Relative atomic mass (Ar)
The weighted average mass of an atom of an element, taking into account its naturally occurring isotopes, relative to one twelfth of an atom of carbon 12
Relative molecular mass (Mr)
The mass of a molecule compared to one twelfth of an atom of carbon 12
Isotopes
Atoms of an element which have identical proton numbers but have a different number of electrons
Isotopes don’t affect the chemistry of an element, because all that’s different is the number of neutrons; the number and configuration of electrons is still the same
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