Homework problem 2A.23 (7th edition) [ENDORSED]

[Caitlin_Murphy_3C]

This problem states:
On the basis of the expected charges on the monatomic ions, give the chemical formula of each of the following compounds: a) magnesium arsenide; b) Indium (III) sulfide; c) aluminum hydride; d0 hydrogen telluride; e) bismuth (III) fluoride.
How would you go about solving this problem? What is the definition of "monatomic"? What are the "expected charges" we are using to solve this problem with?
Thanks!

[Jeffrey Xiao 4A]

Expected charge refers to what the element's charge should theoretically be based on completing the octet of valence electrons. For a), since magnesium has 2 electrons in the s subshell, magnesium is two electron removals away from a complete octet so Mg2+ is the expected charge for magnesium and also arsenide is 3 electron gains away from a complete octet like krypton so As3- is the expected charge for arsenide.

Then with the expected charges, to get the chemical formula, the compound needs to be neutral so
Mg3As2 makes it neutral as 3 Mg ions is a +6 charge which is balanced by 2 As ions with a total of -6 charge.