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: We generated beta2 ( - / - ) mice to investigate the role of beta2 in control of sodium channel density , localization , and function in neurons in vivo .
### Output Text:
We generated beta2 ( - / - ) mice to investigate the role of beta2 in control of <span class="chemical">sodium</span> channel density , localization , and function in neurons in vivo . 