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: In addition to experiencing hypercalcemic episodes with peak calcium values of 2 . 7 to 3 . 8 mmol / L ( 10 . 7 to 15 . 0 mg / dL ) , patients in the hypercalcemic group exhibited a significant increase in the mean calcium concentration obtained during 6 months before the switch , compared with the mean value obtained during the 7 months of observation after the switch ( 2 . 4 + / - 0 . 03 to 2 . 5 + / - 0 . 03 mmol / L [ 9 . 7 + / - 0 . 2 to 10 . 2 + / - 0 . 1 mg / dL ] , P = 0 . 006 ) .
### Output Text:
In addition to experiencing hypercalcemic episodes with peak <span class="chemical">calcium</span> values of 2 . 7 to 3 . 8 mmol / L ( 10 . 7 to 15 . 0 mg / dL ) , patients in the hypercalcemic group exhibited a significant increase in the mean <span class="chemical">calcium</span> concentration obtained during 6 months before the switch , compared with the mean value obtained during the 7 months of observation after the switch ( 2 . 4 + / - 0 . 03 to 2 . 5 + / - 0 . 03 mmol / L [ 9 . 7 + / - 0 . 2 to 10 . 2 + / - 0 . 1 mg / dL ] , P = 0 . 006 ) . 