The correct answer is [tex]NH_4Cl[/tex]. An ionic compound exists because of an electrostatic attraction between oppositely charged ions, cation and an anion. The cation in this case is [tex]NH_4^+[/tex] and the anion is [tex]Cl^-[/tex]. While a covalent compound is brought about by sharing of electrons between two or more atoms. [tex]CCl3[/tex] is a covalent compound between C and Cl. The compound is incomplete because the compound is unstable. Stability is obtained when the octet rule has been achieved.
[tex]Mg_2S[/tex] and [tex]Na(NO_3)_2[/tex] could be ionic compounds but they do not have the correct formula unit. [tex]Mg[/tex] has an oxidation state of +2 while [tex]S[/tex] has an oxidation state of -2, this means it needs one [tex]Mg[/tex] ion to have the correct formula unit. In comparison, [tex]NO_3^-[/tex] has a charge of -1 and there are 3[tex]NO_3^-[/tex] as opposed to one [tex]Na^+[/tex] ion. A correct formula unit will require 3 [tex]Na^+[/tex] ions.
