Element identity / phase data
Reference values and 133Cs identity are evaluated.
Atomic-clock frequency
The SI second fixes the 133Cs hyperfine frequency exactly at 9,192,631,770 Hz.
133Cs vs 137Cs
Stable clock isotope and radioactive fission-product isotope are kept explicitly separate.
Resource geography
Pollucite locality context is selective and not a complete world reserve inventory.
The lens describes evidence status, not confidence theatre. “Unknown” is kept unknown, and teaching schematics are not presented as direct measurements.
Cesium (Cs)
Cesium (also spelled caesium) is element 55, a very reactive alkali metal with one 6s valence electron. Its best-known scientific role is the ¹³³Cs hyperfine transition used to define the SI second.
Cesium atomic number, mass, electron configuration and key properties
Cesium: quick answers
How many protons, neutrons and electrons does cesium have?
Cesium’s atomic number is 55, so every cesium atom has 55 protons, and a neutral atom also has 55 electrons. Its most common natural isotope, cesium-133, has 78 neutrons (other isotopes have different neutron counts).
What is the symbol for cesium?
The chemical symbol for cesium is Cs.
Is cesium a solid, liquid or gas at room temperature?
Cesium is a solid at room temperature (about 25 °C). It melts at about 28.5 °C, just above room temperature.
What family (group) is cesium in?
Cesium is an alkali metal, in group 1, period 6 of the periodic table.
How many valence electrons does cesium have?
Cesium has 1 valence electron, the single electron in its outer shell, which matches its position in group 1.
What is the electron configuration of cesium?
The ground-state electron configuration of cesium is [Xe] 6s¹.
From atomic number to chemistry
Read these as a chain of causes, not as isolated facts. Each step links to the concept hub if you want the underlying idea explained.
55 protons define cesium.
The reference ground-state configuration frames atomic and chemical behavior.
Periodic position organizes recurring trends without replacing element-specific evidence.
The teaching nucleus is one isotope, not the relative atomic mass.
Phase claims are evidence-labelled; unknown superheavy bulk boundaries are not fabricated.
Cesium in its period and family
Cesium is in Group 1, Period 6. Its single 6s electron is weakly held compared with lighter alkali metals, supporting strong +1 chemistry and high reactivity.
Cesium Visual Lab
Compare Cs with Cs⁺, rotate a ¹³³Cs teaching nucleus and BCC metal cell, inspect the 6s probability cloud, then explore low melting, pollucite resources, atomic clocks and the difference between ¹³³Cs and ¹³⁷Cs.
Every mark points to one exact feature
The numbered markers explain the same information system used throughout Element Lookup. Unknown or predicted fields remain visibly labelled rather than being replaced with guesses.
Cesium in one minute
Atomic number 55 means 55 protons.
Neutral cesium ends in 6s¹ and commonly forms Cs⁺.
Cesium melts near 301.7 K, only a little above ordinary room temperature.
The SI second is defined using a specific ¹³³Cs hyperfine transition frequency.
¹³⁷Cs is a radioactive isotope and should not be confused with stable ¹³³Cs.
Shell rings organize electron counts. They are not electron trajectories or orbital shapes.
2 · 8 · 18 · 18 · 8 · 1 electrons
The 6s probability model is spherical and hydrogen-like. It illustrates quantum probability, not a literal electron path around the nucleus.
Cesium metal is represented by a BCC teaching cell. Its very low melting point is handled separately in the temperature explorer.. The viewer is evidence-aware: measured structures are identified as such; unknown bulk structures stay unknown.
Teaching visualization; not a literal finite sample or thermal trajectory.What this model does—and does not—show
The 6s probability model is spherical and hydrogen-like. It illustrates quantum probability, not a literal electron path around the nucleus.
Where do I meet cesium?
Clickable learning cards connect the element to materials, environment, technology, biology or research - depending on what the evidence actually supports.
Atomic clocks
A specific hyperfine transition of ¹³³Cs defines the SI second: 9,192,631,770 cycles per second.
133Cs clock atom ≠ 137Cs radionuclide
Isotope identity matters: stable 133Cs anchors time metrology while 137Cs belongs to radioactive monitoring/environmental contexts.
6s1 alkali-metal atom
Neutral cesium has one weakly held 6s electron and is extremely reactive.
Sodium, potassium and cesium
Compare one-electron outer shells down the alkali metals.
| Outer shell | 3s¹ |
|---|---|
| Melt | 370.9 K |
| Outer shell | 4s¹ |
|---|---|
| Melt | 336.5 K |
| Outer shell | 6s¹ |
|---|---|
| Melt | 301.7 K |
Cesium properties: atomic, physical, thermal and chemical
Categories follow the science of this element rather than a fixed decorative template. Each row carries condition/provenance context and an evidence label; unknown values stay unknown.
