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ChemWiki: The Dynamic Chemistry E-textbook > Inorganic Chemistry > Descriptive Chemistry > Main Group Elements > Group 18: The Noble Gases

Group 18: The Noble Gases

The Noble Gases (Group 18) are located in the far right of the periodic table. These elements were previously referred to as the inert gases due to their one important chemical property: they are extremely nonreactive due to their filled valence shell. The reason for this is because Noble Gases were set apart and distinguished relatively late compared to other element groups.

The History

The first person who set apart and characterized the presence of these noble gases was Henry Cavendish in 1875. Cavendish was able to distinguish these elements by chemically removing all the oxygen and nitrogen from a receptacle containing air. The nitrogen was oxidized to NO2by electric discharges and absorbed by a sodium hydroxide solution. The remaining oxygen was then removed from the mixture. Interestingly, the experiment revealed that 1/120 of the gas volume remained un-reacted in the receptacle. The second person who was able to isolate, but not typify them was William Francis (1855-1925). Francis did this by noting the formation of gas while dissolving Uranium containing minerals in acid. 

Argon

In 1894, John William Strutt discovered that pure nitrogen, chemically obtained, was less dense compared to "pure" nitrogen isolated from air. From this breakthrough, he concluded that something else was in the air. With the aid and collaboration with William Ramsay, the two were able to replicate and modify Cavendish's experiment to get a better understanding of the un-reactive fraction of air in his original experiment. Different from Cavendish, Strutt and Ramsay removed the oxygen by reacting it with copper and nitrogen was removed by reacting it with magnesium. The remaining fraction was properly characterized and the new element was named 'Argon' which came from the Greek word 'inert'.

Helium

Helium was first discovered as a bright yellow line with a wavelength of 587.49 nanometers from the sun in 1868. This discovery was made by Pierre Jansen. Jansen initially assumed that it was sodium. However, later studies by Sir William Ramsay (who isolated helium on Earth by treating a variety of rare elements with acids) confirmed that the bright yellow line that came out of the experiment matched up with the spectrum of the sun. From this, British physicist William Crookes was able to identify it as helium.  

Neon, Krypton, Xenon

These noble gases discovered by Morris W. Travers and Sir William Ramsay in 1898. Ramsay discovered Neon by chilling a sample of the atmosphere until liquid phase, then captured the gases as the boiled off by warming up the liquid. Krypton and Xenon were both discovered through this process.  

Radon

In 1900, while studying radium's decay chain, Friedrich Earns Dorn discovered the last noble gas in group 18; Radon. Through some experiments, Dorn noticed that radium compounds emanated radioactive gas. This radioactive gas was originally named niton after the Latin word "nitens". By 1923, the International Committee for Chemical Elements and International Union of Pure Applied Chemistry (IUPAC) decided to name the element as Radon.  All isotopes of radon are radioactive.  Radon-222 has the longest half-life - less than 4 days, and is an alpha-decay product of Radium-226, part of the U-238 to Pb-206 radioactive decay chain.  

The Electron Configurations for Noble Gases: Group 18

Helium      1s2
Neon        [He] 2s2 2p6
Argon      [Ne] 3s2 3p6
Krypton   [Ar] 3d10 4s2 4p6
Xenon     [Kr] 4d10 5s2 5p6
Radon     [Xe] 4f14 5d10 6s2 6p6  

Trends within Group 18                                   

  Atomic # Atomic mass Boiling point (K) Melting point (K) 1st Ionization E/kJ mol-1      Density (g/dm3) Atomic radius/pm
He 2 4.003 4.216 0.95 2372.3 0.1786 31
Ne 10 20.18 27.1 24.7 2080.6  0.9002 38
Ar 18 39.948 87.29 83.6 1520.4 1.7818 71
Kr 36 83.3 120.85 115.8 1350.7 3.708 88
Xe 54 131.29 166.1 161.7 1170.4 5.851 108
Rn 86 222.1 211.5 202.2 1037.1   9.97 120

