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: OBJECTIVES : To assess changes induced by lithium maintenance therapy on the incidence of thyroid , parathyroid and ion alterations .
### Output Text:
OBJECTIVES : To assess changes induced by <span class="chemical">lithium</span> maintenance therapy on the incidence of thyroid , parathyroid and ion alterations . 