synthesis or composition:
4Na+O2---> 2Na2O
decomposition:
2Na2O--->4 Na + O2
In your fake example you list 2 L's. If both had L, they would not combine.
4Fe+ 3O2--->2 Fe2O3, formation of rust, iron oxide
CO2+ H2O---> H2CO3, carbonic acid synthesis
H2CO3---> CO2+ H2O, decomposition of...