Caesium
Caesium is the element that tells the world what a second is. Since 1967, the second has been defined as 9,192,631,770 cycles of microwave light tied to a transition in caesium-133 atoms. The metal behind that number is a soft, silvery-golden substance with the symbol Cs and atomic number 55. It is also one of the most reactive materials chemists handle. Americans spell it cesium, while the international chemistry body prefers caesium. How did two German scientists find an element nobody had seen before? Why does a metal this soft and gentle-looking explode on contact with cold water? Where on Earth does the world's supply come from, and why does its radioactive cousin still mark the land around Chernobyl? The answers begin with a very large quantity of mineral water.
In 1860, the chemist Robert Bunsen and the physicist Gustav Kirchhoff found caesium in mineral water from Dürkheim, Germany. Their tool was the spectroscope, which they had invented only a year earlier. Caesium became the first element ever discovered this way. Its flame showed bright blue lines, so they named it after the Latin word caesius, meaning bluish grey. The American Chemical Society has used the spelling cesium since 1921, following a dictionary, while the older ligature form cæsium survives as an old-fashioned variant.
Getting a pure sample took enormous patience. The two men evaporated 44,000 litres of mineral water down to 240 kg of concentrated salt solution. They then stripped away the alkaline earth metals, the sodium and the lithium in turn. Fractional crystallization separated caesium from its close relative rubidium. The final haul was 9.2 g of rubidium chloride and just 7.3 g of caesium chloride.
The next step went wrong. From the chloride, they estimated the atomic weight at 123.35, far from today's accepted 132.9. When they tried electrolysis on molten caesium chloride, they got a blue substance that, in their words, "neither under the naked eye nor under the microscope showed the slightest trace of metallic substance". They called it a subchloride. It was probably a colloidal mixture of the metal and its chloride. The Swedish chemist Carl Setterberg finally isolated the true metal in 1882, while working on his doctorate with Kekulé and Bunsen. He used caesium cyanide instead of the chloride.
On the Mohs scale, caesium scores 0.2, making it the softest of all elements that are solid at room temperature. It is also very ductile. Left near trace oxygen, the pale metal darkens. Stored under mineral oil for transport, it loses its lustre and turns a dull grey.
Its melting point is 28.5 C, so it nearly counts as a liquid at room temperature. Only a handful of metals share that trait. Rubidium melts at 39 C, francium is estimated at 27 C, and gallium melts at 30 C. Mercury is the only stable metal with a known melting point below caesium's. Its boiling point of 641 C is also the lowest of any stable metal except mercury. Blended with potassium and sodium in a mix of 41%, 47% and 12% by moles, it forms the lowest-melting metal alloy known.
The gold tint is a quirk of physics. Moving down the alkali metals, the light needed to excite surface electrons drops in frequency. For lithium through rubidium it sits in the ultraviolet. For caesium it reaches the blue-violet edge of the visible range. The metal absorbs some violet and reflects the rest, so it looks yellowish. Its compounds burn blue or violet in a flame.
Caesium also holds a size record. Among elements whose radii have been measured or calculated, it has the largest atom, at about 260 picometres. That bulk drives much of its strange chemistry.
Anything above about 100 g of caesium is shipped in hermetically sealed stainless steel containers. Smaller amounts travel in vacuum-sealed borosilicate glass ampoules or in dry hydrocarbons such as mineral oil. The precautions exist because the metal ignites on its own in air and is classed as a hazardous material. Handling it safely means working under an inert gas such as argon.
It reacts with water more fiercely than any other alkali metal, even when the water is cold. It can react with ice at very low temperatures. Oddly, a caesium explosion is often weaker than a sodium one of similar size. Caesium detonates the instant it touches water, leaving little time for hydrogen to build up.
The metal sits at an extreme of the periodic table. It is the least electronegative stable element, at 0.79 on the Pauling scale, and the most electropositive. Fluorine sits at the opposite end, and the two together form caesium fluoride. Almost always, caesium takes an oxidation state of +1. In compounds called caesides, though, it carries a negative charge. Mixed with gold and heated, it forms yellow caesium auride, in which gold plays the part of a halogen. Theory suggests that above 30 GPa of pressure, caesium might bond in ways no other alkali metal does.
Burning caesium in air yields mainly the superoxide, CsO2. Researchers have also studied several brightly coloured suboxides, including a dark-green one, Cs3O. Caesium hydroxide is the strongest base in water, and it corrodes glass quickly. Its large, soft ion can be separated from potassium in nuclear waste cleanup, which leads to its radioactive side.
Caesium averages only 3 parts per million in the Earth's crust. That makes it the 45th most abundant element and 36th among metals. It is about 30 times scarcer than rubidium. Its big ions fit poorly into most crystals, so during the cooling of magma it stays in the liquid and crystallizes last. The richest deposits are therefore zoned pegmatites, where the metal ends up concentrated in the final-forming rock.
The only economically important ore is pollucite. In theory it holds 42.6% caesium. Pure samples from the richest known deposit hold about 34%, and the average there is 24% by weight. That deposit is the Tanco Mine at Bernic Lake in Manitoba, Canada. It holds an estimated 350,000 metric tons of pollucite, over two-thirds of the world's reserve base. Other sources include the Bikita pegmatite in Zimbabwe, mined mainly for petalite, and the Karibib Desert in Namibia. With world mine output of just 5 to 10 metric tons per year, reserves would last thousands of years.
Processing is small-scale and selective. Workers crush the ore, sort it by hand and grind it. Chemists then extract caesium by acid digestion, alkaline decomposition or direct reduction. Pure metal can also come from reducing caesium halides with calcium or barium, or from heating caesium azide to 390 C. In 2009-99.8% pure caesium metal cost about 10 dollars per gram. Most mined caesium never becomes metal, however. It is turned into a salt called caesium formate.
