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: Mastodynia was reported by 21 patients , and physical examination revealed a light increase in breast firmness in 12 women and a moderate increase in breast nodularity in 2 women .
### Output Text:
Mastodynia was reported by 21 patients , and physical examination revealed a light increase in breast firmness in 12 women and a moderate increase in breast nodularity in 2 women . 