Final answer:
A represents the mass number in a nuclear equation, which is the sum of the number of protons and neutrons in an atom's nucleus.
Step-by-step explanation:
In the given nuclear equation, A represents the mass number of an element in the periodic table, which is the total count of protons and neutrons (nucleons) in the nucleus of an atom. The mass number (A) is equal to the sum of the number of protons (Z) and the number of neutrons (N), expressed as A = N + Z. For instance, in the carbon nucleus with six protons and six neutrons, we have A = 12 because 6 protons (Z) plus 6 neutrons (N) equals 12 nucleons (A). This mass number is nearly equal to the mass of an atom in atomic mass units (u) because the mass of the nucleus is approximately the sum of the masses of its constituent protons and neutrons.