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

Explanation:

Isotopes of an element have the same atomic number (since atomic number defines the element) and same element name. Electric charge depends on electron gain/loss, not isotope difference. Isotopes differ in mass number (sum of protons and neutrons; neutrons vary). So the mass number must be different.

Answer:

the mass number