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: Four patients died in the perioperative period and one died 4 months later .
### Output Text:
Four patients died in the perioperative period and one died 4 months later . 