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: There was no significant difference in the frequency of signs or symptoms between the two groups although neurotoxicity symptoms presented mostly with lower scores of severity in group G .
### Output Text:
There was no significant difference in the frequency of signs or symptoms between the two groups although neurotoxicity symptoms presented mostly with lower scores of severity in group G . 