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: This electrophysiological parameter provides information about subclinical neurotoxic potential of thalidomide but is not helpful in predicting the appearance of sensory symptoms .
### Output Text:
This electrophysiological parameter provides information about subclinical neurotoxic potential of <span class="chemical">thalidomide</span> but is not helpful in predicting the appearance of sensory symptoms . 