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All atoms have a mass no which is derived from calculating the sum of the total no of neutrons and protons in an atomic nucleus of a particular atom.
mass no=no of protons+no of neutrons
1 5 1
Mass number is the number of protons + neutrons in an element.Different elements have different mass numbers.Sometimes they are same also for some isotopes. It is denoted By Z. Eg= hygrogen has 1 mass number, helium has 4 mass number.