Isotopes
Although every atom has equal numbers of protons and electrons, the number of neutrons can vary.
For example, every chlorine atom has 17 electrons and 17 protons; its atomic number is 17. However, three out of four chlorine atoms weigh 35 amu (17 protons and 18 neutrons) and the fourth weighs 37 amu (17 protons and 20 neutrons). These are the isotopes of chlorine. The atomic weight reported in the periodic table is the average weight of 35.5 amu.