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The definition of an atomic number is the number of protons in the nucleus of an atom of an element. Three for lithium and 20 for calcium are each an example of an atomic number.
The mass number is the count of these particles and so the mass number is very close to the atomic mass. Atomic number. Atomic number refers to the number of protons in nucleus. It is often denoted using the symbol Z. The number of electrons in the neutral atom is equal to the number of protons in the nucleus and therefore equal to the atomic number of the atom.