Periodic table › Caesium
1. Identity and classification
Soft, silvery-white, highly reactive metallic solid that tarnishes rapidly in air
- Name and symbol
- Caesium (Caesium) · Cs
- Atomic number
- 55
- Relative atomic weight / mass number
- 132.91 ± 0.01
- Group · period · block
- 1 · 6 · s
- Category
- Alkali Metal
- State near room temperature
- Solid
- Stable isotopes
- At least one stable isotope
- CAS number
- 7440-46-2
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
- 55 · 55
- Neutrons in reference isotope
- 78 (133Cs)
- Full electron configuration
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 6s¹- Abbreviated configuration
[Xe]6s¹- Electrons per shell
- 2, 8, 18, 18, 8, 1
- Listed oxidation states
- +1
- Electronegativity
- 0.79 (Pauling)
- First ionization energy
- 375.714 kJ/mol · 3.894 eV
- Electron affinity
- 45.541 kJ/mol
- Covalent radius
- 244 pm
- van der Waals radius
- 343 pm
Configurations of superheavy elements may include theoretical predictions. Different definitions of atomic radius are not interchangeable.
3. Physical and thermal properties
- Density
- 1.93 g/cm³
- Melting point
- 301.59 K · 28.44 °C · 83.192 °F
- Boiling point
- 944 K · 670.85 °C · 1239.53 °F
- Crystal structure
- BCC
- Specific heat
- 0.242 J/(g·K)
- Thermal conductivity
- 35.9 W/(m·K)
- Enthalpy of fusion
- 2.09 kJ/mol
- Enthalpy of vaporization
- 63.9 kJ/mol
- Magnetic ordering
- Paramagnetic
- Mohs hardness
- 0.2
- Speed of sound
- 1090 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
Usually forms a +1 cation in ionic compounds.
Selected compounds: CsO / Cs₂O₃ (Oxide) · CsClₙ (Chloride) · CsFₙ (Fluoride)
Reaction reference
Readily oxidized in air; storage excludes air and moisture.
Reacts with water to form hydrogen and an alkaline hydroxide solution; vigor varies with the metal.
5. Isotopes
| Isotope | Status | Natural abundance | Half-life |
|---|---|---|---|
| 133Cs | Stable | 100% | Stable |
| 134Cs | Radioactive | — | 2.0652 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
- 1860 CE
- Scientists associated with discovery
- Robert Bunsen & Gustav Kirchhoff
- Location
- Germany (Heidelberg)
- Origin
- Natural
- Listed crustal abundance
- Not established
Minerals and naming reference
Pollucite (Cs(Si₂Al)O₆·nH₂O)
Named from scientific/historical roots associated with Caesium (Cs), discovered or characterized by Robert Bunsen & Gustav Kirchhoff.
7. Applications
SI definition of the second (¹³³Cs hyperfine transition at 9,192,631,770 Hz in atomic clocks), high-density cesium formate oil-drilling fluids
Detailed applications by field
- everyday
- SI definition of the second (¹³³Cs hyperfine transition at 9,192,631,770 Hz in atomic clocks), high-density cesium formate oil-drilling fluids
- industry
- Used in alkali metal processing, specialized alloys, and chemical manufacturing (SI definition of the second (¹³³Cs hyperfine transition at 9,192,631,770 Hz in atomic clocks), high-density cesium formate oil-drilling fluids).
- electronics
- Applied in electronic components, semiconductors, or functional coatings where Caesium's electronic structure ([Xe] 6s¹) provides target conductivity.
- medicine
- Diagnostic imaging, biomedical instrumentation, or pharmaceutical synthesis research.
- energy
- Utilized in power generation, battery materials, catalysts, or nuclear technology involving Caesium.
- aerospace
- High-performance materials, structural alloys, or space instrumentation utilizing Caesium.
- agriculture
- Soil geochemistry analysis, trace micronutrient research, or analytical environmental monitoring.
- science
- Spectroscopic standard, crystallography, and quantum chemistry studies of Period 6 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 non-essential in human and biological systems.