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: Tic disorders can be effectively treated by atypical antipsychotics such as risperidone , olanzapine and ziprasidone .
### Output Text:
Tic disorders can be effectively treated by atypical antipsychotics such as <span class="chemical">risperidone</span> , <span class="chemical">olanzapine</span> and <span class="chemical">ziprasidone</span> . 