spore: profile
id: caesium
spore_version: 0.2
family: alkali-and-alkaline-earth-metals
name: Caesium
abbreviation: Cs
role: atomic-clock and ion-thruster element
category: metals
type: heavy-metals
summary: |
  Caesium (cesium, Cs) is a soft alkali metal that melts just above room temperature, at 28.44 C in PubChem's record. It is pyrophoric in air and reacts violently with water. Its most important use is as the clock element: the SI second is defined by the caesium-133 hyperfine transition, fixed at exactly 9 192 631 770 Hz. Caesium also fed contact-ionization ion thrusters, as NASA Lewis reported in 1967, and its antimonide makes visible-light photocathodes. Rubidium, its neighbour in the periodic table, is covered in a short section.
best_for: Frequency standards and atomic clocks, where the caesium-133 hyperfine transition defines the SI second
weakest_at: 'Handling: it ignites spontaneously in air and reacts violently with water, and it melts at 28 C, so it stays liquid in a warm room'
identity:
  formula: Cs
  cas: 7440-46-2
  source: pubchem-cesium
  where: Names and Identifiers > Other Identifiers > CAS (compound record, CID 5354618)
tags:
  environments: []
  stresses: []
  applications:
    - propulsion-and-tanks
    - electronics-and-chips
    - optics-and-cameras
properties:
  - benchmark: melting_point
    label: Melting point (PubChem)
    value: 28.44
    unit: °C
    scope: material
    conditions:
      basis: melting point as printed by PubChem; no pressure given
    method: Experimental value listed by PubChem
    status: ground
    note: PubChem also prints 28.5 °C and 29 °C for the same property. The Strem supplier sheet gives 28.64 °C (next row).
    source: pubchem-cesium
    where: Chemical and Physical Properties > Experimental Properties > Melting Point
  - benchmark: melting_point
    label: Melting point (supplier sheet)
    value: 28.64
    unit: °C
    scope: material
    conditions:
      form: 99.9+% prescored ampoule, Strem product 55-5551
    method: Supplier safety data sheet value
    status: ground
    source: strem-cs-sds
    where: Section 9, Physical and Chemical Properties
  - benchmark: boiling_point
    label: Boiling point (PubChem)
    value: 671
    unit: °C
    scope: material
    conditions:
      pressure: 760 mmHg
    method: Experimental value listed by PubChem
    status: ground
    note: PubChem also prints 685 °C without a pressure. The supplier sheet gives 690 °C (next row).
    source: pubchem-cesium
    where: Chemical and Physical Properties > Experimental Properties > Boiling Point
  - benchmark: boiling_point
    label: Boiling point (supplier sheet)
    value: 690
    unit: °C
    scope: material
    conditions:
      form: 99.9+% prescored ampoule, Strem product 55-5551
    method: Supplier safety data sheet value
    status: ground
    source: strem-cs-sds
    where: Section 9, Physical and Chemical Properties
  - benchmark: density
    label: Density, solid at 17 C
    value: 1892
    unit: kg/cu m
    scope: material
    conditions:
      temperature: 17 C
      phase: solid
    method: Experimental value listed by PubChem, printed in kg/cu m
    status: ground
    source: pubchem-cesium
    where: Chemical and Physical Properties > Experimental Properties > Density
  - benchmark: density
    label: Density, liquid at 40 C
    value: 1827
    unit: kg/cu m
    scope: material
    conditions:
      temperature: 40 C
      phase: liquid
    method: Experimental value listed by PubChem, printed in kg/cu m
    status: ground
    note: Solid and liquid are measured at different temperatures, so the two values are not a like-for-like comparison.
