Periodic Table
Elements and Compounds

Why do isotopes of the same element have the same atomic number but different mass number?



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An element's atomic number refers to the number of protons in its nucleus, and this number never varies - an atom with a different number of protons becomes a different element; i.e. an element with one proton is called hydrogen, an element with 2 protons is helium, 3 protons means it's lithium etc). Usually there are the same number of neutrons as there are protons, but this number can vary, and when an element doesn't have the standard number of neutrons it's called an isotope. The mass number of an element is the sum of the number of protons plus neutrons, which is why this number can is different for different isotopes.