Physical And Chemical Properties of Noble Gases

In this article, we will discuss the properties of noble gases, which are physical and chemical properties. The physical properties of noble gases will be explained using data of atomic properties and structure of noble gases, whereas the chemical properties will be explained using data of the atomic properties and electron configurations.

Properties of Noble Gases
Tabel 1. Properties of Noble Gases

Physical Properties of Noble Gases

In order to study the trends in the physical properties of the noble gases, take a look at the data of the atomic properties and the structure of the elements.
Except for radon, the noble gas element can be found in the atmosphere although only in very small concentrations. Doe to this, noble gases are also known as “rare gases.”

The Atomic Properties of Noble Gases

Table 1 contains the atomic properties of the elements of noble gases.
From the table above, we can see certain trends in the atomic properties of noble gases from He to Rn.

  • The atomic radius (covalent radius) increases from He to Rn.
  • The ionization energy decreases from He to Rn.
  • The electronegativity of He, Ne, and Ar is not available, While the electronegativity decreases from Kr to Rn.
  • The oxidation number of He, Ne, and Ar is zero, while Kr, Xe, and Rn have several oxidation numbers.
  • Helium actually comes from the decay of radioactive elements. Some of the He gas is released into the air, while other mix with natural gas. The concentration of He in natural gas is -1%, far greater than that in air. This can be understood because Helium gas is too light to remain in the atmosphere.
noble gases percentage in air
Table 2. Noble Gases Percentage in Air

Structure of Noble Gases

Noble gases exist as monoatomic atoms that are held together by London forces. As these London forces only work on monoatomic atoms, the factor that affects the strength of the London force is the size of the atom, which is the radius of the atom. Since the atomic radius increases from He to Rn, the London force become stronger from He to Rn.

We will now take a look at how the atomic properties and the structure of elements serve as the basis for the tendency of the physical properties of noble gases, which include density, melting point, boiling point, the enthalpy change of fusion (ΔHfus), enthalpy change of evaporation (ΔHvap), and heat conductivity (see Table 1).

From the data above, we can see the following patterns:

Density Increases From He to Rn

The density of noble gases is affected by the atomic mass, atomic radius, and London force. The density increases with the increasing atomic mass and strength of London forces but decreases with the increasing atomic radius. As the density of noble gases increases from He to Rn, the increase of the atomic mass and the strength of London forces from He to Rn is more dominant compared to the increase of the atomic radius.

density of noble gases
Table 3. Density of Noble Gases

Note: Noble gases take the form of gas at temperature. As they are colorless, we cannot distinguish them. However, when given sufficient pressure, noble gases can take a liquid form.

Melting Point and ΔHfus increase from He to Rn

This is because the strength of London forces increases from He to Rn thus making atoms of noble gases more difficult to separate. Energy is needed, and in this case, a higher temperature to overcome the increasingly stronger London forces.

Boiling point and ΔHvap increase from He to Rn

This is because the strength of London forces increases from He to Rn thus making atoms of noble gases more difficult to separate. Energy is needed, in this case, a higher temperature to overcome the increasingly stronger London forces.

boiling point of noble gases
Table 4. Boiling Point of Noble Gases

Heat Conductivity Decreases from He to Rn

This is because the strength of London forces increases from He to Rn. In other words, the particles are relatively harder to move so the energy in the form of heat will be more difficult to transfer.

Read more about : is helium a noble gas

Chemical Properties of Noble Gases

The chemical properties or reactivity of noble gases will be discussed using the data of atomic properties and electron configurations. Take a look at the data of the atomic properties of noble gases in Table 1.

Noble gases have a stable electron configuration. Because of this, noble gases tend to experience difficulty reacting and are therefore unreactive. This is supported by the fact that in nature, noble gases always exist as single atoms or monoatomic. However, scientists have succeeded in synthesizing compounds of noble gases in period 3 and above, i.e. Ar, Xe, Kr, and Rn.

This is related to the d subshell that is still vacant in period 3 and above. Meanwhile, He and Ne until today cannot be reacted (to form a compound).

What is the trend in reactivity of noble gases from Ar to Rn?

The trend in reactivity of noble gases increases from Kr to Rn. This is obtained by comparing the conditions needed for the three elements to react with F2 However, the reactivity of Ar cannot be directly compared with these three elements because Ar still cannot react directly with fluorine (F) but with HF at a low temperature.

Despite that, it is expected that Ar has a reactivity that follows the trend above. This is supported by data of the trend in atomic properties of noble gases, i.e. the atomic radius increases from Ar to Rn, which means that a valence electron is bonded more weakly to the nucleus. The reactivity of noble gases increases from Ar to Rn.

Reactions of Noble Gases

As explained, elements of noble gases Ar, Kr, Xe, and Rn can react with elements that are very electronegative, such as F and O. Take a look at several reactions of noble gases below.

Argon (Ar)

Ar(s) + HF ⟶ HArF

Argon hydrofluoride, (HArF) is the first Ar compound synthesized around the year 2000. This compound is produced by photolysis of HF in a dense, stable Ar matrix at low temperature. The stability of the compound lies in the energy needed to break the weak bond between H-Arf. When it is heated or interaction among particles takes place, the molecules spontaneously decompose back into HF and Ar.

Krypton (Kr)

Kr(s) + F2(s) ⟶ KrF2(s)

Kr and F2 are reacted by first cooling them to a temperature of -196°C, then given an electrical charge jump or X-ray. The reaction product is krypton difluoride (KrF2).

Xenon (Xe)

Xe(g) + F2(g) ⟶ XeF2(s)
Xe(g) + 2 F2(g) ⟶ XeF4(s)
Xe(g) + 3 F2(g) excess ⟶ XeF6(s)

Xenon fluorides

XeF2 and XeF4 are synthesized by heating Xe and F2 under a pressure of 6 atm. If the amount of F2 is in excess, XeF6 is obtained. XeF2, XeF4, and XeF6 are stable crystals that are colorless and very reactive.

XeF6(s) + 3 H2O(l) ⟶ XeO3(s) + 6 HF(aq)
6 XeF4(s) + 12 H2O(l) ⟶ 2 XeO3(s) + 4 Xe(g) + 3 02(g) + 24 HF(aq)

Xenon oxides

XeO3 (xenon trioxide) is a white solid that is very explosive.
XeO4 (xenon tetraoxide) is made from a complex disproportionate reaction from Xeo3 solution that is alkaline. Xeo4 is an unstable and very explosive type of gas.

Radon (Rn)

Rn(g) + F2(g) ⟶ RnF2

Reaction takes place spontaneously, reaction of Rn and fluorine gas to produce radon difluoride (RnF2).

Read more about Effects of Radon on Humans

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