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: Maternal lithium and neonatal Ebstein ' s anomaly : evaluation with cross - sectional echocardiography .
### Output Text:
Maternal <span class="chemical">lithium</span> and neonatal Ebstein ' s anomaly : evaluation with cross - sectional echocardiography . 