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Noble Gases

Helium II – The super fluid: Helium II ‐ The super fluid Helium II is the only gas that can not be liquefied by lowering temperature without increasing pressure. Below 2.17 K (lambda temperature) helium is converted to Helium II, the superfluid, a quantum mechanical state of matter with strange properties. Helium II has negligible viscosity. When you store Helium II in an open container it shows fountain effect (escapes from the surface against the gravity). Unlike ordinary liquids Helium II will creep along surfaces. If you keep any small open vessel in a closed container containing He‐II (as shown in the figure), He‐II creeps along the surface and after a short while, the levels in the two containers will equalize. Unless the container is carefully constructed, the helium II will creep along the surfaces and through valves until it reaches somewhere warmer, where it will evaporate.

Learning Objectives

After completing the topic, the student will be able to:

  • List the elements present in group 18(VIIA) or zero group in periodic table.
  • Write the electronic configurations of noble gases.
  • Explain the reason for inert behavior of noble gases.
  • Explain the reason for naming noble gases as such.
  • Recognize the importance of discovery of noble gases in developing chemistry.
  • Describe the physical and chemical methods that are used to isolate the noble gases.
  • Appreciate the scientific methods for purification of noble gases.
  • Discuss the physical and chemical properties of noble gases.
  • Identify the structures of xenon fluorides and xenon oxyfluorides based on VSEPR theory.
  • Describe the preparation methods and properties of xenon compounds in detail.
Neon lights Neon lights Neon lights are tubes filled with neon gas. When electricity runs through them, the molecules get excited and the electrons jump to a higher energy level. When they fall back they emit a certain wavelength of light equal to the distance they fall back. Neon lights are red–orange.
Zero group or Group 18 Elements

Neon lights are the commonly used colorful and bright decoration lights. They are named so because of the gas used in them. Neon emits light when electricity excites the atomic electrons. Neon belongs to a family of gases known as noble gases or inert gases, which means unreactive. They belong to the Group 18 of the periodic table starting with Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe) and Radon (Rn). Mnemonic: Here are the New Arrivals. So, Karan is eXit ed to Ride.

If you see the electrons in the atomic orbit in noble gases, they are completely filled. This family of noble gases has been put as group 18 [or zero], at the end of the periodic table because they were not known when the periodic table was first thought about. Nobody knew that they were present because they were so unreactive. The bond between the atoms is so weak that at ordinary temperatures they exist as gases.

The last orbital in all is full or completed. There is no unpaired electron. So the valency is zero. Helium has two and other gases have eight electrons in their outermost shell. So, these shells are full. This makes them very stable. They do not react with other atoms, so they exist as single atoms or they are monoatomic gases.

Noble gases electronic configuration Noble gases electronic configuration All the nobel gases do not have same valence electron configuration. Valence electrons are electrons in the outer shell. They all don't have the same configuration because the noble gas helium only has 2 electrons in its outer shell. This is the stable configuration for first shell. The others have 8.
Discovery of Noble Gases

This group has very interesting 18th century history. Cavendish, while passing an electric discharge in a mixture of air and oxygen to prepare oxides of nitrogen and then removing them and oxygen completely, discovered that there always remained a gaseous residue [1/120 of the original volume of air]. He thought might be a new element.

After a lapse of a few years, during a total eclipse of the sun on 18th August 1868, spectroscopic examination of the sun's chromosphere indicated a new atomic spectral line, not observed from any of the known elements. This suggested the presence of a new element in the sun and the name Helium from Greek word ‘helios’ (meaning sun) was given to it.

Ramsay in 1895 while making a spectroscopic examination of a gas obtained from the mineral clevite, observed helium lines. Thus its presence on the earth was also established. During the same year another member named “Argon” (the lazy one) was discovered and named so because of its inert character. In the next four years other gases namely Krypton, Xenon and Neon were also isolated.

This family was also called rare gases because they occur in relatively small amounts in the atmosphere. Helium is also found as the occluded gas in some of the radioactive minerals and can be released on heating. Others are obtained by fractional distillation of liquid air.

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