#"Sodium sulfide"#...sodium is a Group 1 metal, and thus has one valence electron to lose to give the #Na^+# ion. On the other hand, sulfur is a Group 16 atom, that is likely to pick up two electrons to form the #S^(2-)#...and thus we got #Na_2S#..
#"Caesium bromide"#...caesium is a Group 1 metal, and thus has one valence electron to lose to give the #Cs^+# ion. On the other hand, bromine is a Group 17 atom, that is likely to pick up ONE electrons to form the #Br^(-)#...and thus we got #CsBr#..
#"Magnesium nitride"#...magnesium is a Group 2 metal, and thus has two valence electron to lose to give the #Mg^(2+)# ion. Nitride anion is #N^(3-)#...i.e. it picks up THREE electrons to fill its valence shell...and so again the salt is electrically neutral, i.e. #Mg_3N_2#.
