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: Therapeutic ranges for antidepressants that have been derived from general adult population studies may not be appropriate for the elderly .
### Output Text:
Therapeutic ranges for <span class="chemical">antidepressants</span> that have been derived from general adult population studies may not be appropriate for the elderly . 