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| Atomic number | 55 | Source-reviewed; see Sources below | Evaluated |
| Relative atomic mass | 132.905 | Source-reviewed; see Sources below | Evaluated |
| Ground-state electron configuration | [Xe] 6s¹ | Source-reviewed; see Sources below | Evaluated |
| Group / period / block | Group 1 · Period 6 · s-block | Periodic-table placement | Evaluated |
| Electronegativity | 0.79 | Source-reviewed; see Sources below | Evaluated |
| Reference isotope | ¹³³Cs | Source-reviewed; see Sources below | Evaluated |
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| State context | Soft gold-coloured solid at 20 °C | Source-reviewed; see Sources below | Evaluated |
| Density | 1.873 g/cm³ | Source-reviewed; see Sources below | Evaluated |
| Material / molecular structure | Cesium metal · body-centred cubic | Cesium metal is represented by a BCC teaching cell. Its very low melting point is handled separately in the temperature explorer. | Measured |
| Classification | Alkali metal | Periodic-table / chemistry classification | Evaluated |
| Structure-model scope | Cesium metal is represented by a BCC teaching cell. Its very low melting point is handled separately in the temperature explorer. | Teaching visualization; exact crystallographic coordinates are not implied unless stated. | Reviewed |
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| Melting / transition reference | 301.7 K | Source-reviewed; see Sources below | Evaluated |
| Boiling / gas reference | 944 K | Source-reviewed; see Sources below | Evaluated |
| Phase-path context | At approximately standard pressure cesium is solid below about 301.7 K, liquid from there to about 944 K, then gaseous. Its melting point lies close to common warm-room temperatures. | Shared phase registry drives the slider, regions and markers. | Evaluated |
| Condition warning | Temperature and pressure define phase behavior; purity/allotropy may matter. | Teaching condition statement | Reviewed |
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| Ordinary electrical behavior | Metallic conductor | Qualitative bulk behavior; exact resistivity depends on temperature, purity and alloy state. | Measured |
| Conduction model | Collective solid-state electrons | Do not interpret isolated-atom orbital clouds as literal current paths. | Reviewed |
| Surface / compound caveat | Oxides, salts and alloys can behave differently from the pure metal | Material context | Reviewed |
| Engineering values | Condition-dependent | Use condition-specific materials data for engineering calculations. | Reviewed |
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| Common oxidation states | +1 | Source-reviewed; see Sources below | Evaluated |
| Ion / common ion context | Cs⁺ | Source-reviewed; see Sources below | Evaluated |
| Periodic chemistry context | Cesium is in Group 1, Period 6. Its single 6s electron is weakly held compared with lighter alkali metals, supporting strong +1 chemistry and high reactivity. | Element-specific interpretation | Reviewed |
| Chemistry caution | Elemental form, ions and compounds are distinct chemical objects. | Interpretive teaching rule | Reviewed |
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| ¹³³Cs | Stable · essentially all natural cesium | Reference teaching nucleus: 55 protons, 78 neutrons; basis of the SI second definition. | Evaluated |
| ¹³⁷Cs | Radioactive fission-product isotope | Important in monitoring/environmental contexts; not the clock isotope. | Evaluated |
| ¹³⁴Cs | Radioactive isotope | Used in specialized tracer/monitoring contexts under controlled conditions. | Evaluated |
| Teaching nucleus | ¹³³Cs · 55 protons + 78 neutrons | Reference isotope used in the nucleus model | Reviewed |
Is Cesium a solid, liquid or gas? State at temperature
At approximately standard pressure cesium is solid below about 301.7 K, liquid from there to about 944 K, then gaseous. Its melting point lies close to common warm-room temperatures.
Where on Earth is Cesium found or produced?
Who discovered Cesium, and when?
Robert Bunsen and Gustav Kirchhoff discovered cesium spectroscopically from distinctive blue lines.
Carl Setterberg isolated metallic cesium by electrolysis.
The SI second was redefined using the ¹³³Cs hyperfine transition.
Cesium remains important in precision timekeeping and specialized chemical/physical applications.
From rare cesium minerals to precision uses: a high-level path
Cesium is not mined as native metal; pollucite is the principal ore mineral in rare pegmatites.
Ore concentration and chemical conversion produce cesium compounds in specialized industrial facilities.