    source: pubchem-cesium
    where: Chemical and Physical Properties > Experimental Properties > Density
  - benchmark: specific_gravity
    label: Specific gravity (supplier sheet)
    value: 1.892
    unit: '-'
    scope: material
    conditions:
      basis: specific gravity as printed; no temperature given
    method: Supplier safety data sheet value
    status: ground
    source: strem-cs-sds
    where: Section 9, Physical and Chemical Properties
  - benchmark: vapor_pressure
    label: Vapor pressure, solid at 25 C
    value: 0.0000015
    unit: mmHg
    scope: material
    conditions:
      temperature: 25 C
      phase: solid
    method: Experimental value listed by PubChem
    status: ground
    note: PubChem also prints this as 1.5X10-6 mm Hg at 25 C (solid).
    source: pubchem-cesium
    where: Chemical and Physical Properties > Experimental Properties > Vapor Pressure
  - benchmark: hyperfine_transition_frequency
    label: Caesium-133 ground-state hyperfine transition frequency
    value: 9192631770
    unit: Hz
    scope: material
    conditions:
      isotope: 133Cs, unperturbed ground state
    method: Exact value fixed by the SI definition of the second (the value is a defined constant, not a measurement)
    status: ground
    source: bipm-second
    where: Definition of the second (Delta nu_Cs = 9 192 631 770 Hz)
  - benchmark: atomic_weight
    label: Standard atomic weight of caesium
    value: 132.90545196
    unit: '-'
    scope: material
    conditions:
      basis: NIST standard atomic weight; printed uncertainty (6) applies to the last digits
    method: Table value from NIST
    status: ground
    source: nist-atomic-weights
    where: Cs entry, standard atomic weight
  - benchmark: atomic_weight
    label: Relative atomic mass of Cs-133 (only stable isotope)
    value: 132.905451961
    unit: '-'
    scope: material
    conditions:
      isotope: 133Cs
    method: Table value from NIST
    status: ground
    note: NIST prints 132.905 451 9610(80), with uncertainty (80) on the last digits.
    source: nist-atomic-weights
    where: Cs entry, relative atomic mass of Cs-133
  - benchmark: quantum_efficiency
    label: Quantum efficiency of a caesium antimonide photocathode film
    value: 1.2
    unit: '%'
    scope: material
    conditions:
      material: CsSb thin film on 3C-SiC or graphene-coated TiO2; not caesium metal
      wavelength: 400 nm
    method: Value stated in the abstract ("up to 1.2% at 400 nm")
    status: ground
    note: The abstract gives a maximum (up to), not a typical value. Two readers of the abstract printed different photoemission thresholds (550 nm and 566 nm), so no threshold is recorded here.
    source: arxiv-parzyck-2023
    where: Abstract
corrosion:
  - environment: Liquid water
    headline: Caesium reacts violently with water, releasing hydrogen that ignites.
    effect: Reacts very violently with cold water and the evolved hydrogen ignites. Caustic cesium hydroxide is produced. PubChem reports that the reaction releases hydrogen gas that explodes.
    rating: severe
    conditions:
      form: bulk metal or fresh surface
      temperature: cold water
    source: pubchem-cesium
    where: Safety and Hazards > Stability and Reactivity > Air and Water Reactions; Hazardous Reactivities and Incompatibilities; Explosive Limits and Potential
  - environment: Ambient room air (clean surface)
    headline: Clean caesium is spontaneously flammable in air at room temperature.
    effect: Spontaneously flammable in air at room temperature if the surface is clean, forming cesium oxides, carbonates and hydroxide. The Strem sheet says it is pyrophoric and may self-ignite.
    rating: severe
    conditions:
      atmosphere: air
      surface: clean
    source: pubchem-cesium
    where: Safety and Hazards > Stability and Reactivity > Air and Water Reactions; Fire Potential
  - environment: Dry argon or mineral oil (storage)
    headline: Stored under argon, or immersed in mineral oil, it is kept from air and moisture.
    effect: The supplier sheet says to handle and store it in a tightly sealed container under argon. PubChem's storage entry says to keep it immersed in mineral oil. Both are printed in the sources, and they are alternatives.
