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: Risperidone is an antipsychotic drug with high affinity at dopamine D2 and serotonin 5 - HT2 receptors .
### Output Text:
<span class="chemical">Risperidone</span> is an antipsychotic drug with high affinity at <span class="chemical">dopamine</span> D2 and <span class="chemical">serotonin 5 - HT2</span> receptors . 