore, the atomic number is smaller than the mass number. But in the nucleus (like that of hydrogen 1H​1), only protons are present. Due to this, the mass number is equal to the atomic number.

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