Cesium
Cesium is a soft, gold-tinged alkali metal so reactive that it bursts into flame or explodes on contact with water, and so soft-melting that it can turn to liquid in a warm room. It's best known today for a very different reason: the vibrations of a cesium atom are so regular that they're used to define the length of a second itself.
- Group · Period
- 1 · 6
- At room temp
- solid
- Melts at
- 301.59 K
- Density
- 1.93 g/cm³
- Discovered
- 1860
Uses
Cesium’s defining modern use is in atomic clocks, where the precise, unchanging frequency of its atoms provides the reference against which the international second is defined; these clocks underpin GPS satellite navigation and global timekeeping standards. In the oil and gas industry, cesium formate is used in dense drilling fluids that help control pressure and stabilize wells during deep drilling operations.
Cesium compounds also find use in specialized photoelectric cells, which generate a small electric current when exposed to light, taking advantage of how easily cesium atoms give up an electron.
History
Cesium was discovered in 1860 by German chemists Robert Bunsen and Gustav Kirchhoff, pioneers of a then-new technique called flame spectroscopy that identified elements by the distinctive colors of light they emit when heated. While analyzing mineral water samples, they spotted two previously unrecorded blue spectral lines and used them to identify a new element, naming it cesium after the Latin word caesius, meaning sky-blue, in reference to those lines. It remained largely a laboratory curiosity until the mid-twentieth century, when its extraordinarily precise atomic vibrations made it the foundation of modern atomic timekeeping.
Fun facts
- The SI second is officially defined by counting a precise number of oscillations of a cesium-133 atom, making cesium atomic clocks the world's timekeeping standard.
- Cesium melts at only about 28°C, just above room temperature, making it one of the few metals that can turn to liquid in your hand.
- Cesium reacts explosively with water, even more violently than sodium or potassium, the more commonly seen reactive alkali metals.
Frequently asked questions
Why is cesium used to define the length of a second?
A cesium-133 atom absorbs and emits radiation at an extremely precise, unchanging frequency when it switches between two specific energy states. That frequency is so stable and repeatable that scientists chose it as the reference standard, and cesium atomic clocks built around it are accurate enough to stay within a second over many millions of years.
Is cesium metal actually dangerous to handle?
Very. Cesium reacts violently and often explosively with water, including the moisture in air, so pure cesium metal has to be stored sealed away from air entirely, typically under inert gas or oil. Its extreme reactivity is part of a broader pattern among the alkali metals, which get more reactive the further down the group you go.
Why does cesium melt so easily compared with other metals?
Cesium's atoms are large and hold onto their outer electron only loosely, which means the metallic bonds tying its atoms together in a solid are unusually weak. That weak bonding is why cesium melts at just under 30°C, making it one of only a handful of metals — alongside gallium, rubidium, and mercury (which is liquid well below room temperature) — that are liquid at or near everyday room temperature.
Compounds
5 notable compounds containing Cs
- Cs2CO3salt
Cesium carbonate
A hygroscopic white salt widely used as a mild, highly soluble base in organic and pharmaceutical synthesis.
Used for: Base reagent in organic synthesis, including palladium-catalyzed couplings
- CsClsalt
Cesium chloride
A simple ionic salt whose crystal structure gives its name to a common packing arrangement, and whose dense solutions are used to separate biomolecules by density.
Used for: Density-gradient medium for separating DNA and other biomolecules
- CsFhalide
Cesium fluoride
A very water-soluble ionic salt that serves as a source of "naked" fluoride ion in organic synthesis, thanks to cesium's weak attraction to it.
Used for: Fluoride source and mild base in organic synthesis
- CsOHbase
Cesium hydroxide
The strongest of the common alkali metal hydroxides, a highly corrosive, deliquescent solid that dissolves readily in water.
Used for: Strong base used in specialty batteries and research chemistry
- CsIsalt
Cesium iodide
A dense ionic salt that, when doped with thallium, forms crystals that scintillate brightly under radiation and are widely used in detectors.
