The First Helium Compounds

Can helium form compounds?
Helium, He, is the first Group 18 element or Noble Gas element.
In the nucleus of a helium atom there are 2 protons, and surrounding the nucleus there are just 2 electrons. These 2 electrons complete the first energy level and require large amounts of energy to remove. For this reason, an atom of helium is very stable and does not undergo chemical reactions on Earth.
But what if you tried to react helium, a non-metal, with a highly reactive metal, such as sodium, at extremely high pressures (greater than 113 GPa)?
Utah University Chemists predicted that under these conditions helium and sodium would form a compound. Then, high pressure synthesis in a diamond anvil actually produced a stable compound with the formula Na₂He.

On the left hand side is a "ball and stick" model of Na₂He in which the purple balls represent sodium and the white balls represent helium.You can see that the structure is cubic.
On the right hand side is a polyhedral representation of Na₂He in which sodium atoms form the cubes (Na₈). Half of these cubes are occupied by helium atoms, and these are shown as grey boxes. But the other half of the cubes are occupied by 2 electrons and these are shown in the diagram as red spheres.
The chemists are predicting that other compounds of helium may also be possible, such as Na₂HeO.

Reference:
 Utah State University. "Up, up and away: Chemists say 'yes,' helium can form compounds: Helium and sodium form stable compound at high pressure." ScienceDaily. www.sciencedaily.com/releases/2017/02/170206111848.htm (accessed February 12, 2017).

Further Reading:
Introduction to the Modern Periodic Table 
 Bohr Model of the Atom 
Evidence for Electron Configuration of an Atom
Subshell Electronic Configuration

Suggested Study Questions:

1. What is the atomic number for
   • helium
   • sodium
2.  How many electrons are present in an atom of
   • helium
   • sodium
3.  Write the simple electronic configuration for an atom of each element below:
   • helium
   • sodium
4.  Write the electron configuration for an atom of each element below using subshell notation:
   • helium
   • sodium
5. Write an equation for the loss of an electron from a gaseous atom of sodium. 
6. Give the simple electronic configuration for the ion of sodium produced above.
7. Give the electronic configuration of the sodium ion produced above in terms of subshells.
8. Explain why the first ionisation of helium is so much higher than the first ionisation energy of sodium.
9. Explain why sodium readily forms compounds.
10. Explain why it is extremely difficult to produce helium compounds, and why it has required such enormous pressures in order to produce the first helium compound.

Argon Hydride in Space

Almost 1,000 years ago, the Chinese recorded a bright new object in the night sky. This is believed to be a star that exploded. The remnant of that exploded star is now called the Crab Nebula and it is about 6,500 light years from Earth.
Europe launched its Herschel Space Observatory in 2009, and in 2012 Astronomers used it to study the emission lines from Crab Nebula. What the scientists found were 2 bright emission lines from an area of shining dust. It was only by consulting databases of the known properties of different molecules that the scientists could finally identify these emission lines as coming from a very special molecule, argon hydride.
Argon hydride is special because it is the first time that molecules of a Noble Gas (Group 18) element have been found in space. On Earth, the naturally occurring Noble Gas (Group 18) elements are monatomic gases. Atoms of Noble Gas elements have a full valence shell of electrons so they tend not to react with other atoms.
Something really interesting must be happening in the Crab Nebula for argon hydride to be formed naturally. The researchers believe that the argon formed when the original star exploded. The intense energy of the explosion stripped electrons off the argon atoms, ionising them. Ionised argon atoms then reacted with hydrogen molecules to form argon hydride molecules.

Reference
M. J. Barlow, B. M. Swinyard, P. J. Owen, J. Cernicharo, H. L. Gomez, R. J. Ivison, O. Krause, T. L. Lim, M. Matsuura, S. Miller, G. Olofsson, E. T. Polehampton. Detection of a Noble Gas Molecular Ion, 36ArH , in the Crab Nebula. Science, 2013; 342 (6164): 1343 DOI: 10.1126/science.1243582

Further Reading
http://ausetute.com.au/trendec.html
http://ausetute.com.au/trendie.html
http://ausetute.com.au/massconv.html
http://ausetute.com.au/members/emissions.html

Suggested Study Questions:

1. 1 light year is approximately 9.5 x 10¹⁵ metres. How far from Earth is the Crab Nebula in kilometres?
2. 1 astronomic unit (1 AU) is approximately 1.5 x 10¹¹ metres. What is the distance between Earth and the Crab Nebula in astronomic units?
3. What sort of instrument can be used to observe the emission lines of hot gases?
4. Explain how these emission lines are produced.
5. Give the electron configuration of an atom of argon.
6. With reference to the electron configuration of argon, explain why compounds of argon are not found naturally on Earth.
7. Write an equation to show the loss of an electron from a gaseous argon atom.
8. Explain how the hydride ion can be formed from a hydrogen atom.
9. Give a likely molecular formula for a molecule of argon hydride.
10. Is argon hydride most likely to an ionic compound or a covalent compound? Explain your answer.