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: All patients were examined preoperatively and 39 were examined at six months ; 27 of the patients were also examined at one year , and 11 at two years .
### Output Text:
All patients were examined preoperatively and 39 were examined at six months ; 27 of the patients were also examined at one year , and 11 at two years . 