The atomic number (Z) represents the number of protons in an atom, so if the atom has an atomic number of 11, it means there are 11 protons.
The neutron number refers to the number of neutrons in the atom. In this case, the neutron number is given as 12. To determine the mass number (A), we need to sum the number of protons and neutrons. Therefore, the mass number can be calculated as follows:
Mass number (A) = Number of protons (Z) + Number of neutrons
A = 11 + 12 = 23
Therefore, the atom has a mass number of 23 and 11 protons.