#20 / 118
20Ca40.078
Alkaline Earth Metalssolid at STPS-block

Calcium

Group: 2Period: 4Standard Atomic Weight: 40.078 u

Why is Calcium in this position?

Understanding the scientific rationale behind Calcium's position in the periodic table:

Group Assignment
Group 2

Belongs to Group 2 because it has 2 valence electrons in its outer s-orbital (4s²).

Period Assignment
Period 4

Belongs to Period 4 because its outermost occupied shell is n=4.

Orbital Block
S-block

Belongs to the s-block because the differentiating electrons fill the 4s subshell.

Chemical Category
Alkaline Earth Metals

Classified as an alkaline earth metal due to its basic oxide and standard +2 oxidation state.

Atomic Structure & Bohr Shell Model

Shell Distribution: [2, 8, 8, 2]

Bohr Atomic Shell Model(2, 8, 8, 2)

Hover or tap any shell orbit ring to inspect electron counts and 2n² capacities.

20 Protons (p⁺)20 Neutrons (n⁰)
KShell K: Electron 1 of 2Shell K: Electron 2 of 2LShell L: Electron 1 of 8Shell L: Electron 2 of 8Shell L: Electron 3 of 8Shell L: Electron 4 of 8Shell L: Electron 5 of 8Shell L: Electron 6 of 8Shell L: Electron 7 of 8Shell L: Electron 8 of 8MShell M: Electron 1 of 8Shell M: Electron 2 of 8Shell M: Electron 3 of 8Shell M: Electron 4 of 8Shell M: Electron 5 of 8Shell M: Electron 6 of 8Shell M: Electron 7 of 8Shell M: Electron 8 of 8NShell N: Electron 1 of 2Shell N: Electron 2 of 2CaZ = 20

Educational Note: This Niels Bohr planetary model visually illustrates principal quantum energy shells ($n=1, 2, 3\dots$) and electron counts. In modern quantum mechanics (Schrödinger model), electrons do not orbit in fixed circular planetary tracks, but exist as 3D probability clouds (orbitals: $s, p, d, f$) governed by the Heisenberg uncertainty principle.

Electron Shell Filling Breakdown

Shell K (n=1):2 / 2 electrons (100%)
Shell L (n=2):8 / 8 electrons (100%)
Shell M (n=3):8 / 18 electrons (44%)
Shell N (n=4):2 / 32 electrons (6%)
Aufbau Electron Configuration
1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

Neutral ground state configuration. Valence electrons: 2.

Atomic & Quantum Properties

Electronegativity (Pauling)1 Pauling
1st Ionization Energy589.8 kJ/mol
Electron Affinity-2.37 kJ/mol
Atomic Radius (empirical)194 pm
Common Oxidation States+2
Crystal StructureFCC

Physical & Thermal Properties

Density at STP1.55 g/cm³
Melting Point842 °C (1115 K)
Boiling Point1484 °C (1757 K)
Magnetic OrderingDiamagnetic
Discovery Year1808
Discovered ByHumphry Davy

Real-World Uses, Occurrence & Compounds

Major Industrial & Everyday Uses
  • Construction cement, concrete, mortar, and plaster (limestone / quicklime)
  • Dietary supplements and milk calcium for bone density
  • Steelmaking deoxidizer and desulfurizer agent
  • Cheese manufacturing, brewing water hardness regulation, and winemaking
Occurrence in Nature

Massive geologic deposits of limestone, marble, and chalk (CaCO₃), gypsum (CaSO₄·2H₂O), and fluorite (CaF₂).

Etymology & Name Origin

From Latin 'calx' meaning lime or limestone

Important Chemical Compounds
CaCO₃ (Calcium carbonate)
CaO (Quicklime)
Ca(OH)₂ (Slaked lime)
CaSO₄·2H₂O (Gypsum)
CaCl₂ (Calcium chloride)
Interesting Chemical Facts
  • Over 99% of the calcium in the human body is stored inside bones and teeth as hydroxyapatite crystals.
  • The White Cliffs of Dover in England are composed almost entirely of microscopic calcium carbonate coccolith shells.
  • Calcium ions (Ca²⁺) serve as essential intracellular signaling messengers triggering muscle contraction and neurotransmitter release.
Safety & Handling Note

Calcium metal reacts with water generating heat and flammable hydrogen; quicklime (CaO) causes severe chemical burns on contact with moisture.