The chemical element chlorine has 17 electrons, 17 protons and 18 neutrons. An atom, which is the fundamental unit of an element, contains an equal number of electrons and protons. The number of neutrons is calculated by subtracting the atomic number from the mass number.
The atomic number is the same as the number of protons while the mass number is the sum of the protons and neutrons. A chlorine atom has a mass number of 35. Subtracting 17 from 35 equals 18, which is the number of neutrons in a chlorine atom. In the periodic table, all elements are represented by their chemical symbol with the mass number written on the top-left portion and the atomic number on the bottom-left.