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: In the preceding months , the patient had a number of admissions with transient unilateral hemiparesis with facial droop , and had been started on valproate for presumed hemiplegic migraine .
### Output Text:
In the preceding months , the patient had a number of admissions with transient unilateral hemiparesis with facial droop , and had been started on <span class="chemical">valproate</span> for presumed hemiplegic migraine . 