Answer
18 protons, 18 electrons, and 22 neutrons.
Work Step by Step
Because Argon is the 18th element on the periodic table, it has 18 protons. There is no charge indicated in this example, so the number of electrons must equal the number of protons. Finally, the mass number is 40, and because mass number is the combined number of protons and neutrons, there must be 22 neutrons (40-18=22).
