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: RATIONALE AND OBJECTIVES : The light - enhanced startle paradigm ( LES ) is suggested to model anxiety , because of the non - specific cue and the long - term effect .
### Output Text:
RATIONALE AND OBJECTIVES : The light - enhanced startle paradigm ( LES ) is suggested to model anxiety , because of the non - specific cue and the long - term effect . 