Answer:
Argon (Ar)
Explanation:
Chlorine has an atomic number of 17. That means that it has 17 protons and 17 electrons, thus being electrically neutral. When it gains 1 electron to form the chloride ion, the total number of electrons is 17 + 1 = 18.
Argon is a noble gas with an atomic number of 18, which means it has 18 electrons.
