molecule既問題...................

The conversion from Cl atoms to Cl2­ molecules is a chemical reaction.
2Cl → Cl2
In the above reaction, the reactant is Cl atoms and the product (new substance) is Cl2 molecules.

Cl atom and Cl2 molecule are two different substances. Cl atom is much more reactive than Cl2 molecule. This is because each unbonded Cl atom has 7 electrons in the outermost shell, while each Cl atom in a Cl2 molecule has 8 (stable octet).
=

其實2粒chlorine bond埋係冇chemical reaction

只不過2粒share electrons

變成chlorine