    rating: low
    conditions:
      storage: sealed under argon or immersed in mineral oil
    source: strem-cs-sds
    where: Section 7, Handling and Storage; PubChem Safety and Hazards > Handling and Storage > Storage Conditions
  - environment: Acids, halogens and oxidising agents
    headline: It reacts violently with acids, halogens and other oxidising materials.
    effect: Violent reaction with acids, halogens and other oxidizing materials. The supplier sheet also lists alcohols, carbon dioxide and halocarbons as incompatible.
    rating: severe
    conditions:
      contact: direct contact
    source: pubchem-cesium
    where: Safety and Hazards > Stability and Reactivity > Hazardous Reactivities and Incompatibilities
  - environment: Heating in air or oxygen
    headline: Heated in air or oxygen, it ignites immediately.
    effect: It ignites immediately in air or oxygen. Interaction with sulfur or phosphorus is incandescent. Interaction with all halogens is violent at ambient temperature.
    rating: severe
    conditions:
      temperature: not stated as a single value; ignition in air on contact
    source: pubchem-cesium
    where: Safety and Hazards > Stability and Reactivity > Hazardous Reactivities and Incompatibilities
  - environment: Construction metals and dry gases
    headline: No corrosion rate for construction metals or dry gases was found.
    effect: No rate or test result for caesium in contact with construction metals or dry gases was found in the sources read for this profile.
    rating: unknown
    conditions:
      basis: no source read
    source: pubchem-cesium
    where: Stability and Reactivity sections read; no construction-metal entry found
risks:
  hazards:
    - text: Classified Danger. In contact with water releases flammable gases which may ignite spontaneously (water-reactive category 1, in 99.4 % of ECHA notifications). Causes severe skin burns and eye damage (skin corrosion 1B, in 99.4 %).
      ghs:
        - H260
        - H314
      source: pubchem-cesium
      where: Safety and Hazards > Hazards Identification > GHS Classification (ECHA C&L, 158 reports from 7 notifications)
    - text: Serious eye damage (eye damage category 1) is notified in 71.5 % of ECHA reports.
      ghs: []
      source: pubchem-cesium
      where: Safety and Hazards > Hazards Identification > Hazard Classes and Categories
    - text: 'Transport: UN1407, Division 4.3 (dangerous when wet), packing group I, label Dangerous When Wet.'
      ghs: []
      source: pubchem-cesium
      where: Safety and Hazards > Hazards Identification > DOT Hazard Classification
    - text: Pyrophoric solid. May self-ignite. Evolves flammable hydrogen with water.
      ghs: []
      source: strem-cs-sds
      where: Section 5, Unusual Fire or Explosion Hazards
    - text: 'Fire: produces irritating, corrosive and toxic gases. Burning metal may produce severely corrosive fumes of cesium oxide and hydroxide.'
      ghs: []
      source: strem-cs-sds
      where: Section 5, Hazardous Combustion and Decomposition Products
    - text: PubChem records the protective action criteria for caesium as PAC-1 5.6 mg/m3, PAC-2 61 mg/m3 and PAC-3 370 mg/m3.
      ghs: []
      source: pubchem-cesium
      where: Safety and Hazards > Exposure Control and Personal Protection > Protective Action Criteria (PAC)
  failure_risks:
    - mode: Ionizer wear-out in a caesium contact ion thruster
      trigger: Operation of the heated porous tungsten ionizer
      consequence: A 1967 NASA Lewis report calls the ionizer one of the most critical parts of a cesium contact-ionization thruster for reliability and lifetime.
      severity: moderate
      source: ntrs-1967-ionizer
      where: Introduction
    - mode: Ignition on exposure to air
      trigger: Handling a clean surface outside argon or oil
      consequence: Spontaneous ignition in air, forming oxides, carbonates and hydroxide. A fire is hard to control with water, so dry Class D agents are needed.
