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: A cross - sectional evaluation of the effect of risperidone and selective serotonin reuptake inhibitors on bone mineral density in boys .
### Output Text:
A cross - sectional evaluation of the effect of <span class="chemical">risperidone</span> and selective <span class="chemical">serotonin</span> reuptake inhibitors on bone mineral density in boys . 