The P-Block Elements (Group 18 - Noble Gases)

Group 18 Elements

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Group 18 elements, commonly known as the noble gases or inert gases, include Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), Radon (Rn), and Oganesson (Og). They are characterized by their very low reactivity.

Occurrence

Atmosphere: Noble gases (except radon and oganesson) are present in the Earth's atmosphere in small quantities.

• Argon is the third most abundant gas in the atmosphere (0.934% by volume).
• Neon, Helium, Krypton, and Xenon are present in much smaller amounts.
• Radon is a radioactive gas that occurs naturally as a decay product of radium.
• Oganesson is a synthetic radioactive element.

Helium: Found in significant concentrations in natural gas obtained from radioactive minerals due to $\alpha$-decay.

Electronic Configuration

General Configuration: All noble gases have a completely filled valence shell, with the general electronic configuration $ns^2np^6$.

• He: $1s^2$
• Ne: $[He] 2s^22p^6$
• Ar: $[Ne] 3s^23p^6$
• Kr: $[Ar] 3d^{10} 4s^24p^6$
• Xe: $[Kr] 4d^{10} 5s^25p^6$
• Rn: $[Xe] 4f^{14} 5d^{10} 6s^26p^6$
• Og: $[Og] 7s^27p^6$ (predicted)

Significance: The completely filled valence shell makes them exceptionally stable and unreactive, as they have no tendency to gain, lose, or share electrons to achieve a noble gas configuration.

Ionisation Enthalpy

Trend: Noble gases have the highest first ionization enthalpies among the elements in their respective periods.

Reason: The completely filled valence shell ($ns^2np^6$) provides very high stability, and the outermost electrons are held tightly by the nucleus.

Trend Down the Group: Ionization enthalpies decrease from He to Rn.

Reasons:

• Increase in atomic size.
• Increased shielding effect of inner electrons.

Anomalous Behavior of Helium: Helium has exceptionally high ionization enthalpy due to its very small size and the stability of the $1s^2$ configuration.

Atomic Radii

Trend: Atomic radii increase down the group from He to Rn.

Reasons:

• Increase in the number of electron shells.
• Increased shielding effect of inner electrons.

Van der Waals Radii: Since noble gases do not form chemical bonds easily, their radii are usually measured by van der Waals radii.

Electron Gain Enthalpy

Trend: Noble gases have almost zero or slightly positive electron gain enthalpies.

Reason: The completely filled valence shell makes it energetically unfavorable to add an electron. Adding an electron would require entering the next higher energy level, which is much less favorable.

Exception: Helium has a positive electron gain enthalpy, indicating that it is very difficult to add an electron to it.

Physical Properties

States of Matter: All noble gases are colorless, odorless, and tasteless gases at room temperature and pressure.

Liquefaction: They have very low boiling points because they exist as individual atoms with only weak van der Waals forces between them. Liquefaction becomes easier down the group due to increasing van der Waals forces.

Solubility: They are sparingly soluble in water.

Radioactivity: Radon (Rn) and Oganesson (Og) are radioactive.

Chemical Properties

Inertness: Noble gases are chemically very inert due to their stable, completely filled valence electron shells ($ns^2np^6$).

Formation of Compounds:

• Early Belief: Initially, they were thought to be completely inert.
• Discovery of Compounds: In 1962, Neil Bartlett synthesized the first known compound of a noble gas, $XePtF_6$. Since then, several compounds of Xenon (with highly electronegative elements like F and O) and some compounds of Krypton and Radon have been prepared.
• Reactivity Trend: Reactivity increases down the group from Ne to Rn.
• Helium, Neon, and Argon are virtually inert.
• Krypton forms some compounds, mainly with fluorine ($KrF_2$).
• Xenon is the most reactive among the lighter noble gases and forms a wide range of fluorides ($XeF_2$, $XeF_4$, $XeF_6$) and oxides ($XeO_3$, $XeO_4$), and derived compounds (oxyfluorides, etc.).
• Radon, being radioactive, also forms compounds, but its radioactivity limits extensive study.
• Factors Affecting Compound Formation: The ability of Xenon to form compounds is attributed to:
• Its relatively low ionization enthalpy (compared to other noble gases except Rn).
• The availability of valence electrons in larger shells ($5s^25p^6$) that are less tightly held.
• The high electronegativity of fluorine and oxygen, which can stabilize the resulting compounds.

Uses:

• Helium (He): Filling balloons and airships (due to its lightness and non-flammability), in cryogenics (as a coolant), in gas-cooled nuclear reactors, and in arc welding.
• Neon (Ne): Used in discharge tubes for lighting (neon signs), and in lasers.
• Argon (Ar): Used to provide an inert atmosphere for welding and in incandescent lamps to prevent filament evaporation.
• Krypton (Kr) and Xenon (Xe): Used in photographic flashes, high-intensity discharge lamps (Xenon), lasers, and anesthesia (Xenon).
• Radon (Rn): Used in radiotherapy (cancer treatment).