Scientist in a lab are working with2 different types of mercury they know the different samples are different isotopes which propeetybof the isotopes must be different

The atomic weight

Explanation:

The property of isotopes that must be different is the atomic weight.

Isotopes are generally referred to as atoms of the same element with the same atomic number (proton number) but different neutron numbers.

Since the atomic weight of an atom is the sum of proton and neutron, atomsof the same element with different number of neutrons will definitely have different atomic weight.