In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Oxidative stress is considered to play an important role in epileptogenesis and cognitive deficits , and antioxidants have a putative antiepileptic potential .
### Output Text:
Oxidative stress is considered to play an important role in epileptogenesis and cognitive deficits , and antioxidants have a putative antiepileptic potential . 