Diatomic elements are written as molecules:
I2, Br2, Cl2, F2, O2, N2, H2, P4, S8
When these form ions, they no longer exist as molecules! They form anions and combine with a cation to form an ionic compound.
We observe the law of conservation of mass by balancing equations using coefficients not by changing subscripts.
The state of all substances must be listed.
Ionic compounds are solids at room temperature.
If the reaction is occurring in water/solution, then an ionic compound will be aqueous (aq) or solid (s) - check your solubility table!
The state of a pure element is indicated by the colour on your periodic table.
In combustion reactions, CO2 and H2O are both formed as gases.