A certain element has only two naturally occurring isotopes: one with 18 neutrons and the other with 20 neutrons. The element forms I- charged ions when in ionic compounds. Predict the identity of the element. What number of electrons does the charged ion have?

[Cl-18]⁻ & [Cl-20]⁻

Explanation:

By definition isotopes are elements with the same number of protons by different number of neutrons. Elements X-18 & X-20 have 17 protons and represent Chlorine isotopes Cl-18 & Cl-20 each with 17 protons and 18 electrons to give the isotopes a -1 oxidation state. Both isotope of chlorine have 7 electron in its valence shell and 10 electrons in its core structure.  Gaining 1 electron fills the valence octet and establishes a -1 oxidation state.

the answer is na (sodium)