The Rules:

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.