The Atomic and Physical Properties

  • Atomic mass, boiling point and atomic radii INCREASE down a group in the periodic table. 
  • The first ionization energy, DECREASES down a group in the periodic table. 
  • The noble gases have the largest ionization energies which reflects their chemical inertness.
  • As you go down group 18, atomic radius and inter-atomic forces INCREASES resulting in an INCREASED melting point, boiling point, enthalpy of vaporization and solubility. 
  • Group 18's INCREASE in density is correlated to the INCREASE in atomic mass.
  • Because they are INCREASING in atomic size, the electron clouds of these non polar atoms become increasingly polarized which leads to weak van Der Waals forces among the atoms.  Thus, the formation of liquids and solids is more easily attainable for these heavier elements because of their melting and boiling points.
  • Because noble gases’ outer shell is full, they are extremely stable, not tending to form chemical bonds and have a low tendency to gain or lose electrons.
  • Under standard conditions the members of the noble gas group behave similarly.
  • All are monotomic gases under standard conditions.
  • Noble gas atoms, like other atoms in other groups, INCREASE steadily in atomic radius from one period to the next due to the INCREASING number of electrons. 
  • The size of the atom is positively correlated to several properties of noble gases.  The ionization potential DECREASES with an INCREASING radius, because the valence electrons in the larger noble gases are further away from the nucleus, so they are not being held as tightly together by the atom. 
  • The attractive force INCREASES with the size of the atom as a result of an INCREASE in polarizability and thus a DECREASE in ionization potential.
  • Overall, noble gases have weak interatomic forces, and therefore very low boiling and melting points compared to other group elements. 

For covalently-bonded diatomic and polyatomic gases, heat capacity consists of translational, rotational, and vibrational fractions.  Since monatomic gases such as noble gases have no bonds, they cannot absorb heat as bond vibrations.  Since the center of mass of monatomic gases is at the nucleus of the atom, and the mass of the electrons is negligible compared to the nucleus, the kinetic energy due to rotation is negligible compared to the kinetic energy of translation, unlike in di- or polyatomic molecules where rotation of nuclei around the center of mass of the molecule contributes to the heat capacity.  Therefore, the internal energy per mole of a monatomic noble gas equals its translational contribution = (3/2) R T, where R = universal gas constant and T = absolute temperature. 

For monatomic gases at a given temperature, the average kinetic energy due to translations of the atoms is practically equal regardless of the element.  Therefore at a given temperature, the heavier the atom, the more slowly its gas atoms will move.  Monatomic gas mean velocity DECREASES with increasing molecular mass, and given the simplified heat capacity situation, noble gaseous thermal conductivity DECREASES with increasing molecular mass.  

 The Chemical Properties

  • They all are odorless, colorless, nonflammable, and monotonic gases that have low chemical reactivity.
Electron Configurations

Helium.JPG neon.JPG Argon.JPG

Krypton.JPG Zenon.JPG Radon.JPG

Correction: Xenon

 

Atomic Number Element Number of Electrons/Shell
2 Helium 2
10 Neon 2,8
18 Argon 2,8,8
36 Krypton 2,8,18,8
54 Xenon 2,8,18,18,8
86 Radon 2,8,18,32,18,8
  • Their full valence electron shells make noble gases extremely stable and least likely to form chemical bonds because they have little tendency to gain or lose electrons.
Compounds
  • However, although noble gases do not tend to react with other elements to form compounds, there are some exceptions! Xe may form compounds with Fluoride and Oxide.

Example  

XeF2 (Xenon Difluoride)

  • Dense white crystalized solid
  • Powerful fluorinating agent
  • Covalent inorganic fluorides
  • Stable xenon compound
  • Decomposes on contact with light or water vapor
  • Linear structure
  • Moister sensitive
  • Low vapor pressure
  • XeF2.JPG

XeF4(Xenon Tetrafluoride)

  • Colorless Crystals
  • Square planar structure
  • Found in 1963
  • XeF4.JPG

XeF6(Xenon Hexafluoride)

  • Strongest fluorinating agent
  • Colorless solid
  • Highest of the three binary fluorides of xenon (XeF2 and XeF6)
  • Exergonic and stable at normal temperatures
  • Readily sublimes into intense yellow vapors
  • Structure lacks perfect octahedral symmetry
  • XeF6.JPG

XeO4 (Xenon Tetroxide)

  • Yellow crystalline solid
  • Relatively stable compound of the noble gases
  • Oxygen is the only element that can bring xenon up to its highest oxidation state.
  • XeO4.JPG

IMPORTANT! Xenon reacts directly ONLY with Fluorine.


