Why do all atoms of an element have the same atomic number, although they may have different mass numbers?. What do we call atoms of the same element with different mass numbers?

(1) Answers

The isotopes of an element differ in the number or neutrons whereas the number of protons or electrons of a neutral atom stays the same. The molar mass of the element is based on the percent abundances of different isotopes and the individual molar mass of each isotope. 

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