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: Lithium - associated cognitive and functional deficits reduced by a switch to divalproex sodium : a case series .
### Output Text:
<span class="chemical">Lithium</span> - associated cognitive and functional deficits reduced by a switch to <span class="chemical">divalproex sodium</span> : a case series . 