Xenon Compounds

For a long time, scientists believed that you could not make covalent compounds using Noble Gas, Group 18, elements because they are just not reactive. All these elements have a full valence shell of electrons (s²p⁶ for all the Noble Gases except helium, which has no p electrons) so there is no tendency to gain or lose electrons.

In 1933, Linus Pauling used his new concept of electronegativity, the power of an atom to attract electrons, to predict the possibility of making compounds using the heavier Noble Gas elements and fluorine or oxygen.

Fluorine, with an an electronegativity of 4.0 is the most electronegative element in the Periodic Table, so atoms of fluorine have the greatest power to attract electrons. Oxygen is the second most electrongetative element with an electronegativity of 3.5. So, if you want to try to make a compound with an "unreactive" non-metal like a Noble Gas, you should probably use a reactive non-metallic element like fluorine or oxygen.

Now, as you go down any Group of the Periodic Table from top to bottom, the electrons in the highest energy levels (the valence electrons) are less strongly attracted to the nucleus because they are further away from the nucleus. So, if  you want your "unreactive" Noble Gas atom to share electrons with your more reactive fluorine or oxygen atom, you will want to use a Noble Gas from the bottom of the Group, that is, xenon or radon.

So why didn't Linus Pauling predict radon compounds instead of xenon compounds then?

Radon is a radioactive element which occurs naturally as the nuclear decay product of uranium or thorium.

The real problem, as far as the production of radon compounds is concerned, is that its most stable isotope, radon-222, has a half-life of just 3.8 days, not much time for the preparation, isolation, and structural determination of your new compounds!

Xenon, on the other hand, occurs in the Earth's atmosphere in trace amounts, about 0.09 ppm (by volume) and naturally occurring xenon includes 8 stable isotopes.

In June 1962, Neil Bartlett reported that xenon reacted with platinum hexafluoride to produce xenon compounds ( a mixture of XeFPtF₆ and XeFPt₂F₁₁).

In September 1962, Howard Claasen reported the production of XeF₄ by reacting xenon and fluorine at high temperature.

XeF₂ can actually be produced simply by exposing xenon and fluorine gases to sunlight ! 

Many xenon compounds have now been synthesized, and many of these compounds include the electronegative elements fluorine and/or oxygen.

Further Reading

 Periodic Table

Electron Configuration

Trends in Electron Configuration

Trends in Electronegativity

Nuclear Decay

Parts Per Million Concentration

Isotopes

Ligands and Complex Ions

Suggested Study Questions

1. Write the  electron configuration for each of the following elements:
• helium
• neon
• argon
• fluorine
• oxygen
2.  Which of the Noble Gas, Group 18, elements is expected to have
• the largest atomic number
• the largest number of electrons in an atom
• the largest atomic radius
• the lowest first ionization energy
• the lowest electronegativity
3. Uranium-234 undergoes alpha decay to produce thorium-232. Write a nuclear decay equation for this reaction.
4. Thorium-232 undergoes alpha decay to produce radium-228. Write a nuclear decay equation for this reaction.
5. Radium-228 undergoes spontaneous alpha decay to produce radon-224. Write a nuclear decay equation for this reaction.
6. If you are given 25 grams of radon-222 today, how much radon-222 will remain in 38 days time?
7. Radon-222 emits an alpha particle to produce polonium-218. If you were give 10 grams of radon-222 19 days ago, what mass of polonium-218 has been produced since then?
8. Write the formula for platinum hexafluoride.
9. Give the systematic names for
• XeF₂
• XeF₄
• XeF₆
10. Explain why no compounds of neon have so far been produced.

Defining Electronegativity

Electronegativity is defined as the power of an atom to attract electrons to itself, but it cannot be directly measured so it must be calculated using other properties of atoms. There are many different ways of calculating electronegativity.

Pauling Electronegativity
In 1932 Pauling proposed the concept of electronegativity to explain why the covalent bond formed between two different atoms, eg A-B bond, is stronger than you might expect if you were to take an average of the A-A and B-B bond strengths. Since Pauling's electronegativities are based on the differences in bond strengths between atoms, it is necessary to choose an arbitrary reference point so that these electronegativities can be compared and used to make predictions. Hydrogen, with an electronegativity of 2.20, has been used as the reference to build up a table of electronegativities. Pauling electronegativities have no units.

Mulliken Electronegativity
Mulliken electronegativities are based on the mean of the first ionization energy and the electron affinity and has the units kJ/mol or eV.

Allred-Rochow Electronegativity
Allred-Rochow electronegativity is related to the charge experienced by an electron on the 'surface' of an atom and is calculated using the ratio of effective nuclear charge experienced by the valence electrons and the square of the covalent radius.

Sanderson Electronegativity
Similar to the Allred-Rochow electronegativity, Sanderson's calculations use atomic volume instead of the square of the covalent radius.

Allen Electronegativity
Allen electronegativity is related to the average energy of the valence electrons in a free atom.

Solid State Energy Scale
In 2011, Oregon State University scientists created a new method to describe electronegativity. In this approach, electronegativity is characterised as the solid state energy of elements in a compound, and shows that electrons simply move from a higher energy to a lower energy.

Reference
Brian D. Pelatt, Ram Ravichandran, John F. Wager, Douglas A. Keszler. Atomic Solid State Energy Scale. Journal of the American Chemical Society, 2011; : 111003131629001 DOI: 10.1021/ja204670s

Further Reading
Electronegativity Trends
Electronegativity and Bond Polarity
Interactive Periodic Table of the Elements
Ionization Energy and Electron Configuration

Suggested Study Questions

1. Use the Interactive Periodic Table to find the Pauling Electronegativity for each of the following elements:
   • sodium
   • potassium
   • magnesium
     
   • calcium
   • oxygen
   • sulfur
   • chlorine
   • fluorine
2. Use the values for the electronegativities above to describe each of the following compounds as ionic or covalent:
   • sodium chloride
   • magnesium oxide
   • potassium fluoride
   • calcium oxide
   • sulfur dioxide
   • chlorine dioxide
   • ozone
3. For each of the covalent compounds above, describe the compound as non-polar or polar covalent.
4. What is meant by the term ionization energy?
5. What is meant by the term electron affinity?
6. How is electronegativity differen to electron affinity?
7. If an element has a very low first ionization energy, do you expect it to have a high of low Pauling electronegativity? Explain your answer.
8. If an element has a high Pauling electronegativity, do you expect it to have a high or low value for its electron affinity? Explain your answer.