write the chemical symbols for three different atoms or atomic anions with 18 electrons.

Explanation:

Step1: Recall neutral atom with 18 electrons

Argon (Ar) is a neutral atom with 18 electrons as its atomic number is 18.

Step2: Consider anions with 18 electrons

Chloride ion ($Cl^-$) is formed when a chlorine atom (atomic number 17) gains one electron, resulting in 18 electrons.

Step3: Find another anion

Sulfide ion ($S^{2 -}$) is formed when a sulfur atom (atomic number 16) gains two electrons, giving it 18 electrons.

Answer:

Ar, $Cl^-$, $S^{2 -}$