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Mass Number and Isotopes
The mass number, Ar, of an atom is the total number of protons and neutrons in the nucleus of
one atom of the element.
The atomic (proton) number, Z, of an atom is the number of protons in the nucleus of an atom.
The atomic number is also equal to the number of electrons in an atom.
Isotopes are atoms of the same element with the same atomic number but with different
The chemical properties of isotopes are almost identical as they have the same electronic
configuration as one another.
The only differences between isotopes are their physical properties, such as their rates of
diffusion as they depend on the mass of the atom, and in nuclear properties such as
radioactivity and the ability to absorb neutrons.
Different isotopes of the same element also have slightly different boiling points.