The number of protons in the nucleus of an atom determines its atomic number. Each element on the periodic table has a different number of protons in its nucleus and its own atomic number.
For instance, the element chlorine contains 17 protons in its nucleus, so it has an atomic number of 17. Depending on the layout of the specific periodic table, the atomic number is usually listed in either the top or bottom left corner. The other number that is given is the element's atomic mass or weight, which is a measurement of the total number of protons and neutrons in the nucleus. The atomic mass of chlorine is 35, which means that it has 18 neutrons in addition to its 17 protons.
While the atomic number technically refers to the number of protons in the atom's nucleus, it also tells how many electrons the atom contains outside the nucleus. This is because the number of electrons is always equal to the number of protons, as elements theoretically have no electrical charge. However, an atom can become either positively or negatively charged by gaining or losing an electron. When this happens, a plus or minus sign is put alongside the chemical symbol to designate it. A negatively charged chlorine molecule would be written as Cl-.