These are five atoms that are important in living things. The nucleus of each atom has a characteristic number of protons, which are positively charged, and neutrons, which are electrically neutral. Each atom of a particular element has the same unique number of protons. This is the element's atomic number. The number of particles in the nucleus-- protons plus neutrons-- is called the mass number. Most elements have two or more isotopes, whose atoms vary a bit in neutron number and mass number.