In short
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, so they have different mass numbers.
The element is fixed by its atomic number, the number of protons. Isotopes have the same chemical properties because they have the same electrons, but slightly different physical properties. Hydrogen has three isotopes: protium (no neutrons), deuterium (1 neutron) and tritium (2 neutrons). Chlorine occurs as chlorine-35 and chlorine-37, which is why its relative atomic mass is .
Calcium-40 has protons, electrons and neutrons. Calcium-44 has protons and neutrons.
Also searched as: calcium 40 protons neutrons electrons, the isotope deuterium of hydrogen has.
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