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: The 2 patients with the higher serum calcium concentrations received pamidronate intravenously ( 60 and 30 mg , respectively ) , which caused severe hypocalcemia .
### Output Text:
The 2 patients with the higher serum <span class="chemical">calcium</span> concentrations received <span class="chemical">pamidronate</span> intravenously ( 60 and 30 mg , respectively ) , which caused severe hypocalcemia . 