Answer
Ammonium dichromate: (NH$_4$)$_2$Cr$_2$O$_7$
Ammonium nitrate: NH$_4$NO$_3$
Trinitrotoluene: C$_7$H$_5$N$_3$O$_6$
Work Step by Step
Ammonium dichromate is an ionic compound of ammonium, NH$_4^+$, and dichromate, Cr$_2$O$_7^{2-}$ ions. The ammonium ion has a charge of +1, but the dichromate ion has a charge of -2, so twice as many ammonium ions are needed to balance the charge. The ammonium ion is placed in parentheses and subscripted with a 2 to demonstrate that the ions are in a 2:1 ratio.
Ammonium nitrate is an ionic compound of ammonium and nitrate, NO$_3^-$ ions. They have the same charge so no balancing is needed, they will be in a 1:1 ratio.
Trinitrotoluene is a molecular compound. Each atom is simply subscripted with the number of each atom in one molecule.
