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: Seven different life - threatening toxicities were observed in patients while receiving treatment .
### Output Text:
Seven different life - threatening toxicities were observed in patients while receiving treatment . 