Atomic Number and Atomic Mass Number

The number of protons in an atom's nucleus fixes the chemical identity of the element and is called the atomic number, symbol Z. Since the positive charge on the nucleus comes entirely from its protons (total nuclear charge $= +Ze$), and an atom overall is electrically neutral, every atom must carry exactly Z electrons outside the nucleus as well (total electronic charge $= -Ze$). The total count of protons and neutrons together — the particles that actually contribute measurable mass — is called the atomic mass number, symbol A, and the neutron count itself is written N. These three quantities are related by $A = Z + N$, so $N = A - Z$. Because a proton and a neutron each weigh about 1 u while an electron's mass is negligible in comparison, the atomic mass in daltons works out numerically equal to the total nucleon count A. An atom's full composition — a specific combination of Z and A — is written using the element symbol X with …