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: OBJECTIVES : The present study was designed to answer these questions , using a rodent model of early childhood Pb exposure and treatment with succimer , a widely used chelating agent for the treatment of Pb poisoning .
### Output Text:
OBJECTIVES : The present study was designed to answer these questions , using a rodent model of early childhood <span class="chemical">Pb</span> exposure and treatment with <span class="chemical">succimer</span> , a widely used chelating agent for the treatment of Pb poisoning . 