The atomic number of an element is the number of protons that are present in its nucleus. The exact atomic mass of an element is derived from the weighted average of the mass of its different isotopes. For a particular isotope, the approximate atomic mass is the sum of the mass of the protons and the mass of the neutrons in an atom of the element's nucleus. Though the mass of the neutron is more than that of the proton, they are considered to be equal and the mass of one particle is denoted by amu or atomic mass unit.
For chromium, the atomic number is 24. Therefore there are 24 protons in the nucleus of a chromium atom. The number of electrons in an atom is equal to the number of protons as atoms are electrically neutral. An atom of chromium has 24 electrons. The atomic weight of chromium is approximately equal to 52. As this comprises the mass of protons and neutrons and the number of protons is 24, the number of neutrons is equal to 52 - 24 = 28.