Input (grade 10): The number of neutrons, N, is known as the neutron number of the atom; thus, A = Z + N/O. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes), the atomic mass of an atom is roughly equal to A.
Output (grade 7): In chemistry and physics, the atomic number of an atom is the number of protons in the atom.