      severity: high
      source: pubchem-cesium
      where: Safety and Hazards > Stability and Reactivity > Air and Water Reactions
    - mode: Hydrogen release and explosion on contact with water
      trigger: Water or moisture reaching the metal
      consequence: Violent reaction that releases hydrogen gas, which explodes.
      severity: critical
      source: pubchem-cesium
      where: Safety and Hazards > Safety and Hazard Properties > Explosive Limits and Potential
    - mode: Melting in a warm environment
      trigger: Temperature above the melting point of 28.44 C (PubChem's physical description says it melts at 85 F)
      consequence: The metal turns liquid. The sources read do not describe the hazard of a melted sample beyond its reactivity.
      severity: moderate
      source: pubchem-cesium
      where: Chemical and Physical Properties > Experimental Properties > Physical Description
  side_effects:
    - text: Corrosive solutions form on contact with water, and burning cesium gives off corrosive fumes of oxide and hydroxide.
      affects: Nearby equipment, people and surfaces exposed to fumes or spray
      source: strem-cs-sds
      where: Section 5
    - text: Certain cesium compounds may be toxic if consumed. Manufactured radioactive isotopes of cesium have caused adverse health effects; the stable element is not radioactive.
      affects: People who ingest or are exposed to cesium compounds or isotopes
      source: usgs-mcs-2026-cesium
      where: Domestic Production and Use (health and environmental paragraph)
  handling:
    - text: Handle and store in a tightly sealed container under an inert atmosphere of argon.
      source: strem-cs-sds
      where: Section 7, Handling and Storage
    - text: 'Storage alternative: keep immersed in mineral oil.'
      source: pubchem-cesium
      where: Safety and Hazards > Handling and Storage > Storage Conditions
    - text: 'Small fire: dry chemical, soda ash, lime or sand. Large fire: dry sand, dry chemical, soda ash or lime. Do not use water or foam.'
      source: pubchem-cesium
      where: Safety and Hazards > Fire Fighting (ERG 2024, Guide 138)
    - text: 'Spill: isolate the area in all directions for at least 25 m (75 ft) for solids and 50 m for liquids; keep water away; eliminate ignition sources.'
      source: pubchem-cesium
      where: Safety and Hazards > Accidental Release Measures > Isolation and Evacuation (ERG 2024, Guide 138)
    - text: 'Firefighters: positive-pressure self-contained breathing apparatus, per the emergency guide.'
      source: pubchem-cesium
      where: Safety and Hazards > Exposure Control and Personal Protection (ERG 2024, Guide 138)
  mitigations:
    - text: Keep the metal sealed under argon or in mineral oil, away from moisture and air, so that it cannot ignite or react with water.
      source: strem-cs-sds
      where: Section 7 and Section 10
    - text: Use a dry Class D agent (dry sand or dry salt) to put out a metal fire, not water or foam.
      source: strem-cs-sds
      where: Section 5, Extinguishing Medium
uses:
  - application: propulsion-and-tanks
    device: Caesium contact-ionization ion thruster
    example: 'NASA Lewis (1967): caesium is fed to a heated porous tungsten ionizer, ionized on its downstream surface and accelerated out of the thruster to produce thrust. The report calls the ionizer one of the most critical parts for reliability and lifetime.'
    source: ntrs-1967-ionizer
    where: Introduction
  - application: electronics-and-chips
    device: Caesium fountain primary frequency standard (NIST-F2)
    example: NIST-F2 is a cryogenic caesium fountain primary frequency standard. The SI second is fixed by setting the caesium-133 hyperfine transition frequency at exactly 9 192 631 770 Hz.
    source: nist-f2-bipm-2013
    where: Opening paragraph (first evaluation report, 45-day period)
  - application: electronics-and-chips
    device: Atomic clocks for global positioning systems
    example: The USGS 2025 critical minerals list names cesium as used in atomic clocks for global positioning systems.