Used for: Scintillation crystals in radiation and medical imaging detectors
Isotopes
41 known isotopes
Swipe to see all columns →
| Isotope | Natural abundance | Half-life | Decay mode | Mass excess | Binding energy / nucleon | Decay width |
|---|---|---|---|---|---|---|
| Cs-133 stable | 100% | Stable | — | -88,070.94 keV | 8,409.98 keV | — |
Show all 41 isotopes
| Cs-112 | — | 0.49 ms | Proton emission (100%), Alpha decay (0.26%) | -46,415 keV | 8,102 keV | 9.31 × 10-13 eV |
| Cs-113 | — | 17.7 us | Proton emission (100%) | -51,764.54 keV | 8,148.62 keV | 2.58 × 10-11 eV |
| Cs-114 | — | 0.57 s | Electron capture / beta-plus decay (99.982%), ECP (8.7%), ECA (0.19%) | -54,685.93 keV | 8,173.57 keV | 8.00 × 10-16 eV |
| Cs-115 | — | 1.4 s | Electron capture / beta-plus decay (100%), Beta-plus, proton emission (0.07%) | -59,699 keV | 8,216 keV | 3.26 × 10-16 eV |
| Cs-116 | — | 0.70 s | Electron capture / beta-plus decay (100%), ECP (2.8%), ECA (0.049%) | -62,043 keV | 8,235 keV | 6.52 × 10-16 eV |
| Cs-117 | — | 8.4 s | Electron capture / beta-plus decay (100%) | -66,493.1 keV | 8,271.87 keV | 5.43 × 10-17 eV |
| Cs-118 | — | 14 s | Electron capture / beta-plus decay (100%), ECP (0.042%), ECA (0.0024%) | -68,409.38 keV | 8,286.41 keV | 3.26 × 10-17 eV |
| Cs-119 | — | 43.0 s | Electron capture / beta-plus decay (100%) | -72,305.06 keV | 8,317.33 keV | 1.06 × 10-17 eV |
| Cs-120 | — | 61.3 s | Electron capture / beta-plus decay (100%), Beta-plus, alpha emission (0.00002%), Beta-plus, proton emission (0.000007%) | -73,888.65 keV | 8,328.48 keV | 7.44 × 10-18 eV |
| Cs-121 | — | 155 s | Electron capture / beta-plus decay (100%) | -77,102.34 keV | 8,352.92 keV | 2.94 × 10-18 eV |
| Cs-122 | — | 21.18 s | Electron capture / beta-plus decay (100%) | -78,144.77 keV | 8,359.15 keV | 2.15 × 10-17 eV |
| Cs-123 | — | 5.86 m | Electron capture / beta-plus decay (100%) | -81,043.66 keV | 8,380.38 keV | 1.30 × 10-18 eV |
| Cs-124 | — | 30.9 s | Electron capture / beta-plus decay (100%) | -81,741.06 keV | 8,383.51 keV | 1.48 × 10-17 eV |
| Cs-125 | — | 46.7 m | Electron capture / beta-plus decay (100%) | -84,089.74 keV | 8,399.8 keV | 1.63 × 10-19 eV |
| Cs-126 | — | 1.643 m | Electron capture / beta-plus decay (100%) | -84,350.68 keV | 8,399.27 keV | 4.63 × 10-18 eV |
| Cs-127 | — | 6.25 h | Electron capture / beta-plus decay (100%) | -86,240.03 keV | 8,411.56 keV | 2.03 × 10-20 eV |
| Cs-128 | — | 3.640 m | Electron capture / beta-plus decay (100%) | -85,931.77 keV | 8,406.5 keV | 2.09 × 10-18 eV |
| Cs-129 | — | 32.06 h | Electron capture / beta-plus decay (100%) | -87,499.05 keV | 8,416.05 keV | 3.95 × 10-21 eV |
