Donald B. Sullivan, a physicist and chief of the time and frequency division of the National Institute of Standards and Technology, explains.

The atomic number of cesium (Cs) is 55. If an atom of cesium has 78 neutrons, what is the atomic mass of cesium? Dec 16, 2002 In a collaboration between Essen and Markowitz, the relative durations of the astronomical and atomic (cesium) seconds were measured over an averaging time of 2.75 years with a final determination. Cesium Cesium is a chemical element with symbol Cs and atomic number 55. Classified as a n alkali metal, Cesium is a solid at room temperature.

When the cesium second was defined in 1967, it was based on a measurement of the number of cycles of the radiation from a particular cesium-133 transition with reference to the second commonly used in civilian timekeeping, which at that time was based on astronomical observations. The objective was to improve the stability of timekeeping in a manner that would be invisible to the general population. The decision to redefine the second was ultimately that of the International Committee of Weights and Measures, an organization that works to standardize and coordinate measurements. At its 13th official meeting in 1967, the committee adopted the following definition: 'The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.'

In making this decision, the committee relied primarily on a measurement first reported in 1958 that compared the cesium transition frequency to the second of ephemeris time, which is defined by the orbital motion of the earth about the sun. A collaboration between the National Physical Laboratory (NPL) in England and the United States Naval Observatory (USNO) produced the measurement. Louis Essen of NPL had just developed the worlds first reliable cesium-beam atomic clock, and William Markowitz of USNO had developed a moon-position camera that provided a way to easily access ephemeris time, something that had been previously very difficult to do.

For years prior to this measurement people had recognized that the earths motions were not sufficiently predictable for highly accurate timekeeping, and alternatives based on atomic clocks were under study. Harold Lyons and his collaborators at the National Bureau of Standards (NBS) (now renamed the National Institute of Standards and Technology (NIST)) made one of the first accurate assessments of the cesium frequency relative to earth-based time in 1952. The cesium-beam standard used for this measurement could be operated for only short periods, so the uncertainty of Lyons measurement was too large for it to serve as the basis for a new definition.

NPL holds the distinction of developing the first operational cesium-beam atomic clock. Essen received funding for his clock project in 1953 and had a very reliable version running within two years. Modemunlock. In a collaboration between Essen and Markowitz, the relative durations of the astronomical and atomic (cesium) seconds were measured over an averaging time of 2.75 years with a final determination that the cesium frequency was 9,192,631,770 20 Hz. The definition of the second accepted internationally uses the exact number produced by this measurement. It is interesting to note that the timing comparison across the Atlantic Ocean was made using a method based on simultaneous reception of the shortwave time signals broadcast by the NBS radio station WWV, which was then located on the east coast of the United States.

The story of these measurements is nicely detailed in Splitting the Second: The Story of Atomic Time, by Tony Jones (Institute of Physics, 2000). The report on the measurement of the cesium frequency appeared in Physical Review Letters, vol. 1, p. 105, 1958.

Overview

Cesium is a member of the alkali family, which consists of elements in Group 1 (IA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. The alkalis include lithium, sodium, potassium, rubidium, and francium. Cesium is considered the most active metal. Although in theory francium is more active than cesium, francium is too rare to have any commercial uses.

Cesium was discovered in 1861 by German chemists Robert Bunsen (1811-99) and Gustav Kirchhoff (1824-87). They found the element using a method of analysis they had just invented: spectroscopy. Spectroscopy is the process of analyzing light produced when an element is heated. The light produced is different for every element. The spectrum (plural: spectra) of an element consists of a series of colored lines.

SYMBOL
Cs

ATOMIC NUMBER
55

ATOMIC MASS
132.9054

FAMILY
Group 1 (IA)
Alkali metal

>

PRONUNCIATION
SEE-zee-um

Cesium is not a common element, and it has few commercial uses. One of its radioactive isotopes, cesium-137, is widely used in a variety of medical and industrial applications.

Discovery and naming

The invention of spectroscopy gave chemists a powerful new tool. In many cases, the amount of an element present in a sample is too small to see. But the element is much easier to detect by spectroscopy. When the substance is heated, the hidden elements give off characteristic spectral lines. Using spectroscopy, a chemist can identify the elements by these distinctive lines.

Such was the case with the discovery of cesium. In 1859, Bunsen and Kirchhoff were studying a sample of mineral water taken from a spring. They saw spectral lines for sodium, potassium, lithium, calcium, and strontium. These elements were already well known.

After Bunsen and Kirchhoff removed all these elements from their sample, they were surprised to find two beautiful blue lines in the spectrum of the 'empty' spring water. The water contained an unknown element. Bunsen suggested calling the element cesium, from the Latin word caesius for 'sky blue.' For many years, the name was also spelled caesium.

Physical properties

Cesium is a silvery-white, shiny metal that is very soft and ductile. Ductile means capable of being drawn into thin wires. Its melting point is 28.5°C (83.3°F). It melts easily in the heat of one's hand, but should never be handled that way! Cesium's boiling point is 705°C (1,300°F), and its density is 1.90 grams per cubic centimeter.

