Question

\(\mathrm{PCl}_{5}\) in the solid state cxists as \(\mathrm{PCl}_{4}^{+}\) and \(\mathrm{PCl}_{6}^{-}\) because (1) Solid \(\mathrm{PCl}_{5}\) is a conduetor. (2) \(\mathrm{PCl}_{4}\) and \(\mathrm{PCl}_{6}\) have stable symmetrical structures unlike \(\mathrm{PCl}_{5}\) that has asymmetrical strueture. (3) lon pairs are more stable than neutral molecules. (4) Phosphorous belongs to \(\mathrm{V}\) group in the periodic table.

Short answer

Option (2): \(\text{PCl}_4\) and \(\text{PCl}_6\) have stable symmetrical structures.

Step-by-step solution

- Analyze the options

Read through the given options to understand what is being stated about \(\text{PCl}_5\). The goal is to determine why \(\text{PCl}_5\) exists as \(\text{PCl}_4^+\) and \(\text{PCl}_6^-\) in the solid state.

- Understand the structure stability

Consider that \(\text{PCl}_5\) can break into \(\text{PCl}_4^+\) and \(\text{PCl}_6^-\) due to structural stability. \(\text{PCl}_4\) and \(\text{PCl}_6\) have symmetrical structures, making them more stable compared to the asymmetrical \(\text{PCl}_5\).

- Evaluate each option

1. Solid \(\text{PCl}_5\) being a conductor doesn't directly explain the ion formation. \ 2. Stability due to symmetrical structures of \(\text{PCl}_4\) and \(\text{PCl}_6\) can be a valid reason. \ 3. Ion pairs being more stable than neutral molecules is not necessarily always true. \ 4. Phosphorous belonging to group V in the periodic table is related to element properties but not directly to the formation of these ions.

- Select the best option

Given the reasoning above, the most logical explanation for \(\text{PCl}_5\) existing as \(\text{PCl}_4^+\) and \(\text{PCl}_6^-\) in solid state is that \(\text{PCl}_4\) and \(\text{PCl}_6\) have stable symmetrical structures unlike \(\text{PCl}_5\), making option 2 the correct choice.

Key concepts

Solid state chemistry

Solid state chemistry is the study of the synthesis, structure, properties, and applications of solid materials. This scientific area helps us understand why certain compounds form stable structures in their solid forms. The behavior of PCl5 in the solid state is a perfect example. PCl5 prefers to exist as PCl4^+ and PCl6^- because these ions arrange themselves in a more stable, ordered structure. This is a key aspect of solid state chemistry - understanding how atoms and ions organize themselves and why some configurations are more stable than others. For example:

• PCl4^+ forms one type of stable structure with four chlorine atoms around the phosphorus.
• PCl6^- forms another type of stable structure with six chlorine atoms surrounding the phosphorus.

Breaking up PCl5 into these components reduces asymmetry, which is energetically favorable for the solid state structure. So, solid state chemistry principles guide us to the correct answer by showing us the importance of stable and symmetrical arrangements.

Ionic compounds

Ionic compounds are formed from positive and negative ions that are held together by strong electrostatic forces. In the case of PCl5, it breaks down into PCl4^+ and PCl6^- in the solid state. These are ionic forms of the compound. The positive ion PCl4^+ and the negative ion PCl6^- attract each other and arrange themselves into a stable crystalline structure. Here are some important aspects of ionic compounds:

• They typically form when metals react with non-metals.
• They have high melting and boiling points due to strong ionic bonds.
• They conduct electricity when molten or dissolved in water.

In the solid state, PCl5 forms an ionic compound because this arrangement is more stable compared to its molecular form. This stability is due to the symmetrical positive and negative ions forming a tightly packed, orderly structure.

Molecular symmetry

Molecular symmetry refers to the balanced, mirror-image arrangement of a molecule's atoms. Symmetry plays a crucial role in determining the stability of a molecule or ion. For PCl5 in solid state, the symmetrical structures of PCl4^+ (with its tetrahedral shape) and PCl6^- (with its octahedral shape) contribute to the overall stability. Symmetry helps to evenly distribute the electrons and minimize the repulsion between them, leading to:

• Lower energy and higher stability of the molecule or ion.
• More predictable chemical reactions and bonding.
• Better packing in the solid-state, contributing to stronger ionic bonding.

Understanding the significance of symmetry allows us to comprehend why PCl5 prefers to exist as PCl4^+ and PCl6^- in solid forms. The symmetrical nature of these ions makes them more stable than the asymmetrically arranged PCl5 molecule.