    source: usgs-crit-2025
    where: Explore the 2025 List of Critical Minerals
  - application: optics-and-cameras
    device: Caesium antimonide photocathode for electron sources
    example: A thin CsSb film reaches up to 1.2% quantum efficiency at 400 nm, and its authors say it resists oxidation up to 15 times better than Cs3Sb. The USGS also notes that cesium metal is used to make photoelectric cells.
    source: arxiv-parzyck-2023
    where: Abstract
supply:
  - subject: Caesium
    abundance: 'No official production data. USGS 2026: pollucite is the principal cesium ore, and cesium is also recovered as a byproduct of lithium production from lepidolite; Australia, Canada, China and Namibia were estimated to hold reserves totalling less than 200,000 tons. Namibia''s lepidolite project had a measured and indicated resource of 3,100 tons. In 2025 one Canadian company reported intermittent production at Tanco Mine, and tailings recovery at Bikita in Zimbabwe for shipment to China was restarted. The United States was 100 % net import reliant for cesium minerals in 2025. Crustal abundance was not found (cesium is not in USGS PP 1802-A Table A1).'
    producers: []
    critical_lists:
      - list: USGS 2025 List of Critical Minerals (names cesium, used in atomic clocks for global positioning systems)
        listed: true
      - list: EU critical raw materials list (Regulation (EU) 2024/1252, 34 items; caesium is not on the list)
        listed: false
    recycling: 'USGS 2026: one company developed a recycling program for X-ray panels in 2020 that recovered cesium iodide for reuse in new panels.'
    substitutes:
      - 'USGS 2026: cesium and rubidium can be used interchangeably in many applications because they have similar physical properties and atomic radii. Cesium is more electropositive and is preferred for some applications.'
    source: usgs-mcs-2026-cesium
    where: 'Domestic Production and Use; World Mine Production and Reserves; Recycling; Substitutes. Critical lists: USGS 2025 list page and EU critical raw materials page (see sources).'
media: []
documents:
  - title: Cesium (99.9+%) prescored ampoule, Strem Chemicals safety data sheet (product 55-5551, hosted by Purdue)
    type: sds
    url: https://engineering.purdue.edu/Intranet/Groups/Schools/ECE/Admin/Safety/SDS/ECE00066_MSDS_Cesium_Ampoule.pdf
    year: 2009
  - title: 'PubChem: Caesium (CID 5354618), physical properties and hazard record'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/Caesium
    year: 2026
  - title: 'PubChem: Rubidium (CID 5357696), physical properties and hazard record'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/5357696
    year: 2026
  - title: 'USGS Mineral Commodity Summaries 2026: Cesium'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-cesium.pdf
    year: 2026
  - title: 'NASA Technical Note D-3997: Status of Porous Ionizers for Contact Ionization of Cesium'
    type: report
    url: https://ntrs.nasa.gov/api/citations/19670018969/downloads/19670018969.pdf
    year: 1967
  - title: BIPM evaluation report of NIST-F2 cesium fountain (MJD 56489 to 56534)
    type: report
    url: https://webtai.bipm.org/ftp/pub/tai/data/PSFS_reports/nist-f2_56489-56534.pdf
    year: 2013
  - title: NIST Atomic Weights and Isotopic Compositions, element Cs
    type: database
    url: https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=Cs&all=all
    year: 2026
  - title: 'Parzyck et al., Atomically smooth films of CsSb: a chemically robust visible light photocathode (arXiv:2305.19553; APL Materials 11, 101125, 2023)'
    type: paper
    url: https://arxiv.org/abs/2305.19553
    year: 2023
gaps:
  - 'Crystal structure and space group of caesium metal: no source opened for this profile.'
  - 'Crustal abundance of caesium: cesium is not listed in the USGS PP 1802-A Table A1 read for this profile.'
  - 'Country shares of caesium production: USGS 2026 states that there are no official production data, so no share is given.'
