Calcium

Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to its heavier homologues strontium and barium.
Symbol: Ca
Electron configuration: [Ar] 4s²
Atomic number: 20
Atomic mass: 40.078 u
Van der Waals radius: 231 pm
Electronegativity: 1
ref: Wikipedia