Every atom has the same number of electrons and protons. This number is equivalent to the atomic number of the element. Sodium has an atomic number of 11, which means that sodium has 11 protons and 11 electrons. The amount of neutrons in an atom is equivalent to the atomic number subtracted by the mass number. Sodium has a mass number of 23. Twenty-three minus eleven gives sodium a total of 12 neutrons. The atomic number of an element can be found on the upper-left hand corner of the element's symbol on the periodic table, while the mass number is located directly below the element's name.