The molecular mass of copper is 63.5g, whereas the mass number is always an integer. The reason for such variation is:A.Presence of impuritiesB.Presence of isobarsC.Presence of isotopesD.All of the above

D. All of the above

The reason for the variation in the mass number of copper (Cu) is due to the presence of isotopes. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Since the mass number of an atom is the sum of its protons and neutrons, different isotopes of an element will have different mass numbers.

Copper has two stable isotopes: copper-63 (which has 29 protons and 34 neutrons) and copper-65 (which has 29 protons and 36 neutrons). The average atomic mass of copper is calculated based on the relative abundances of these isotopes in a naturally occurring sample. This results in a molecular mass that is not a whole number but is approximately 63.5 g.