Calcium

Calcium is atom number 20 — every Calcium atom has exactly 20 protons in its center, and that number is what makes it Calcium and nothing else. Its symbol is Ca. At room temperature it is a solid.

Calcium is a chemical element with symbol Ca and atomic number 20. Calcium is a soft gray alkaline earth metal, fifth-most-abundant element by mass in the Earth's crust. The ion Ca2+ is also the fifth-most-abundant dissolved ion in seawater by both molarity and mass, after sodium, chloride, magnesium, and sulfate.

Summary from the source dataset, derived from Wikipedia (CC BY-SA 3.0) · original article

Calcium sits in period 4, group 2 of the s-block. Its 20 electrons fill shells of 2, 8, 8, 2 — written as [Ar] 4s2. Its electronegativity is 1 on the Pauling scale, and pulling the first electron off takes 589.8 kJ/mol. The outermost shell holds 2 electrons — the ones that do all the chemistry.

Full configuration: 1s2 2s2 2p6 3s2 3p6 4s2
Ionization energies (kJ/mol): 589.8, 1145.4, 4912.4, 6491, 8153, 10496, 12270, 14206 … (20 published)
Electron affinity: 2.37 kJ/mol · Electronegativity (Pauling): 1
Density: 1.55 g/cm³ · melts 1115 K · boils 1757 K

Values exactly as published — see where the data comes from.