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: Six patients received a normal - sodium diet and four a low - sodium ( 17 mmol / day ) diet .
### Output Text:
Six patients received a normal - <span class="chemical">sodium</span> diet and four a low - <span class="chemical">sodium</span> ( 17 mmol / day ) diet . 