  - 'Electrical resistivity, hardness, first ionization energy and work function of caesium metal: no source read gave a number.'
  - 'Atomic clock frequency uncertainty today: the NIST-F2 report read is a 2013 evaluation, and its values are printed in exponent notation, so they are not recorded here.'
  - 'Caesium flight history after the 1960s contact-ionization thrusters: only the 1967 report was read.'
  - The photoemission threshold of CsSb films is printed differently by two readers of the same abstract (550 nm and 566 nm), so it is not recorded.
  - 'Corrosion of caesium by construction metals and dry gases: no sourced data found.'
  - 'Toxicity values (LD50 or occupational limits) for cesium metal: none read; PubChem gives only protective action criteria.'
  - Property names vapor_pressure, hyperfine_transition_frequency, specific_gravity and quantum_efficiency are not in spores/property-canon.yaml and are used as short snake_case names.
  - 'Type: no alkali-metal type exists under the metals category in src/data/taxonomy.ts; heavy-metals is used as the closest type.'
  - 'Rubidium numbers read but not profiled: melting point 39.5 C, boiling point 688 C at 760 mmHg and GHS H260 (98.8 %) and H314 (91.4 %), from the PubChem rubidium record (CID 5357696). A separate rubidium profile would hold them.'
  - 'Rubidium: only melting point, boiling point, hazards and substitution were read; no rubidium density is recorded because the source prints no unit.'
  - 'Images: no caesium images were downloaded or licensed for this profile.'
sources:
  pubchem-cesium:
    title: PubChem compound record, Caesium (CID 5354618), with HSDB, ERG 2024 and ECHA C&L excerpts
    id: PubChem CID 5354618
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/5354618/JSON
    year: 2026
    kind: database
    license: public domain (NCBI); contributed data keep their own terms
  pubchem-rubidium:
    title: PubChem compound record, Rubidium (CID 5357696), with HSDB and ECHA C&L excerpts
    id: PubChem CID 5357696
    url: https://pubchem.ncbi.nlm.nih.gov/compound/5357696
    year: 2026
    kind: database
    license: public domain (NCBI); contributed data keep their own terms
  bipm-second:
    title: 'SI brochure: the second (BIPM), definition with the caesium-133 hyperfine transition frequency'
    id: BIPM SI base units, second
    url: https://www.bipm.org/en/si-base-units/second
    year: 2019
    kind: standard
    license: BIPM web content
  nist-atomic-weights:
    title: NIST Atomic Weights and Isotopic Compositions for All Elements (element Cs)
    id: NIST Physical Measurement Laboratory
    url: https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=Cs&all=all
    year: 2026
    kind: database
    license: public domain (US government work)
  strem-cs-sds:
    title: Strem Chemicals safety data sheet, Cesium (99.9+%) prescored ampoule, product 55-5551 (hosted by Purdue University)
    id: Strem product 55-5551; print date 1/26/2009
    url: https://engineering.purdue.edu/Intranet/Groups/Schools/ECE/Admin/Safety/SDS/ECE00066_MSDS_Cesium_Ampoule.pdf
    year: 2009
    kind: datasheet
    license: supplier document, linked not reproduced
  ntrs-1967-ionizer:
    title: Status of Porous Ionizers for Contact Ionization of Cesium (NASA Technical Note D-3997, Lewis Research Center, Headley)
    id: NASA TN D-3997; NTRS 19670018969
    url: https://ntrs.nasa.gov/api/citations/19670018969/downloads/19670018969.pdf
    year: 1967
    kind: report
    license: public domain (US Government work)
  nist-f2-bipm-2013:
    title: BIPM evaluation report of the NIST-F2 cryogenic cesium fountain primary frequency standard (45-day period MJD 56489 to 56534)
    id: BIPM PSFS report
    url: https://webtai.bipm.org/ftp/pub/tai/data/PSFS_reports/nist-f2_56489-56534.pdf
    year: 2013
    kind: report
    license: BIPM report, cited not reproduced
  usgs-mcs-2026-cesium:
    title: Mineral Commodity Summaries 2026, Cesium
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-cesium.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  usgs-crit-2025:
    title: About the 2025 List of Critical Minerals (USGS, final list)
    id: Final 2025 List of Critical Minerals, November 2025
    url: https://www.usgs.gov/programs/mineral-resources-program/science/about-2025-list-critical-minerals
    year: 2025
    kind: web
    license: public domain (USGS)
  ec-crm:
    title: Critical raw materials (European Commission, list of 34 critical raw materials under Regulation (EU) 2024/1252)
    id: Regulation (EU) 2024/1252
    url: https://single-market-economy.ec.europa.eu/sectors/raw-materials/areas-specific-interest/critical-raw-materials_en
    year: 2024
    kind: web
    license: European Commission web page
  arxiv-parzyck-2023:
    title: 'Atomically smooth films of CsSb: a chemically robust visible light photocathode (Parzyck et al.)'