Of 41 known caesium isotopes, with mass numbers from 112 to 152, only caesium-133 is stable. It has 78 neutrons. Caesium-135 lasts longest among the radioactive ones, with a half-life of about 1.33 million years. Caesium-137 has a half-life of 30.04 years, and caesium-134 has 2.065 years. Most others vanish within seconds or fractions of a second.
Caesium-137 and strontium-90 are the main medium-lived products of nuclear fission. Together they supply most of the radioactivity in spent fuel from several years to several hundred years after removal. They are also the largest source of leftover radioactivity around the Chernobyl disaster. Neutron capture cannot destroy caesium-137 efficiently, so the only option is to wait for it to decay. Because its parent iodine and xenon are volatile, radioactive caesium often forms far from where the fission took place.
Nuclear weapons testing from the 1950s through the 1980s sent caesium-137 into the atmosphere, and it fell back as fallout. That became a useful marker. Year-to-year differences in the fallout let scientists date soil and sediment layers and trace their movement. Caesium-134 and caesium-135 serve in hydrology to track output from nuclear power, since they come only from human activity.
In living things, radiocaesium follows potassium. About 10% of an absorbed dose leaves quickly in sweat and urine. The other 90% has a biological half-life of 50 to 150 days. Plants take it up unevenly, and mushrooms from contaminated forests collect it in their fruiting bodies. After Chernobyl, buildup in lakes became a major worry. In dogs, 3.8 millicuries per kilogram proved lethal within three weeks. The International Atomic Energy Agency has also warned that caesium-137 could be used in a so-called dirty bomb.
Louis Essen built the first accurate caesium clock in 1955 at the National Physical Laboratory in the United Kingdom. The design counts the electromagnetic transitions inside caesium-133 atoms and uses them as a steady beat. A quote describes these clocks as "the most accurate realization of a unit that mankind has yet achieved."
Their early error was 2 to 3 parts in 10 to the 14th power. That works out to about 2 nanoseconds per day, or one second lost in 1.4 million years. The latest versions do better than 1 part in 10 to the 15th power, or roughly one second in 20 million years. Caesium clocks set the timing for cell phone networks and the internet.
The 1967 redefinition did not end the story. In 2018, the international bureau of weights and measures restated the second at its 26th conference. The new wording fixes the numerical value of the caesium frequency, the unperturbed ground-state hyperfine transition of the caesium-133 atom, and expresses the second from that. The change was one of phrasing rather than of the atom being used.
Caesium-133 has another timing-related role in laboratories. Researchers can cool it with lasers to probe problems in quantum physics. Its Feshbach spectrum is particularly convenient for studying ultracold atoms whose interactions can be tuned.
Before the 1920s, almost nobody had a use for caesium. Then it entered radio vacuum tubes in two roles. As a getter, it soaked up leftover oxygen after manufacture. As a coating on the heated cathode, it improved electrical conductivity. Only in the 1950s was it recognized as a high-performance industrial metal. It went into photoelectric cells, photomultiplier tubes, infrared spectrophotometers and scintillation counter crystals, among other devices.
The biggest use today is in oil drilling. In the mid-1990s, engineers developed caesium formate brines as drilling and completion fluids. These brines reach a density of up to 2.3 g/cm3, which reduces the need for toxic suspended solids. They can be blended with potassium and sodium formates to match the density of water. They are biodegradable and recyclable, which matters given a cost of about $4,000 per barrel in 2001. In 1997, the Cabot Corporation built a plant at the Tanco mine with a capacity of 12,000 oil barrels per year of formate solution.
Safety is part of the appeal. The lethal dose of caesium chloride in mice, 2.3 g/kg, is comparable to potassium chloride and sodium chloride.
Other uses range widely. Caesium chloride solutions separate viruses, organelles and nucleic acids in ultracentrifuges. Caesium nitrate burns silicon in infrared flares such as the LUU-19. Caesium-137 sources have sterilized food and surgical equipment, and measured moisture, density and thickness in industry. They also treated some cancers until safer alternatives and contamination risks from caesium chloride pushed them aside. Early ion engines for spacecraft used caesium as fuel, but corrosion drove designers to xenon, which flew on Deep Space 1 in 1998.
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Common questions
What is caesium and what is its atomic number?
Caesium is a soft, silvery-golden alkali metal with the symbol Cs and atomic number 55. It is the least electronegative stable element, at 0.79 on the Pauling scale.
Who discovered caesium and when?
Robert Bunsen and Gustav Kirchhoff discovered caesium in 1860 in mineral water from Dürkheim, Germany. It was the first element found with a spectroscope.
Why is caesium used to define the second?
Since 1967, the second has been defined by 9,192,631,770 cycles of the hyperfine transition of caesium-133 atoms. Caesium clocks are accurate to about one second in 1.4 million years or better.
Is caesium liquid at room temperature?
Caesium melts at 28.5 C, so it is liquid at or near room temperature. It is one of only a few metals with that property.
Where is caesium mined?
The richest known source is the Tanco Mine at Bernic Lake in Manitoba, Canada, which holds an estimated 350,000 metric tons of pollucite ore. Other sources include the Bikita deposit in Zimbabwe and the Karibib Desert in Namibia.
What is caesium-137 and how long does it last?
Caesium-137 is a radioactive fission product with a half-life of 30.04 years. It is a major source of leftover radioactivity near Chernobyl and has been used in medical and industrial gauges.
What is the main use of caesium today?
The largest use of nonradioactive caesium is caesium formate in oil drilling fluids, developed in the mid-1990s. These dense brines are less toxic than many alternatives.
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