Problem 129 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following noble gases do not form any compound? (a) \(\mathrm{Kr}\) (b) \(\mathrm{He}\) (c) \(\mathrm{Xe}\) (d) \(\mathrm{Ne}\)

Step-by-Step Solution

Answer

Helium (He) and Neon (Ne) do not form compounds; however, of the choices, Helium (b) is more inert.

Step 1: Understand Noble Gases

Noble gases are found in Group 18 of the periodic table. They are known for having a complete valence electron shell, which makes them very stable and unreactive under normal conditions. However, some heavier noble gases can form compounds under specific conditions because their outer electrons can be influenced by larger nuclear charge.

Step 2: Identify Known Compounds

The heavier noble gases like Xenon (Xe) and Krypton (Kr) have been observed to form compounds, such as xenon hexafluoroplatinate (\(\mathrm{XePtF_6}\)) and krypton difluoride (\(\mathrm{KrF_2}\)). These compounds can be synthesized under special laboratory conditions.

Step 3: Focus on Lighter Noble Gases

Helium (He) and Neon (Ne) are much lighter compared to Xenon and Krypton. Their tightly packed electrons and smaller sizes make them even less reactive. To date, no stable compounds of Helium or Neon have been successfully synthesized under standard laboratory conditions.

Step 4: Select the Correct Answer

Based on the information that only heavier noble gases like Xenon and Krypton have confirmed compounds, and lighter noble gases like Helium and Neon do not form stable compounds, we can determine which noble gas from the options given does not form any compound at all.

Key Concepts

Reactivity of Noble Gases

Noble gases are renowned for their remarkable lack of reactivity. Found in Group 18 of the periodic table, these elements—including Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn)—have full valence electron shells. This complete electron arrangement gives these atoms little tendency to gain, lose, or share electrons in chemical reactions. There are a few specific conditions under which some of the heavier noble gases exhibit reactivity, forming compounds. However, no stable compounds of the lighter noble gases like Helium or Neon are known. They are incredibly stable under normal conditions due to their small size and lower number of electrons, which contribute to a tightly bound electron cloud around the nucleus. The larger the noble gas, the more easily its outer electrons can be influenced to react, due to the larger atomic radius and greater shielding effect from inner electrons.

Noble Gas Compounds

Although the atoms of noble gases generally resist combining with other elements to form compounds, certain conditions can coax reactivity, especially in heavier noble gases. For instance, Xenon (Xe) and Krypton (Kr) have been observed to participate in compound formation. These compounds typically involve highly electronegative elements or elements capable of forming strong bonds. One known compound is Xenon hexafluoroplatinate ( XePtF_6 ), which can be synthesized with a powerful oxidizing agent like fluorine under specific conditions. Similarly, Krypton can form krypton difluoride ( KrF_2 ), where krypton exhibits a +2 oxidation state. Despite these exceptions, the majority of noble gases, especially lighter ones like Helium and Neon, remain non-reactive and do not form stable compounds.

Periodic Table Group 18

In the periodic table, Group 18 comprises the noble gases. This group stands out due to its elements' lack of propensity to partake in chemical reactions under normal conditions. The reason for this lies in their electronic configuration—each noble gas has a full valence shell: - Helium (He) has 2 electrons, filling its first shell. - Neon (Ne) contains 8 outer electrons. - Successive noble gases follow the trend with full outer p-orbitals in their respective periods. These complete valence shells result in maximal stability, often called "inertness," because there are no "gaps" for additional electrons. Elements in Group 18 don't easily gain, lose, or share electrons, making them the epitome of chemical stability.

Stability of Electron Shells

The stability of noble gases is directly linked to their electron shell configuration. A full valence shell means that an atom doesn’t seek to achieve stability through a reaction, as it's already in an energetically favorable state. Most elements tend to react in a way to attain a complete outer shell, like that of noble gases, as seen in the "octet rule." Since noble gases already possess this full outer shell naturally, they resist engaging in chemical bonding. For lighter noble gases, like Helium and Neon, this stability is profound, and no known compounds exist that incorporate them. Heavier noble gases like Xenon, despite having a full outer shell, can still form compounds under extreme conditions due to the greater ability of their outer electrons to participate in bonding because of weaker attraction by the nucleus in larger atoms.