PTL:Cesium

From ChemEd Collaborative

Cesium

Cesium is a Group IA or alkali metal in the 6th period. Other members of the group are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and francium (Fr).

1 Discovered
2 Name
3 Characteristics
4 Physical Data
5 Atomic Data
6 Found
7 Prepared
8 Cesium Compounds
9 Reactions
10 Uses
11 "Cesium Atomic Clocks"
12 "Cesium Effect"
13 Emission Spectrum
14 References

Discovered

Cs was discovered in 1860 by Bunsen and Kirchhoff, and was the first element discovered spectroscopically.

Name

The name, from the Latin word caesius for "sky blue", refers to the characteristic blue lines in the emission spectrum of the element.

Characteristics

Pure Cs is a silvery-white, soft, ductile metal. Of the five Group IA metals it:

has the highest vapor pressure,
has the lowest boiling point (670°C),
has the biggest mass,
has the highest electrical resistivity,
is the most electropositive,
has the lowest ionization energy, and
it is the least abundant in the earth's crust.

Cs, with a melting point of only 28.6°C, is one of only three metals - Hg, Cs, and Ga - that can easily be liquids at room temperature.

Cesium exists in nature only as the stable isotope Cs-133, but several radioactive isotopes of cesium can be produced artificially in nuclear reactors during reactions involving fission of uranium.

The wavelengths of the cesium spectrum that are observed with the most intense lines are 4560 Å and 4597 Å.

Like other metals, cesium exhibits a photoelectric effect.

Physical Data

State at 25°C: solid

Density (g/cm^3): 1.9

Hardness (Mohs): 0.2

Melting Point (K): 301.65

Boiling Point (K): 944

Heat of Fusion (kJ/mol): 2.09

Heat of Vaporization (kJ/mol): 63.9

Heat of Atomization (kJ/mol): 76.06

Thermal Conductivity (J/m sec K): 35.9

Electrical Conductivity (1/mohm cm): 48.8

Abundance, Solar System: -0.42945706

Abundance, Earth's Crust: 3.0

Pure Cost ($/100g): 3928

Bulk Cost ($/100g): 1370

Atomic Data

Number of isotopes: 1

Electron affinity (kJ/mol): 44.5617

First Ionization Energy (kJ/mol): 375.7041

Second Ionization Energy (kJ/mol): 2234.3535

Third Ionization Energy (kJ/mol): n/a

Electronegativity: 0.79

Polarizability (Å3): 59.6

Atomic Volume (cm3/mol): 68.86

Ionic Radius2- (pm): no data

Ionic Radius1- (pm): no data

Atomic Radius (pm): 265

Ionic Radius1+ (pm): 167

Ionic Radius2+ (pm): no data

Ionic Radius3+ (pm): no data

Common Oxidation Numbers: +1

Other Oxidation Numbers: none

Found

Cs is widely distributed in the earth's crust, but at very low concentrations. However, it is an important constituent of the mineral pollucite [(Cs,Na)₂Al₂Si₄O₁₂·(H₂O)], a hydrated cesium aluminosilicate. One of the world's richest sources is in Manitoba, Canada where there are an estimated 300,000 tons of pollucite averaging 20% cesium.

Minerals Containing Cesium
Pollucite

Another mineral that contains cesium is rhodzite, which is a hydrated borate of aluminum, beryllium, sodium and cesium; unlike pollucite, the mineral doesn't contain a very large percentage of cesium.

Lepidolites also contain cesium, but the concentration of cesium in the lepidolites is very low.

Cesium salts can be found in their dissolved state in sea water and in mineral springs.

Prepared

First, cesium has to be extracted from wherever it is located; the easiest place to get cesium is obviously pollucite (as mentioned in the above section). There are many ways to obtain cesium; an example method is listed below:

~Concentrated hydrochloric acid gets added to crushed pollucite, and the solution is then diluted in order to eliminate silicic acid which gets filtered off. Next, cesium gets precipitated from solution as a double salt with antimony chloride. After this, boiling an aqueous suspension of the double salt hydrolyzes the double salt. Finally, as the solution becomes heated, cesium chloride gets converted into cesium nitrate, the cesium nitrate gets converted into cesium oxalate, and the cesium oxalate gets converted into cesium carbonate.

Next, the element is prepared by the reduction of one of its salts, often CsCl, with an electropositive metal, usually Ca.

2 CsCl + Ca CaCl₂ + 2 Cs

Listed below are other examples in which cesium may be obtained.

~Reducing cesium oxide with magnesium.

