Chemical change - decomposing single element?
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.