Periodic table › Calcium
1. Identity and classification
Silvery-white, lustrous, moderately soft metallic solid
- Name and symbol
- Calcium (Calcium) · Ca
- Atomic number
- 20
- Relative atomic weight / mass number
- 40.078 ± 0.004
- Group · period · block
- 2 · 4 · s
- Category
- Alkaline Earth Metal
- State near room temperature
- Solid
- Stable isotopes
- At least one stable isotope
- CAS number
- 7440-70-2
Square brackets [A] indicate a representative isotope mass number, not an average atomic weight. Neutron counts differ between isotopes.
2. Atomic structure and electron configuration
- Protons · electrons in a neutral atom
- 20 · 20
- Neutrons in reference isotope
- 20 (40Ca)
- Full electron configuration
1s² 2s² 2p⁶ 3s² 3p⁶ 4s²- Abbreviated configuration
[Ar]4s²- Electrons per shell
- 2, 8, 8, 2
- Listed oxidation states
- +2
- Electronegativity
- 1 (Pauling)
- First ionization energy
- 589.815 kJ/mol · 6.113 eV
- Electron affinity
- Not established
- Covalent radius
- 176 pm
- van der Waals radius
- 231 pm
Configurations of superheavy elements may include theoretical predictions. Different definitions of atomic radius are not interchangeable.
3. Physical and thermal properties
- Density
- 1.54 g/cm³
- Melting point
- 1115 K · 841.85 °C · 1547.33 °F
- Boiling point
- 1757 K · 1483.85 °C · 2702.93 °F
- Crystal structure
- FCC
- Specific heat
- 0.647 J/(g·K)
- Thermal conductivity
- 201 W/(m·K)
- Enthalpy of fusion
- 8.54 kJ/mol
- Enthalpy of vaporization
- 154.7 kJ/mol
- Magnetic ordering
- Paramagnetic
- Mohs hardness
- 1.75
- Speed of sound
- 3810 m/s
Density and structure depend on temperature, pressure and allotrope. 25°C is a room-temperature reference, not STP. Unestablished values are not zero.
4. Chemical behavior and compounds
The +2 oxidation state is common; beryllium compounds show substantial covalent character.
Selected compounds: CaO / Ca₂O₃ (Oxide) · CaClₙ (Chloride) · CaFₙ (Fluoride)
Reaction reference
Oxide layers form in air; the behavior depends on temperature and the metal.
Reactivity with water increases broadly down Group 2; beryllium and magnesium differ from the heavier members.
5. Isotopes
| Isotope | Status | Natural abundance | Half-life |
|---|---|---|---|
| 40Ca | Stable | 96.941% | Stable |
| 41Ca | Radioactive | — | 9.94E+4 Y |
| 42Ca | Stable | 0.647% | Stable |
| 43Ca | Stable | 0.135% | Stable |
| 44Ca | Stable | 2.086% | Stable |
| 46Ca | Stable | 0.004% | Stable |
| 48Ca | Radioactive | 0.187% | 2.9E19 Y |
Half-life units: s seconds, m minutes, h hours, d days, Y years. A dash means no natural-abundance value is given, not zero. Listed records are selected ground states; metastable states are distinct.
6. Discovery and occurrence
- Discovery
- Known since antiquity; no exact discovery year
- Scientists associated with discovery
- Known since antiquity
- Location
- United Kingdom (London)
- Origin
- Natural
- Listed crustal abundance
- Not established
Minerals and naming reference
Limestone / Calcite (CaCO₃), Gypsum, Dolomite, Fluorite
Named from scientific/historical roots associated with Calcium (Ca), discovered or characterized by Humphry Davy.
7. Applications
Human bones and teeth (hydroxyapatite), Portland cement & mortar, steelmaking flux, muscle contraction signaling
Detailed applications by field
- everyday
- Human bones and teeth (hydroxyapatite), Portland cement & mortar, steelmaking flux, muscle contraction signaling
- industry
- Used in alkaline earth metal processing, specialized alloys, and chemical manufacturing (Human bones and teeth (hydroxyapatite), Portland cement & mortar, steelmaking flux, muscle contraction signaling).
- electronics
- Applied in electronic components, semiconductors, or functional coatings where Calcium's electronic structure ([Ar] 4s²) provides target conductivity.
- medicine
- Diagnostic imaging, biomedical instrumentation, or pharmaceutical synthesis research.
- energy
- Utilized in power generation, battery materials, catalysts, or nuclear technology involving Calcium.
- aerospace
- High-performance materials, structural alloys, or space instrumentation utilizing Calcium.
- agriculture
- Soil geochemistry analysis, trace micronutrient research, or analytical environmental monitoring.
- science
- Spectroscopic standard, crystallography, and quantum chemistry studies of Period 4 s-block elements.
8. Biological role and safety
Hazard depends on the isotope, chemical form, dose and exposure route.
Elemental substances, ions, compounds and radioisotopes can have different health effects. This is an educational reference; laboratory work requires the material safety data sheet and trained supervision.
Biological role reference
Classified as essential in human and biological systems.