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If I had a molecule of a single element such as atmospheric oxygen O2, and I added energy to decompose it into two separate oxygen atoms (i.e. O2 + ENERGY –– 2 O ) is this a chemical or physical change?

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  • $\begingroup$ Please see my response on chemical and physical changes chemistry.stackexchange.com/questions/131474/… $\endgroup$ – M. Farooq May 26 at 16:40
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    $\begingroup$ At an elementary level, your decomposition of O2 molecule into O atoms is a chemical change. $\endgroup$ – M. Farooq May 26 at 16:42
  • $\begingroup$ Thanks for the feedback. I assumed that this was a chemical reaction because the chemical formula is changing. It does seem the distinction between physical and chemical changes can get a bit nebulous though. $\endgroup$ – John May 26 at 17:14
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    $\begingroup$ Yes and in the long term this labeling is useless. $\endgroup$ – M. Farooq May 26 at 17:32
  • $\begingroup$ Something like physical and chemical "change" has only notion in some introductory textbooks. Chemical reaction is the important term. $\endgroup$ – Mithoron May 27 at 19:19
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It is a chemical change

The aspect of this that is probably confusing you is that both the product and the result in the case of an element are still a form of the element.

But elements can exist in different chemical forms and moving between them is still a chemical process. A relevant example is the reaction occurring mostly in the upper atmosphere that converts dioxygen (O2) to ozone (O3). This is a specific example of what happens when you add energy (in this case from UV light) to an oxygen molecule in the presence of other oxygen molecules: the split-apart oxygen atoms react to form ozone (which is more stable than a free oxygen atom). This is clearly a chemical change as the molecular structure is different, though ozone is still elementary oxygen so the element has not changed.

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