If any of these elements are FREE, that is ALL ALONE, then they are written with the subscripted 2 .
Example 1: 2H2 + O2 ---> 2H2O . The hydrogen and oxygen are FREE and get the subscripted two's. The water is a compound and its elements are NOT FREE. The 2 after the hydrogen is because of formula valence rules.
Example 2: 3H2 + N2 ---> 2NH3 . The hydrogen and nitrogen are FREE and get the subscripted two's. Ammonia is a compound and its elements are NOT FREE. No subscripted two's!
Example 3: NH4Cl (s) -----> NH4+1 (aq) + Cl-1 (aq)
The chloride is an aqueous ION and it is NOT FREE! [it has water molecules attached it (aq)]. NO subscripted two!
Example 4: HBr(g) ----> H+1(aq) + Br-1(aq). The hydrogen and bromine in the gas are NOT FREE and DO NOT get subscripted two's. The hydrogen aqueous ion and the bromide aqueous ion are NOT FREE. NO two's.