Chemical properties

Cesium is a very reactive metal. It combines quickly with oxygen in the air and reacts violently with water. In the reaction with water, hydrogen gas is released. Hydrogen gas ignites immediately as a result of the heat given off by the reaction. Cesium must be stored under kerosene or a mineral oil to protect it from reacting with oxygen and water vapor in the air.

Cesium also reacts vigorously with acids, the halogens, sulfur, and phosphorus.

Occurrence in nature

The abundance of cesium in the Earth's crust has been estimated at about 1 to 3 parts per million. It ranks in the middle

of the chemical elements in terms of their abundance in the earth.

Cesium occurs in small quantities in a number of minerals. It is often found in an ore of lithium called lepidolite. The mineral containing the largest fraction of cesium is pollucite (Cs ₄ Al ₄ Si ₉ O ₂₆ ). This ore is mined in large quantities at Bernic Lake, in the Canadian province of Manitoba. Cesium is also found in small amounts in a mineral of boron called rhodizite.

Isotopes

Only one naturally occurring isotope of cesium is known, cesium-133. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

A number of artificial radioactive isotopes of cesium are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

One radioactive isotope of cesium is of special importance, cesium-137. It is produced in nuclear fission reactions. Nuclear fission is the process in which large atoms break apart. Large amounts of energy and smaller atoms are produced during fission. The smaller atoms are called fission products. Cesium-137 is a very common fission product.

Nuclear fission is used in nuclear power plants. The heat produced by nuclear fission can be converted into electricity. While this process is going on, cesium-137 is being produced as a by-product. That cesium-137 can be collected and used for a number of applications.

As an example, cesium-137 can be used to monitor the flow of oil in a pipeline. In many cases, more than one oil company may use the same pipeline. How does a receiving station know whose oil is coming through the pipeline? One way to solve that problem is to add a little cesium-137 when a new batch of oil is being sent. The cesium-137 gives off radiation. That radiation can be detected easily by holding a detector at the end of the pipeline. When the detector shows the presence of radiation, a new batch of oil has arrived.

This isotope of cesium can also be used to treat some kinds of cancer. One procedure is to fill a hollow steel needle with cesium-137. The needle can then be implanted into a person's body. The cesium-137 gives off radiation inside the body. That radiation kills cancer cells and may help cure the disease.

Cesium Atomic Mass

When a hollow steel needle filled with cesium-137 is implanted into a person's body, the isotopes's radiation can kill cancer cells.

Cesium-137 is often used in scientific research also. For example, cesium tends to stick to particles of sand and gravel. This fact can be used to measure the speed of erosion in an area. Cesium-137 is injected into the ground at some point. Some time later, a detector is used to see how far the isotope has moved. The distance moved tells a scientist how fast soil is being carried away. In other words, it tells how fast erosion is taking place.

Cesium-137 has also been approved for the irradiation of certain foods. The radiation given off by the isotope kills bacteria and other organisms that cause disease. Foods irradiated by this method last longer before beginning to spoil. Wheat, flour, and potatoes are some of the foods that can be preserved by cesium-137 irradiation.

Extraction

Cesium can be obtained in pure form by two methods. In one, calcium metal is combined with fused (melted) cesium chloride:

In the other, an electric current passes through a molten (melted) cesium compound:

Uses

Cesium has a limited number of uses. One is as a getter in bulbs and evacuated tubes. The bulb must be as free from gases as possible to work properly. Small amounts of cesium react with any air left in the bulb. Black keys discography. It converts the gas into a solid cesium compound. Cesium is called a getter because it gets gases out of the bulb.

Cesium is also used in photoelectric cells, devices for changing sunlight into electrical energy. When sunlight shines on cesium, it excites or energizes the electrons in cesium atoms. The excited electrons easily flow away, producing an electric current.

An important use of cesium today is in an atomic clock. An atomic clock is the most precise method now available for measuring time. Here is how an atomic clock works:

Cesium-137 is used in atomic clocks, the most precise method for measuring time.

A beam of energy is shined on a very pure sample of cesium-133. The atoms in the cesium are excited by the energy and give off radiation. That radiation vibrates back and forth, the way a violin string vibrates when plucked. Scientists measure the speed of that vibration. The second is officially defined as that speed of vibration multiplied by 9,192,635,770.

Atomic clocks keep very good time. The best of them lose no more than one second in a million years.

Compounds

What Is Cesium Atomic Number

Cesium compounds have relatively few commercial uses. Cesium bromide is used to make radiation detectors and other measuring devices. Cesium carbonate and cesium fluoride are used to make specialty glasses. Cesium carbonate and cesium chloride are used in the brewing of beers. Cesium compounds are also used in chemical research.

Cesium Atomic Number Electrons

Health effects

Cesium is not regarded as essential to the health of plants or animals, nor does it present a hazard to them.