Here's why:

* Atomic Mass Number: This number represents the total number of protons and neutrons in an atom's nucleus.

* Zinc: Zinc has an atomic number of 30, meaning it has 30 protons.

* Zinc-70: Since the atomic mass number is 70, and zinc always has 30 protons, this isotope has 40 neutrons (70 - 30 = 40).

Therefore, "zinc-70" specifically refers to the zinc isotope with 30 protons and 40 neutrons.