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 mass number

Explanation:

Isotopes are numerous forms of one single element. In simple terms, atomic weights are different for the isotopes. Isotopes refer to the atoms that have the same number of protons but different neutron numbers. The physical properties of the isotopes vary because these properties also based on mass. These variations can be used to distinguish isotopes of an element from each other by applying methods such as fractional distillation and diffusion.

why do the isotopes of a particular element behave differently in nuclear reactions but the same in chemical reactions? chemical reactions involve changes to electrons in an atom; isotopes do not differ in electrons, and so they react the same way during chemical changes.