Metallic cesium is highly reactive and handled only in controlled systems; no preparation procedure is provided here.
High-value uses include precision frequency standards and specialty chemical/optical applications.
What is cesium used for?
Time & frequency
Cesium atomic clocks realize the SI second with extreme precision.
Specialty fluids
Dense cesium formate brines have specialized drilling/completion uses.
Optics & electronics
Cesium compounds appear in selected optical, vacuum and photoemissive systems.
Research & monitoring
Different cesium isotopes are used in distinct metrology, research and monitoring contexts.
Cesium isotopes and natural abundance
¹³³Cs
Stable · essentially all natural cesiumReference teaching nucleus: 55 protons, 78 neutrons; basis of the SI second definition.
¹³⁷Cs
Radioactive fission-product isotopeImportant in monitoring/environmental contexts; not the clock isotope.
¹³⁴Cs
Radioactive isotopeUsed in specialized tracer/monitoring contexts under controlled conditions.
Five-question Cesium check
What is cesium’s atomic number?
What is the common cesium ion?
Which isotope defines the SI second?
What is the US/international spelling relationship?
What makes ¹³⁷Cs different from ¹³³Cs?
Cesium questions students commonly ask
Each answer starts with the direct fact, then explains the chemistry, evidence or material context so the result is understandable rather than merely memorized.
What is cesium’s atomic number?
Short answer: 55. Every cesium nucleus contains 55 protons.
Atomic number is defined by proton count, so 55 protons are what make an atom cesium. A neutral cesium atom also has 55 electrons, while isotopes can have different neutron counts without changing the element.
Key point: Atomic number = proton count.
Is cesium a metal?
Short answer: Yes. It is a very reactive Group 1 alkali metal.
This guide classifies Cesium as an alkali metal. Its periodic position is Period 6, s-block, Group 1. Cesium is in Group 1, Period 6. Its single 6s electron is weakly held compared with lighter alkali metals, supporting strong +1 chemistry and high reactivity.
Key point: Periodic-table classification describes recurring atomic/chemical behavior; it does not make every element in a column physically identical.
Are cesium and caesium the same element?
Short answer: Yes. “Cesium” is the US spelling; “caesium” is widely used internationally. Both refer to Cs, element 55.
Cesium (also spelled caesium) is element 55, a very reactive alkali metal with one 6s valence electron. Its best-known scientific role is the ¹³³Cs hyperfine transition used to define the SI second. Specialty chemistry Cesium compounds are used in selected catalysts, optical materials, vacuum devices and specialized fluids.
Key point: Use the direct answer together with the material, isotope and evidence context shown elsewhere on the page.
How many valence electrons does cesium have?
Short answer: One, from its 6s¹ outer configuration.
The neutral-atom ground-state reference used on this page is [Xe] 6s¹. This is an isolated-atom reference: bonding and ion formation can change which outer electrons are present or chemically active. The listed common oxidation-state context is +1, which helps connect the atomic configuration to ordinary chemistry without treating electron counting as a single universal rule.
Key point: Electron configuration is a ground-state atomic reference, not a literal picture of every compound.
What is cesium-137?
Short answer: ¹³⁷Cs is a radioactive isotope. It is different from stable ¹³³Cs used in atomic-clock time standards.
Stable ¹³³Cs in atomic clocks and radioactive ¹³⁷Cs are different isotopes with very different contexts. The element page keeps timekeeping, ordinary chemistry and radioisotope questions separate. Time & frequency Cesium atomic clocks realize the SI second with extreme precision.
Key point: Use the direct answer together with the material, isotope and evidence context shown elsewhere on the page.
Why is cesium-133 used in atomic clocks?
Short answer: The SI second is defined by fixing the frequency of the unperturbed ground-state hyperfine transition of the cesium-133 atom at exactly 9,192,631,770 hertz.
Atomic clocks do not use a tiny mechanical “tick.” They tune electromagnetic radiation to a sharply defined quantum transition between hyperfine levels of neutral ¹³³Cs and count cycles of that radiation. Cesium-133 was chosen historically because the transition could be reproduced with exceptional precision; modern cesium fountain clocks realize the SI definition while other atomic-clock species can be even more precise for research.
Key point: Cesium-133 is central because its specified quantum transition defines the unit of time—not because cesium atoms literally oscillate like pendulums.
Scientific sources for Cesium
- Royal Society of Chemistry - Caesium
- BIPM - SI Brochure
- USGS - Cesium mineral commodity context
- NIST — SI base units: second
Questions to ask next about Cesium
A good element lesson should lead to the next useful question, not end after a list of facts.
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