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: PATIENTS : One hundred and one patients ( 28 men and 73 women ) with bipolar disorder receiving lithium maintenance therapy ranging from 1 year ' s to 32 years ' duration .
### Output Text:
PATIENTS : One hundred and one patients ( 28 men and 73 women ) with bipolar disorder receiving <span class="chemical">lithium</span> maintenance therapy ranging from 1 year ' s to 32 years ' duration . 