8.8
Noble Gases
The elements in group 18 are noble gases (helium, neon, argon, krypton, xenon, and radon). They earned the name “noble” because they were assumed to b…
The nonmetallic elements categorized under group 18 – helium, neon, argon, krypton, xenon, and radon – are called noble gases. These elements occur as monatomic species and exist as gases under room-temperature. Radon is the only radioactive element from group 18.
Moving down the group, the elements exhibit an increase in boiling points, densities, and atomic radii, which consequently leads to the decline of ionization energies of each successive element.
Yet, noble gases have high first ionization energies compared to all other elements in the periodic table. This is because these elements have stable electron configurations with complete octets. The removal of an electron requires the input of a large amount of energy, which is unfavorable.
Noble gases also have positive electron affinity values. Meaning, energy is required to add an additional electron to a gaseous atom. Noble gases resist electron additions as their valence shells are already complete, and the incoming electron needs to enter a higher principal quantum shell.
The high stability of noble gases attests to their chemical inertness, which finds many industrial applications. For instance, argon is used to manufacture gas-filled electric light bulbs to prevent the oxidation of tungsten filaments, prolonging the bulb’s life. Helium is used to create an inert atmosphere during the melting and welding of easily oxidizable metals.
Noble gases were Initially thought to be entirely chemically unreactive and were called inert gases. However, in the early sixties Neil Barlett discovered some exceptions. For example, xenon, with the lowest ionization energy from the noble gases, was found to react with the most electronegative element, fluorine.
Xenon difluoride, obtained by heating an excess of xenon gas with fluorine gas, is a stable, crystalline material. Other compounds like xenon tetrafluoride and xenon hexafluoride can also be prepared similarly.
Xenon-compounds with the electronegative element, oxygen, can be produced by replacing fluorine atoms in xenon fluorides with oxygen. For instance, xenon hexafluoride reacts with water, yielding a solution of xenon trioxide.
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Q1: Why do noble gases have such high first ionization energies?
Noble gases possess complete valence shells with stable electron configurations, making electron removal extremely unfavorable. Their filled s and p orbitals create exceptional stability. Removing an electron requires breaking this complete octet, necessitating a large energy input. This is why noble gases exhibit the highest ionization energies periodic variation and exceptions across the periodic table.
Q2: How do noble gas properties change moving down group 18?
Descending group 18, atomic radii increase while first ionization energies decrease. Boiling points and densities also increase due to stronger London dispersion forces between larger atoms. Despite these changes, all noble gases maintain high ionization energies compared to other elements, reflecting their stable electron configurations and chemical inertness.
Q3: What does the positive electron affinity of noble gases indicate?
Positive electron affinity values mean noble gases resist gaining electrons. Their complete valence shells make adding electrons unfavorable, as incoming electrons must enter higher principal quantum shells. This resistance to electron addition, combined with high ionization energies, demonstrates why noble gases are chemically inert and rarely form compounds under normal conditions.
Q4: Why was xenon able to form compounds when other noble gases could not?
Xenon has the lowest ionization energy among noble gases, making electron removal slightly less difficult. When reacted with fluorine, the most electronegative element, xenon can form stable compounds like xenon difluoride. Neil Bartlett's 1962 discovery proved noble gases were not entirely unreactive, challenging the assumption of complete chemical inertness.
Q5: What are the main industrial applications of argon and helium?
Argon prevents tungsten filament oxidation in electric light bulbs, prolonging bulb life. It also fills fluorescent tubes mixed with mercury vapor. Helium creates inert atmospheres during welding of easily oxidizable metals and serves as a cryogenic coolant for superconducting materials. Both gases exploit noble gas chemical inertness for practical purposes.
Q6: How are xenon fluoride compounds converted to xenon oxide compounds?
Xenon oxide compounds form by replacing fluorine atoms in xenon fluorides with oxygen. For example, xenon hexafluoride reacts with water to produce xenon trioxide solution, with xenon remaining in the +6 oxidation state. Dry xenon trioxide is extremely explosive and spontaneously detonates, making these compounds highly unstable.
Q7: Why do noble gases have extremely low boiling and melting points?
Noble gases experience only weak London dispersion forces between atoms since they are monatomic and have no polar bonds. These weak intermolecular forces can hold atoms together only at very low temperatures when molecular motion is minimal. This explains why noble gases remain gaseous at room temperature despite their relatively high atomic masses.