What element has 18 protons, 20 neutrons, and 18 electrons?

The element is Ar- 38. If you look at the periodic table, argon (Ar) has 18 protons and thus 18 electrons. We can use the number of neutrons (20 neutrons) to find the mass number of the element. The mass number of the element is the sum of the protons and the neutrons. So 18 +20 = 38. read more

The number of protons never changes and that is why they use them to define elements. Two things can change: number of neutrons and number of electrons. The number of neutrons defines an isotope an the number of electrons defines an ion. The number of neutrons+protons goes on the top left of the element letters. read more