Problem 11 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following halides is least stable and has doubtful existence? [1996-1 Mark] (a) \(\mathrm{CI}_{4}\) (b) \(\mathrm{GeI}_{4}\) (c) \(\mathrm{SnI}_{4}\) (d) \(\mathrm{PbI}_{4}\)

Step-by-Step Solution

Answer

The least stable halide is PbI4, as it has doubtful existence.

Step 1: Analyze the Stability of the Compounds

The stability of halides generally decreases down the group in the periodic table. This is because as we move down, the atom size increases, leading to weaker bonds between the central atom and the iodine atoms.

Step 2: Consider Lead's Oxidation States

Pb is known to have stable +2 oxidation states, rather than +4, especially when forming halides. The larger size of Pb leads to ineffective overlap with large iodine orbitals, forming weak Pb-I bonds.

Step 3: Evaluate the Known Stabilities

Other halides like \( \mathrm{CI}_{4} \), \( \mathrm{GeI}_{4} \), and \( \mathrm{SnI}_{4} \) are known compounds with credible existence. However, \( \mathrm{PbI}_{4} \) is not stable; Pb usually forms stable \( \mathrm{PbI}_{2} \).

Step 4: Conclude the Least Stable Compound

From the analysis above, \( \mathrm{PbI}_{4} \) is the least stable and has doubtful existence as it prefers to exist in the \( \mathrm{PbI}_{2} \) form.

Key Concepts

Oxidation States

Understanding oxidation states is crucial in determining the stability of chemical compounds. Simply put, the oxidation state represents the degree of oxidation or reduction of an atom within a compound. Different elements have preferred oxidation states where they are most stable.
Lead (Pb), for instance, is more stable in the +2 oxidation state rather than the +4 oxidation state when forming halides. This is vital when examining compounds like PbI enabling us to anticipate their stability.
For example, PbI doesn't easily exist because lead in the +4 oxidation state forms weak bonds with iodine. This causes PbI to be less stable and not generally observed, preferring instead to form PbI"

Periodic Table Trends

The periodic table reveals much about element behaviors, especially trends in stability. As one moves down a group in the periodic table, the atomic size increases. This has a direct impact on the stability of compounds they form.
The increase in size can lead to weaker bonds because the larger atoms have valence electrons that are further away from the nucleus, reducing the overlap with other atoms involved in bonding.
In our specific case here, as we look at the halides like CI\(_4\), GeI\(_4\), SnI\(_4\), and PbI\(_4\), we notice the stability decreases with the increase in size of the central atom.

• Carbon (C) is smaller than Germanium (Ge), Tin (Sn), and Lead (Pb).
• Therefore, CI\(_4\) is more stable compared to PbI\(_4\) based on size alone.
• This trend shows why PbI\(_4\) tends to be less stable and questionable in existence.

Chemical Bonding

Chemical bonds are the forces that hold atoms together in compounds. The type and strength of these bonds significantly affect a molecule's stability. In the context of halides, the size and oxidation state of the central atom play pivotal roles.
With lead halides, Pb-I bonds are typically weak due to the size and oxidation state of lead. A larger atomic radius leads to less effective overlap of orbitals, which is necessary for strong bonds. Hence, PbI\(_4\) is less stable compared to other compounds like SnI\(_4\) and GeI\(_4\).
The preferred stable existence of lead is typically found as PbI\(_2\) instead of PbI\(_4\), further emphasizing the influence of chemical bonding on molecular stability.
Correctly understanding these bonding principles is essential to predict which compounds would be likely to exist under standard conditions.