| Cs-130 | — | 29.21 m | Electron capture / beta-plus decay (98.4%), Beta-minus decay (1.6%) | -86,899.75 keV | 8,408.78 keV | 2.60 × 10-19 eV |
| Cs-131 | — | 9.689 d | Electron capture (100%) | -88,055.57 keV | 8,415.03 keV | 5.45 × 10-22 eV |
| Cs-132 | — | 6.480 d | Electron capture / beta-plus decay (98.13%), Beta-minus decay (1.87%) | -87,152.69 keV | 8,405.59 keV | 8.15 × 10-22 eV |
| Cs-134 | — | 2.0652 Y | Beta-minus decay (99.9997%), Electron capture (0.0003%) | -86,891.17 keV | 8,398.65 keV | 7.00 × 10-24 eV |
| Cs-135 | — | 2.3E+6 Y | Beta-minus decay (100%) | -87,581.96 keV | 8,401.34 keV | 6.29 × 10-30 eV |
| Cs-136 | — | 13.01 d | Beta-minus decay (100%) | -86,338.86 keV | 8,389.77 keV | 4.06 × 10-22 eV |
| Cs-137 | — | 30.08 Y | Beta-minus decay (100%) | -86,545.77 keV | 8,388.96 keV | 4.81 × 10-25 eV |
| Cs-138 | — | 32.5 m | Beta-minus decay (100%) | -82,887.03 keV | 8,360.14 keV | 2.34 × 10-19 eV |
| Cs-139 | — | 9.27 m | Beta-minus decay (100%) | -80,701.09 keV | 8,342.34 keV | 8.20 × 10-19 eV |
| Cs-140 | — | 63.7 s | Beta-minus decay (100%) | -77,049.74 keV | 8,314.32 keV | 7.16 × 10-18 eV |
| Cs-141 | — | 24.84 s | Beta-minus decay (100%), Beta-minus, neutron emission (0.035%) | -74,477.35 keV | 8,294.36 keV | 1.84 × 10-17 eV |
| Cs-142 | — | 1.684 s | Beta-minus decay (100%), Beta-minus, neutron emission (0.09%) | -70,514.56 keV | 8,264.88 keV | 2.71 × 10-16 eV |
| Cs-143 | — | 1.791 s | Beta-minus decay (100%), Beta-minus, neutron emission (1.64%) | -67,675.52 keV | 8,243.67 keV | 2.55 × 10-16 eV |
| Cs-144 | — | 0.994 s | Beta-minus decay (100%), Beta-minus, neutron emission (3.03%) | -63,271.36 keV | 8,211.89 keV | 4.59 × 10-16 eV |
| Cs-145 | — | 0.587 s | Beta-minus decay (100%), Beta-minus, neutron emission (14.7%) | -60,054.42 keV | 8,188.73 keV | 7.77 × 10-16 eV |
| Cs-146 | — | 0.3220 s | Beta-minus decay (100%), Beta-minus, neutron emission (14.2%) | -55,310.38 keV | 8,155.44 keV | 1.42 × 10-15 eV |
| Cs-147 | — | 0.2295 s | Beta-minus decay (100%), Beta-minus, neutron emission (28.5%) | -51,920.07 keV | 8,131.8 keV | 1.99 × 10-15 eV |
| Cs-148 | — | 146 ms | Beta-minus decay (100%), Beta-minus, neutron emission (25.1%) | -46,910.95 keV | 8,097.55 keV | 3.12 × 10-15 eV |
| Cs-149 | — | 107 ms | Beta-minus decay (100%), Beta-minus, neutron emission (25%) | -43,300 keV | 8,073 keV | 4.26 × 10-15 eV |
| Cs-150 | — | 84 ms | Beta-minus decay (100%), Beta-minus, neutron emission (20%) | -38,170 keV | 8,039 keV | 5.43 × 10-15 eV |
| Cs-151 | — | 69 ms | Beta-minus decay (100%), Beta-minus, neutron emission | -34,280 keV | 8,013 keV | 6.61 × 10-15 eV |
| Cs-152 | — | 50 ms | Beta-minus decay (100%), Beta-minus, neutron emission, B-2N | -29,130 keV | 7,980 keV | 9.12 × 10-15 eV |