~Reducing cesium carbonate with magnesium in a current of hydrogen.

~Reducing cesium hydroxide with iron or nickel.

~Reducing cesium cyanide with iron or nickel oxide.

~Decomposing cesium azide.

~Reacting barium azide with cesium chloride.

~Reacting zirconium with cesium chloride or cesium chromate.

Cesium Compounds

Cesium auride (CsAu)- This substance is an ionic intermetallic compound, and has semiconductor properties due to the high electropositivity of cesium. CsAu reacts vigorously with water; the reaction results in the production of cesium hydroxide and metallic gold.

Cesium uranate- This substance contains cesium, uranium, and oxygen. Cs137 can be produced from cesuim uranate during fuel rod reprocessing.

Cesium graphite- This is an interlamellar alkali metal compound. Cesium graphite is very reactive; it ignites when it makes contact with air, and explodes when it is in water. In addition, it has a higher electrical conductivity than pure graphite.

Cesium chloride- This substance is used in self-contained irradiators at hospitals and universities for blood irradiation, biomedical research, radiation research, and other industrial uses. Around 1,300 radioactive high-activity cesium chloride devices, with most of them in self-contained irradiators, are being used in the United States. The National Research Council has requested that radioactive cesium chloride radiation sources be replaced due to concern that cesium-137, the form of cesium in cesium chloride, could be used in a terrorist attack. The cesium in the compound, if used in an attack, could contaminate an area and make it uninhabitable for a long period of time. The National Research Council suggested that the government stop licensing new cesium chloride irradiators or make incentives to buy back used irradiators, and also suggested that the government create incentives for the introduction of replacement irradiators. Cesium chloride has a melting point of 645°C, and a boiling point of 1303°C. The substance can form a cesium mixed halide with a molecular formula of CsCl₂I by adding iodine to a hot solution of cesium chloride in the presence of excess chlorine gas.

Cesium hydroxide- This substance, which is a strong base, can be made by metathesis of barium hydroxide and cesium sulfate. (However, electrolysis of cesium chloride is the main method for preparing a pure hydroxide solution.) Cesium hydroxide is a highly deliquescent, crystalline solid, and it is soluble in water. (The reaction with water gives off heat.) Cesium hydroxide attacks many metals, and promotes the alkylation of alcohols with haloalkanes. In addition, cesium hydroxide can be used as a catalyst; for example, cesium hydroxide can catalyze the reaction in which amines and alcohols add to phenylacetylene.

Cesium permanganate- This substance can be prepared by adding cesium nitrate to a saturated solution of potassium permanganate at 60°C. On cooling, it crystallizes out as an anhydrous salt. Cesium permanganate is the least soluble of all the alkali permanganates. Cesium permanganate decomposes at 320°C.

Cesium schonites- The cesium schonites have the formula Cs₂M(SO₄)₂·6H₂O, in which M may be an element such as Zn, Ni, Co, Fe, Cu, Mn, and V. The cesium schonites form isomorphous monoclinic crystals and are moderately soluble in water. Many schonites can be prepared, and these schonites have been studied in investigation of the effect of replacing atoms in a crystal lattice by other similar atoms.

Cesium nitrite- This substance can be prepared by decomposing cesium nitrate. Cesium nitrite is a strong electrolyte, melts on heating to a yellow liquid which decomposes at higher temperatures, and hydrolyzes to nitrous acid on boiling with water. Cesium nitrite forms only very small crystals and is hygroscopic.

Cesium fluoride- This substance shows the highest reactivity out of all of the alkali metal fluorides, and has a low solubility in aprotic solvents. Cesium fluoride can be used with bromine to fluorinate perfluoroalkylnitrile, and it is commonly used in desilylation reactions. Cesium fluoride can be used as a base in the synthesis of crown compounds from phenols and the ditosylates of polyethylene glycols. The substance has a melting point of 684°C, and has a boiling point at 1251°C. Cesium fluoride can become cesium polyfluoride with a molecular formula of CsF₃ by reacting with gaseous fluorine at 140-200°C.

Cesium fluoroxysulfate (CsOSO₂OF)- This substance can be used in electrophilic fluorination reactions; for example, alkenes and phenol can be fluorinated electrophilically with cesium fluoroxysulfate. Also, cesium fluoroxysulfate can be used to convert aldehydes into acid fluorides in great yields.

Cesium carbonate- This substance is used often as a base. Cesium carbonate is able to deprotonate tosyl amides. In additon, cesium carbonate can be used to help make calix[4]arenes and carcerands.