    id: arXiv:2305.19553; doi:10.1063/5.0166334 (APL Materials 11, 101125)
    url: https://arxiv.org/abs/2305.19553
    year: 2023
    kind: paper
    license: arXiv preprint (abstract cited, not reproduced)
license: CC-BY-4.0
updated: '2026-10-26'
review:
  status: draft
  method: assistant
  checked_values: 12
  unreachable: 0
  caveats: 2
  note: Read by an AI assistant on 2026-10-26. PubChem values were read through its REST API after the browser check blocked the web fetch. The NIST-F2 evaluation is from 2013.
story:
  recipe:
    line: Caesium metal is the pure element, Cs, with one atom per formula unit. It is not a compound or alloy in this profile.
    parts:
      - name: Caesium
        symbol: Cs
        atoms: 1
        atomic_weight: 132.905
        role: The only element in the metal. NIST gives a standard atomic weight of 132.90545196 (6) and Cs-133 is the only stable isotope.
  why: Caesium is a soft alkali metal that melts at 28.44 C, so it is liquid in a warm room. Its importance comes from its atomic transition. The SI second is fixed by setting the caesium-133 hyperfine transition frequency at exactly 9 192 631 770 Hz, which makes caesium the reference for the SI second. Caesium also fed contact-ionization ion thrusters, and its antimonide forms visible-light photocathodes. It is pyrophoric in air and reacts violently with water, so every use depends on sealed handling.
  uses:
    - where: Ion thrusters
      line: NASA Lewis reported in 1967 that caesium is fed to a heated porous tungsten ionizer, ionized on the downstream surface and accelerated out to produce thrust. The ionizer was one of the most critical parts for reliability and lifetime.
    - where: Atomic clocks
      line: NIST-F2 is a cryogenic caesium fountain primary frequency standard. The USGS 2025 critical minerals list names cesium as used in atomic clocks for global positioning systems.
    - where: Photocathodes and photoelectric cells
      line: A thin caesium antimonide film reaches up to 1.2 % quantum efficiency at 400 nm, and its authors say it resists oxidation up to 15 times better than Cs3Sb. USGS notes that cesium metal is used to make photoelectric cells.
  can_do:
    - label: Clock standard
      value: 9192631770 Hz
      line: 'The caesium-133 ground-state hyperfine transition is fixed at exactly 9 192 631 770 Hz, '
    - label: Melting
      value: 28.44 C
      line: Caesium melts at 28.44 C (PubChem), while the supplier sheet gives 28.64 C. It is a liquid at a warm room temperature, and it boils at 671 C at 760 mmHg.
    - label: Density
      value: 1892 kg/cu m solid
      line: The solid is 1892 kg/cu m at 17 C and the liquid is 1827 kg/cu m at 40 C (PubChem).
