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.

Chemistry in Eclipses

The 14th November 2012 excites students of physics and those interested in astronomy. This is the date of a total eclipse of the sun. The area of totality will pass over northern Australia, from east of Darwin in the Northern Territory to the Cape York Peninsula of Far North Queensland, turning morning into darkness. The rest of Australia will see a partial eclipse.

But why would chemists get excited about a solar eclipse?

The story begins more than 200 years ago ...

Gaps in the Solar Spectrum?

In 1802 an English Chemist, William Hyde Wollaston, was the first person to record the appearance of a number of dark lines in the emission spectrum of light from the sun.

In 1814, German physicist Joseph von Fraunhofer began measuring the wavelengths of over 570 of these lines.

Fingerprinting the Sun

Robert Gustave Kirchhoff and Robert Bunsen, developed a better prism-based spectroscope and observed that the spectral lines emitted by a gas occurred at the same wavelength as the absorption lines observed when incandescent light from Bunsen's burner shone through the same gas heated at the same temperature.

Then Kirchhoff,  proposed the laws of spectroscopy which bear his name:

1. A hot solid object produces light with a continuous spectrum
2. A hot tenuous gas produces light with spectral lines at discrete wavelengths (an emission spectrum)
3. A hot solid object surrounded by a cooler tenuous gas produces light with an almost continuous spectrum with gaps at discrete wavelengths (an absorption spectrum)

A star, like the sun, will create an absorption line spectrum because the continuous spectrum emitted by the dense, opaque gas that makes up most of the star passes through the cooler, transparent atmosphere of the star.

In 1859, Kirchhoff  demonstrated that all pure substances display their own characteristic spectrum, so it is possible to use the spectrum of elements to identify elements in a mixture, just like each person's fingerprints are unique and can be used to identify them. He proposed  that the lines in the solar spectrum are caused by the absorption of light by elements in the solar atmosphere and set out to identify the elements present in our sun.

New Element Discovered

On the 18th August 1868 there was a total solar eclipse. In India, French astronomer Pierre Janssen observed this eclipse using a spectroscope. He recorded a bright yellow line with a wavelength of 587.49 nm in the spectrum of the solar prominences. The same result was also recorded by British astronomer Norman Lockyer. This line could not be due to sodium, because although sodium produces a bright yellow line (actually more than 1), the wavelength of sodium's 'line' is about 589.3 nm. Lockyer proposed that this line was due to a new element which he called helium after the greek word 'helios' meaning 'sun'.

About 10 years later, Scottish chemist William Ramsay isolated helium on earth ...... but that's another story.

References:

http://eclipse.aaq.org.au/

http://www.csiro.au/en/Outcomes/Understanding-the-Universe/Tracking-spacecraft/History-of-total-solar-eclipses.aspx

Further Reading:

Emission Spectra 

Suggested Study Questions:

1. speed of light (m/s) = frequency (s^-1) x wavelength (m)
   If the speed of light is 3 x 10⁸ ms^-1 calculate:
• find the frequency of the 'yellow line' in sodium's spectrum
• find the frequency of the yellow line for the new element found in the solar spectrum
2.  speed of light (m/s) = frequency (s^-1) x wavelength (m)
   If the speed of light is 3 x 10⁸ ms^-1 calculate:
• wavelength of blue light with a frequency of 6.9 x 10¹⁴ s^-1
• wavelength of red light with a frequency of 4.6 x 10¹⁴ s^-1
  
3. The energy of light emitted, E, is Planck's constant,h, multiplied by the speed of light divided by the wavelength of light emitted. Write a mathematical equation to represent this.
4. Use your equation above to calculate
   • energy of the blue light in question 2 above
   • energy of the red light in question 2 above
5. Complete the following generalizations:
   • The longer the wavelength of light, the ___________ energy it has
   • The shorter the frequency of light, the _________ energy it has.
6. Compare the wavelength of the 'yellow line' in sodium's spectrum and the yellow line for the 'new element'. Which element has
   • the longest wavelength
   • the shortest frequency
   • the most energy
7. Describe the difference in the spectrum of light from the sun as seen in a spectroscope compared to the spectrum of light from a fluorescent light as seen in a spectroscope.
8. Explain the differences between the two spectrum in question 7 above.

Uniqueness of Helium

Helium. He, is used to fill balloons, in lasers for eye surgery, as a cooling agent in nuclear reactors, and as a pressurizing agent for liquid fuel rockets in space exploration.

Helium has a number of characteristics that make it special. It is the most stable of all the elements and has the lowest boiling point. It becomes a fluid at temperatures close to absolute zero (0K) while most other materials are solid. In fact, helium is a liquid even at 0K and becomes a solid only under high pressure, and, helium is the only substance to exhibit superfluidity.

Of all the elements, helium is closest to the ideal gas. Two helium atoms form the weakest bound diatomic molecule, or dimer. All the properties of temperature, a measure of the kinetic energy of particles in matter, can be modeled if the force acting between a pair of helium atoms is known.
University of Delaware scientists have now predicted that the average separation between the helium atoms is 47 angstroms, compared to a typical bond length of 1 angstrom (one ten billionth of a meter or 0.0001 micron), and that the binding energy is 6,790 times smaller than the potential depth.

Reference:
M. Przybytek, W. Cencek, J. Komasa, G. %u0141ach, B. Jeziorski, K. Szalewicz. Relativistic and Quantum Electrodynamics Effects in the Helium Pair Potential. Physical Review Letters, 2010; 104 (18): 183003 DOI: 10.1103/PhysRevLett.104.183003