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An element's atomic number helps to identify an element from others, and is equal to the number of protons an element has.
Each element on the periodic table has the same number of electrons as its atomic number.
Why can't an element be identified by the number of electrons or neutrons it has?
For guidance in answering the above question or for more information on atomic number, look here.
Mass Number and Isotopes
While an atom of an element may be identified by the number of protons it has, two atoms of the same element may not necessarily have the same mass.
This is due to variation in isotopes, or atoms with varying numbers of neutrons.
An atom's mass number is equal to the sum of the number of protons and neutrons.
Thus, if you are given the identity of an atom and its mass number, explain how you can determine how many protons and neutrons it has.