    - label: Substitution
      value: Rubidium
      line: USGS notes that cesium and rubidium can often be used interchangeably, but cesium is more electropositive and is preferred for some applications.
  compare: Caesium melts at 28.44 C, far below the melting point of lithium metal (lithium-metal profile), so it is a liquid at a temperature far below the melting point of lithium metal (lithium-metal profile). Both react violently with water. Caesium is the clock element, while lithium is a battery anode.
  compare_rows:
    - label: Melting point
      benchmark: melting_point
      scope: material
    - label: Density
      benchmark: density
      scope: material
    - label: Boiling point
      benchmark: boiling_point
      scope: material
  rubidium: 'Rubidium is the neighbouring alkali metal. PubChem''s record gives a melting point and a boiling point, and classifies it Danger: it releases flammable gases in water and causes severe skin burns. It reacts with air and water and is stored in kerosene or a similar liquid, according to the PubChem hazard summary. USGS says cesium and rubidium can often be used interchangeably, and that rubidium is mined as a byproduct of cesium and lithium mining.'
  handling:
    - Store and handle under argon in a tightly sealed container, or immersed in mineral oil. Keep it away from water, moisture, acids and oxidisers.
    - Put out a metal fire with a dry Class D agent such as dry sand or dry salt. Do not use water or foam.
    - Isolate a spill for at least 25 m for solids and 50 m for liquids, and keep water away.
    - Ship only as UN1407, Division 4.3, packing group I (dangerous when wet).
  good_for:
    - Atomic clocks and frequency standards, where the caesium-133 hyperfine transition defines the SI second.
    - Photocathodes, where the antimonide film gives up to 1.2 % quantum efficiency at 400 nm.
  watch_out:
    - It ignites spontaneously in air and reacts violently with water, releasing hydrogen that can explode.
    - It melts at 28.44 C, so it is a liquid in a warm room.
    - Burning caesium gives off corrosive oxide and hydroxide fumes, and water and foam must not be used on the fire.
  verdict: Caesium is the most important clock element and a proven ion thruster propellant, but it is one of the hardest metals to handle safely.
  risk_groups:
    - title: For people
      level: 4
      verdict: Pyrophoric, water-reactive and corrosive
      lead: Caesium is classified Danger. It ignites in air, reacts violently with water and causes severe burns.
      points:
        - label: Hazard classification
          line: In contact with water it releases flammable gases that may ignite spontaneously (water-reactive category 1, in 99.4 % of ECHA notifications). It causes severe skin burns and eye damage (skin corrosion 1B, in 99.4 %). Serious eye damage category 1 is notified in 71.5 % of reports.
        - label: Air
          line: The supplier sheet calls it a pyrophoric solid that may self-ignite. PubChem records that clean caesium is spontaneously flammable in air at room temperature.
        - label: Fire
          line: Burning metal produces corrosive oxide and hydroxide fumes. Water and foam are not to be used. Dry sand, dry salt, dry chemical, soda ash or lime are the agents named in the sources.
        - label: Transport
          line: The metal ships as UN1407, Division 4.3 (dangerous when wet), packing group I.
    - title: If a fire or spill happens
      lead: These results come from safety sheets and emergency guides, not from incident reports.
      points:
        - label: Spill
          line: Isolate the area for at least 25 m for solids and 50 m for liquids. Eliminate ignition sources and keep water away.
        - label: Firefighting
          line: Firefighters need positive-pressure breathing apparatus and chemical protective clothing. The emergency guide names dry chemical, soda ash, lime or sand for small fires.
    - title: Handling and storage
      points:
        - label: Storage
          line: Store under argon in a tightly sealed container, or immersed in mineral oil, away from moisture, acids and oxidisers.
        - label: Exposure
          line: PubChem gives protective action criteria of 5.6 mg/m3 (PAC-1), 61 mg/m3 (PAC-2) and 370 mg/m3 (PAC-3). Certain cesium compounds may be toxic if consumed.