Final answer:
Index A in the formula ZX signifies the mass number, which is the total count of protons and neutrons in an atom's nucleus.
Step-by-step explanation:
In the context of chemistry, specifically nuclear chemistry, the index A refers to the mass number of a chemical element. The mass number A is the total number of protons and neutrons in an atom's nucleus. In the notation ZX, Z represents the atomic number which is the number of protons, and X is the symbol for the chemical element. To find the number of neutrons N, you can subtract the atomic number Z from the mass number A (N = A - Z). This is possible because for any element, once the symbol X is known, the atomic number Z is a constant and can be looked up in the periodic table.