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: This report summarizes our experience in switching bipolar patients from lithium to divalproex sodium to alleviate such cognitive and functional impairments .
### Output Text:
This report summarizes our experience in switching bipolar patients from <span class="chemical">lithium</span> to <span class="chemical">divalproex sodium</span> to alleviate such cognitive and functional impairments . 