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: Group 2 received diazepam alone followed by vehicle .
### Output Text:
Group 2 received <span class="chemical">diazepam</span> alone followed by vehicle . 