Final answer:
Calcium is the element whose ion exists solely as Ca2+, due to its configuration of two valence electrons that when lost, create an ion with a stable noble gas electron configuration.
Step-by-step explanation:
The element whose ion exclusively exists as a 2+ charge is calcium. Calcium is an alkaline earth metal and its most stable ion is Ca2+. This is because calcium has two valence electrons in its outer shell, and by losing both, it achieves a full outer electron configuration, which resembles that of the noble gases, providing it with stability. None of the other options provided (radon, lead, carbon, cesium) have ions that exist only as 2+. For instance, radon is a noble gas and does not readily form ions, lead can form multiple ions such as Pb2+ and Pb4+, carbon typically forms covalent compounds, and cesium as an alkali metal forms a 1+ ion.