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: These mechanisms are not well understood because , in most cases , the patients were not monitored at the moment of the event .
### Output Text:
These mechanisms are not well understood because , in most cases , the patients were not monitored at the moment of the event . 