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: BACKGROUND : Lithium remains a first - line treatment for the acute and maintenance treatment of bipolar disorder .
### Output Text:
BACKGROUND : <span class="chemical">Lithium</span> remains a first - line treatment for the acute and maintenance treatment of bipolar disorder . 