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: On the other hand , apoptotic marker , synaptic markers , mitochondrial activity and Ca ( 2 + ) levels remained unaffected .
### Output Text:
On the other hand , apoptotic marker , synaptic markers , mitochondrial activity and <span class="chemical">Ca</span> ( 2 + ) levels remained unaffected . 