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: A frequency of bradycardia of 50 % was noted in the control group , but this was not significantly different from the frequency with the active drugs .
### Output Text:
A frequency of bradycardia of 50 % was noted in the control group , but this was not significantly different from the frequency with the active drugs . 