Cesium silicide- This substance has a slight coppery color. Cesium silicide is extremely sensitive to moisture, and ignites explosively on direct contact with water and dilute acids.

Cesium germanide- This substance has dark colored crystals, and is stable only if it is in a dry inert gas. Cesium germanide decomposes when it contacts air, water, or dilute acids; however, cesium germanide is more stable than cesium silicide.

Reactions

The element is similar in its reactivity to potassium and rubidium, except that it is oxidized much more readily than any other Group IA metal. It reacts rapidly with air to give the monoxide (Cs₂O), the peroxide (Cs₂O₂), but primarily the superoxide (CsO₂).

Cs(s) + O₂(g) CsO₂(s)

Formation of peroxides is characteristic of potassium and rubidium as well.

Cesium reacts strongly with hydrogen to produce cesium hydride.

The element reacts violently with water to give cesium hydroxide, CsOH, and H₂.

2 Cs(s) + 2 H₂O(liq) H₂(g) + 2 CsOH(aq)

The hydroxide is the strongest base known; it readily attacks glass. (More information is given in "Cesium Compounds" section.)

Cesium dissolves in liquid ammonia.

Cesium can react with numerous elements to form compounds. Listed below are some of the compounds that can be formed:

~Cesium azide (CsN₃) can be prepared by reacting hydrazoic acid with either cesium carbonate or cesium hydroxide. (The compound could also be formed by reacting cesium sulfate with barium nitride.)

~Under vacuum cesium vapors react with phosphorus to form Cs₂P₅.

~Cesium silicide can be made by direct synthesis from cesium and silicon.

~Cesium germanide can be made by direct synthesis from cesium and germanium.

~A compound can be made that has the molecular formula Na₂Cs by having cesium react with sodium. (Other intermetallic compounds with cesium and the other alkali metals can be formed as well.)

Salts of cesium are similar to those of other alkali metals and are generally quite water-soluble. (Some examples of these salts include cesium fluoride, cesium chloride, cesium bromide, and cesium iodide. These salts can be made from cesium carbonate and the hydrogen halide acids.) Listed below are some some salts of cesium and how they are prepared:

~Cesium chlorate can be prepared by reacting a concentrated solution of cesium hydroxide with gaseous chlorine.

~Cesium bromate can be prepared by reacting cesium hydroxide with HBrO₃.

~Cesium iodate can be prepared by passing gaseous chlorine into a solution with cesium iodide and cesium hydroxide.

~Cesium sulfate can be prepared by reacting sulfuric acid with cesium carbonate or cesium chloride.

~Cesium nitrate can be prepared by reacting nitric acid with cesium carbonate or cesium chloride.

~Cesium metagermanate (Cs₂GeO₃) can be prepared by melting cesium carbonate with germanium dioxide.

Cesium salts can also react to form various double salts. For example, cesium nitrate forms a double salt with lead nitrate in aqueous solution. Typically, the double salts of cesium are sparingly soluble.

Uses

Cesium is the basis of atomic clocks which are used as the international time standard

Cesium is used in making photocells

Because it reacts so readily with air, Cs is used as a "getter" in vacuum tubes (see also Rubidium). Furthermore, since it is so electropositive, it has been used in [photocells].

The element is also the basis of so-called "atomic clocks", which are accurate to 5 seconds in 300 years and serve as the [international time standard]. (More information about cesium atomic clocks is given in the "Cesium Atomic Clocks" section.)

The principal use of the element has been in research on ion propulsion engines for use in outer space. It is known that, in principle, 1 lb of Cs will propel a vehicle 140 times farther than by burning the same quantity of any other known liquid or solid.

Cesium ions have been used to influence numerous cyclization reactions and cause various substances to appear in reactions in higher yields; this influence is referred to as the "cesium effect." (More information about substances formed from the cesium effect can be found in the "Cesium Effect" section.)

"Cesium Atomic Clocks"

A "cesium atomic clock" very accurately measures time in seconds. Notably, the SI time unit of seconds measured by these atomic clocks was based on a frequency measurement of cesium-133 atoms; specifically, the length of a second was defined by the 13th General Conference on Weights and Measures in 1967 as "the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state undisturbed by external fields." The frequency of cesium-133 atoms can be accurately measured because the atoms are kept in a vacuum of about 10 trillionths of an atmosphere, and this causes the atoms to radiate in a narrow spectral line whose frequency can be measured. There are two main types of cesium clocks: a "laboratory standard" that's as big as a railroad flatcar and a "commercial standard" that's as big as a suitcase; both standards do an excellent job of measuring time.