Question

Scientists in a lab are working with two different samples of the element mercury. They know that the different samples are different isotopes.

Which property of the isotopes must be different?

a.the atomic number
b.the electric charge
c.the element name
d.the mass number

Answer 1

The answer to this question is D, the mass number.
An isotope only changes the amount of neutrons, not the amount of protons. Since atomic number and the element name are dependent on the number of protons, those answer choices won't work. The electric charge can be different, but remember the wording of the question: it asks for what MUST be different, not what can. Since an isotope involves neutrons, it doesn't have to have a different electric charge. Therefore, since neutrons change the mass number, it is the only thing which must change if you have different isotopes.