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: After stopping the beta - agonist , and after a week with the atenolol , the arrhythmia disappeared .
### Output Text:
After stopping the beta - agonist , and after a week with the <span class="chemical">atenolol</span> , the arrhythmia disappeared . 