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? the atomic number the electric charge the element name the mass number

Answer: the mass number.

Isotopes are atoms that differ only in the number of neutrons.

That means that isotopes of a same element will still have tha same atomic number (which is the number of protons), they are both neutral (do not have net electric charge) because the number of protons = the number of electrons for all the isotopes, and the name of the element is still mercury.

The isotope that has more neutrons will be heavier, because the mass of the atoms = number of protons + number of neutrons.

The answer is mass numbers