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: Of the 262 patients , 14 ( 18 % ) of 76 who had stopped treatment at the time of the study experienced documented adverse effects .
### Output Text:
Of the 262 patients , 14 ( 18 % ) of 76 who had stopped treatment at the time of the study experienced documented adverse effects . 