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 24 hour excretion of iodine was 3657 ( mcg ( 25 - 756 ( mcg ) .
### Output Text:
The 24 hour excretion of <span class="chemical">iodine</span> was 3657 ( mcg ( 25 - 756 ( mcg ) . 