KRYPTON

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Photo by: OlesiaRu

Overview

Krypton was one of three noble gases discovered in 1898 by Scottish chemist and physicist Sir William Ramsay (1852-1916) and English chemist Morris William Travers (1872-1961). Ramsay and Travers discovered the gases by allowing liquid air to evaporate. As it did so, each of the gases that make up normal air boiled off, one at a time. Three of those gases—krypton, xenon, and neon, were discovered for the first time this way.

The term noble gas refers to elements in Group 18 (VIIIA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. These gases have been given the name "noble" because they act as if they are "too arrogant" to react with other elements. Until the 1960s, no compound of these gases was known. Since they are so inactive, they are also called the inert gases. Inert means inactive.

Krypton has relatively few commercial uses. All of them involve lighting systems in one way or another.

Discovery and naming

By 1898, two members of the noble gas family had been discovered. They were helium (atomic number 2) and argon (atomic number 18). But no other elements in the family had been found. The periodic table contained empty boxes between helium and argon and below argon. The missing noble gases had atomic numbers 10, 36, 54, and 86. Chemists think of empty boxes in the periodic table as "elements waiting to be discovered."

Since the two known noble elements, helium and argon, are both gases, Ramsay and Travers hoped the missing elements were also gases. And if they were, they might be found in air. The problem was that air had already been carefully analyzed and found to be about 99.95 percent oxygen , nitrogen , and argon. Was it possible that the missing gases were in the last 0.05 percent of air?

To answer the question, the chemists worked not with air itself, but with liquid air. Air becomes liquid simply by cooling it far enough. The colder air becomes, the more gases within it turn into liquids. At -182.96°C (-297.33°F), oxygen changes from a gas into a liquid. At -195.79°C (-320.42°F), nitrogen changes from a gas into a liquid. And so on. Eventually, all the gases in air can be made to liquefy (change into a liquid).

But the reverse process also takes place. Suppose a container of liquid air holds 100 liters. The liquid air will warm up slowly. When its temperature reaches -195.79°C, liquid nitrogen changes back to a gas. Since about 78 percent of air is nitrogen, only 22 percent of the original liquid air (22 liters) will be left.

When the temperature reaches -182.96°C, oxygen changes from a liquid back to a gas. Since oxygen makes up 21 percent of air, another 21 percent (21 liters) of the liquid air will evaporate.

The work of Ramsay and Travers was very difficult, however, because the gases they were looking for are not abundant in air. Krypton, for example, makes up only about 0.000114 percent of air. For every 100 liters of liquid air, there would be only 0.00011, or about one-tenth of a milliliter of krypton. A tenth of a milliliter is about a drop. So Ramsay and Travers—although they didn't know it—were looking for one drop of krypton in 100 liters of liquid air!

Amazingly, they found it. The discovery of these three gases was a great credit to their skills as researchers. They suggested the name krypton for the new element. The name was taken from the Greek word kryptos for "hidden."

Physical properties

Krypton is a colorless, odorless gas. It has a boiling point of -152.9°C (-243.2°F) and a density of 3.64 grams per liter. That makes krypton about 2.8 times as dense as air.

Chemical properties

For many years, krypton was thought to be completely inert. Then, in the early 1960s, it was found to be possible to make certain compounds of the element. English chemist Neil Bartlett (1932-) found ways to combine noble gases with the most active element of all, fluorine. In 1963, the first krypton compounds were made—krypton difluoride (KrF ₂ ) and krypton tetrafluoride (KrF ₄ ). Other compounds of krypton have also been made since that time. However, these have no commercial uses. They are only laboratory curiosities.

Occurrence in nature

The abundance of krypton in the atmosphere is thought to be about 0.000108 to 0.000114 percent. The element is also formed in the Earth's crust when uranium and other radioactive elements break down. The amount in the Earth's crust is too small to estimate, however.

Isotopes

Six naturally occurring isotopes of krypton exist. They are krypton-78, krypton-80, krypton-82, krypton-83, krypton-84, and krypton-86. 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.

At least sixteen radioactive isotopes of krypton 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 krypton is used commercially, krypton-85. It can be combined with phosphors to produce materials that shine in the dark. A phosphor is a material that shines when struck by electrons. Radiation given off by krypton-85 strikes the phosphor. The phosphor then gives off light. The same isotope is also used for detecting leaks in a container. The radioactive gas is placed inside the container to be tested. Since the gas is inert, krypton will not react with anything else in the container. But if the container has a leak, some radioactive krypton-85 will escape. The isotope can be detected with special devices for detecting radiation.

Krypton-85 is also used to study the flow of blood in the human body. It is inhaled as a gas, and then absorbed by the blood. It travels through the bloodstream and the heart along with the blood. Its pathway can be followed by a technician who holds a detection device over the patient's body. The device shows where the radioactive material is going and how fast it is moving. A doctor can determine whether this behavior is normal or not.

Extraction

Krypton is still obtained by allowing liquid air to evaporate.

Uses

The only commercial uses of krypton are in various kinds of lamps. When an electric current is passed through krypton gas, it gives off a very bright light. Perhaps the most common application of this principle is in airport runway lights. These lights are so bright that they can be seen even in foggy conditions for distances up to 300 meters (1,000 feet). The lights do not burn continuously. Instead, they send out very brief pulses of light. The pulses last no more than about 10 microseconds (10 millionths of a second). They flash on and off about 40 times per minute. Krypton is also used in slide and movie projectors.

Krypton gas is also used in making "neon" lights. Neon lights are colored lights often used in advertising. They are similar to fluorescent light bulbs. But they give off a colored light because of the gas they contain. Some neon lights do contain the gas neon, but others contain other noble gases. A neon light filled with krypton, for example, glows yellow.

Compounds

Compounds of krypton have been prepared in the laboratory but do not exist in nature. The synthetic (artificial) compounds are used for research purposes only.

Health effects

There is no evidence that krypton is harmful to humans, animals, or plants.