The Uses of Noble Gases in Everyday Life

Helium

is used as a component of breathing gases due to its low solubility in fluids or lipids.  For example, gases are absorbed by the blood and body tissues when under pressure during scuba diving. Because of its reduced solubility, little helium is taken into cell membranes, when it replaces part of the breathing mixture, helium causes a decrease in the narcotic effect of the gas at far depths.  The reduced amount of dissolved gas in the body means fewer gas bubbles form decreasing the pressure of the ascent. Helium and Argon are used to shield welding arcs and the surrounding base metal from the atmosphere during welding. 

Helium is used in very low temperature cryogenics, particularly for maintaining superconductors at a very low temperature.  Superconductivity is useful for creating very strong magnetic fields. Helium is also the most common carrier gas in gas chromatography.  

Neon

is used for many applications that we see in daily life. For examples: Neon lights, fog lights, TV cine-scopes, lasers, voltage detectors, luminous warnings and also advertising signs. The most popular applications of Neon would be the Neon tubes that we see for advertisement or elaborate decorations. These neon tubes consist with neon and helium or argon under low pressure submitted to electrical discharges. The color of emitted light shown is dependent on the composition of the gaseous mixture and also with the color of the glass of the tube. Pure Neon within a colorless tube can obtain a red light, which reflects a blue shine. These reflected light are also known as fluorescent light.

blue-neon-lights.jpg

One of the many colors of neon lights.

Argon

Argon is used for a diverse group of applications in the growing industries of : electronics, lighting, glass, and metal fabrications. Argon is used in electronics to provide a protective heat transfer medium for ultra-pure semiconductors from silicon crystals and for growing germanium. Argon can also fill fluorescent and incandescent light bulbs; creating the blue light found in neon type lamps. By utilizing argon's low thermal conductivity, window manufacturers can provide a gas barrier needed to produce double-pane insulated windows. This insulation barrier improves the windows' energy efficiency. Argon also creates an inert gas shield during welding; to flush out melted metals to eliminate porosity in casting; and to provide an oxygen-and-nitrogen free environment for annealing and rolling metals and alloys.

Argon light bulb.jpg

Argon plasma light bulb.

Krypton 

Just like argon, krypton can be found in energy efficient windows. It is also found in fuel sources, lasers and headlights. It is estimated that 30% of energy efficient windows sold in Germany and England are filled with krypton; approximately 1.8 liters of krypton. Being more thermally efficient, krypton is sometimes chosen over argon as a choice for insulation. Krypton can be found in lasers which works as a control for a desired optic wavelength. It is usually mixed with a halogen (most likely fluorine) to produce typically called "excimer" lasers. Krypton is sometimes used within halogen sealed beam headlights. These headlights produces up to double the light output of standard headlights for a brighter gleam. Also, Krypton is used for high performance light bulbs which have higher color temperatures and efficiency because it reduces the rate of evaporation of the filament.

krypton laser.jpg

Krypton laser.

Xenon

Xenon us used for various applications. From incandescent lighting, to development in x-rays, to plasma display panels (PDPs) and much more. Incandescent lighting uses Xenon because less energy can be used to produce the same amount of light output as a normal incandescent lamp. Xenon has also made it possible to obtain better x-rays with reduced amounts of radiation. When mixed with oxygen, it can enhance the contrast in CT imaging. The revolutionize the health care industries. Plasma display panels (PDPs) using xenon as one of the fill gases may one day replace the large picture tube in television and computer screens. This promises a revolution in the television and computer industries.

Nuclear fission products may include a couple of radioactive isotopes of xenon, which also absorb neutrons in nuclear reactor cores. The formation and elimination of radioactive xenon decay products can be a factor in nuclear reactor control.  

Radon

Radon has been said to be the second most frequent cause of lung cancer, after cigarette smoking. However, it can be found in various beneficial applications as well. For examples through: radiotherapy, relief from arthritis, and bathing. In radiotherapy, radon has been used in implantable seeds, made of glass or gold, primarily used to treat cancers. For arthritis, its been said that exposure to radon mitigates auto-immune diseases such as arthritis. Those who have arthritis have actually sought limited exposure to radioactive mine water and radon to relief their pain. However, radon has nevertheless found to induce beneficial long-terms effect. Some places actually have "Radon Spas". For examples: Bad Gastern, Austria and Japanese Onsen in Misasa, Tottori. "Radon Spa" is a relieving therapy where people sit for minutes to hours in a high-radon atmosphere, believing that low doses of radiation will boost up their energy.

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