write the chemical symbol for three different atoms or atomic cations with 18 electrons…?

Again I have no idea on how to do or start this problem. Any help will be appreciated! Thank you!

A neutral atom of argon has 18 electrons, so do the ions: K+, Ca2+ and Sc3+. Note, cations are metal ions with positive charges.

Its simple.

Argon is the only atom with 18 electrons. You can tell by checking out the periodic table.

Chlorine has 17 electrons. It preferably gains an electron and becomes an ion with 18 electrons.
But this ion is an “ANION (negative ion)”.

The possible cations (positive ions) with 18 electrons i can think of are potassium K+ and Calcium Ca++.

K has 19 electrons. It preferably loses an electron and finally has 18 and becomes K+
Ca has 20. Loses 2, finally has 18 and becomes Ca++.

The neutral atom which has 18 electrons will also have 18 protons, and this is Argon (Ar). Now, a chlorine atom has 17 protons and normally 17 electrons, but if you add an additional electron to it, you have a chloride ion (Cl-) which will also have 18 electrons. A third one is a potassium ion (K+). A K atom would have 19 electrons, but if you remove one to form a potassium ion, you are left with something with 18 electrons.