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: In contrast , mood switches were less frequent in patients receiving lithium ( 15 % , 4 / 26 ) than in patients not treated with lithium ( 44 % , 8 / 18 ; p = . 04 ) .
### Output Text:
In contrast , mood switches were less frequent in patients receiving <span class="chemical">lithium</span> ( 15 % , 4 / 26 ) than in patients not treated with <span class="chemical">lithium</span> ( 44 % , 8 / 18 ; p = . 04 ) . 