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: The results indicate that rat atrial spontaneous beating is more dependent on [ Na ] o than on [ Ca ] o in a range of + / - 50 % of their normal concentration .
### Output Text:
The results indicate that rat atrial spontaneous beating is more dependent on [ <span class="chemical">Na</span> ] o than on [ <span class="chemical">Ca</span> ] o in a range of + / - 50 % of their normal concentration . 