As a short-hand way of writing information about both nuclei and nuclear particles we have developed the following system. The symbol of the element is placed in the center and then numbers are placed around the symbol at the four possible corners. The atomic number (the number of protons) is always placed at the bottom left. The mass number (the sum of the protons and neutrons) is placed at the top left. The charge is placed at the top right, and the number of atoms present is placed at the bottom left. Of course, not all of these positions are occupied at all times, as the examples below show.