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?
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.