why there is a need to write a subscript with non-metals like O subscript 2 in a chemical reaction?
See, if you write O it means oxygen atom.
Oxygen does not exist as oxygen atom but as oxygen molecule.
You know that oxygen is a non- metal i.e. it is an element . In elemental state or existing as a non- metal, oxygen exists as oxygen molecule. SO, we write O2. Oxygen molecule is oxygen gas.
Similary . hydrogen element , hydrogen non - metal or hydrogen gas we write H2 and not H.
Also, chlorine element , chlorjne non - metal or chlorine gas we write H2 and not H.
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