Question

Which of the following statements is incorrect for silicon? (1) Forms an oxide \(\left(\mathrm{SiO}_{2}\right)\) that is amphoteric and has a Ga, As structure. (2) Does not undergo coordination number expasion. (3) Forms strong but unconjugated multiple bonds of the \(\mathrm{p} \pi-\mathrm{d} \pi\) variety, especially with \(\mathrm{O}\) and \(\mathrm{N}\). (4) Forms molecular halides that are not hydrolysed. (a) 2,4 (b) \(1,2,4\) (c) \(1,2,3\) (d) \(2,3,4\)

Short answer

Option (b) \(1,2,4\) is incorrect.

Step-by-step solution

Analyze Statement (1)

Statement (1) says that silicon forms an oxide (\(\text{SiO}_2\)) that is amphoteric and has a Ga, As structure. Silicon dioxide (\(\text{SiO}_2\)) is known to be an acidic oxide and has a tetrahedral structure similar to other covalent oxides. However, the claim about having a 'Ga, As structure' is incorrect as \(\text{SiO}_2\) does not have this structure.

Analyze Statement (2)

Statement (2) says that silicon does not undergo coordination number expansion. Silicon, typically with an oxidation state of +4, can sometimes expand its coordination number like in hexacoordinate compounds (e.g., silicon hexafluoride, \(\text{SiF}_6^{2-}\)). In this statement, the claim is incorrect because silicon can expand its coordination number.

Analyze Statement (3)

Statement (3) suggests that silicon forms strong but unconjugated multiple bonds of the \(p\pi-d\pi\) variety, especially with \(\text{O}\) and \(\text{N}\). Silicon does not readily form \(p\pi-d\pi\) bonds because it lacks the needed empty \(d\) orbitals; this is typically seen in heavier elements like phosphorus or sulfur. This statement is largely incorrect as well.

Analyze Statement (4)

Statement (4) claims that silicon forms molecular halides that are not hydrolyzed. Silanes are silicon halides known to be easily hydrolyzed to form silicic acid and hydrogen halides. Hence, this statement is incorrect as silicon halides are typically hydrolyzed.

Identify the Incorrect Statements

From the analysis, statements (1), (2), and (4) have been identified as incorrect. This matches with option (b) \(1,2,4\).

Key concepts

Silicon Oxides

Silicon oxides are an important component of silicon chemistry, primarily known as silicon dioxide (\(\text{SiO}_2\)). Silicon dioxide is a common compound found in sand, quartz, and various minerals. In its natural form, \(\text{SiO}_2\) is typically a crystalline material with a tetrahedral structure, not similar to the 'Ga, As structure' that was mentioned in some incorrect interpretations. The crystalline form bonds silicon to oxygen atoms forming a lattice that is difficult to disrupt without the addition of significant energy.

A critical property of silicon dioxide is that it is an acidic oxide. Unlike amphoteric oxides, which can react with both acids and bases, silicon dioxide predominantly reacts with strong bases, aligning it with typical acidic behavior in oxides. This means in everyday interactions, \(\text{SiO}_2\) doesn’t behave as amphoteric, as originally pointed out in the faulty statement.

Coordination Number

When discussing the chemistry of elements like silicon, the term "coordination number" refers to the number of atoms directly bonded to a central atom within a molecule or crystal. Silicon primarily exhibits a coordination number of 4 in many of its common compounds due to the tetrahedral shape formed by its covalent bonds.

However, there are scenarios where silicon can expand its coordination number up to six. A notable example is in the case of \(\text{SiF}_6^{2-}\) (silicon hexafluoride ion), where silicon undergoes coordination number expansion. This capability stems from the ability of silicon to bond with more than four atoms, often facilitated by specific conditions or ligands capable of donating additional lone pairs. Thus, the statement claiming silicon does not undergo coordination number expansion is incorrect, as silicon demonstrates this property in several specialized compounds.

Silicon-Halide Reactions

Silicon halides, otherwise known as silanes, are molecular compounds consisting of silicon bonded to halogen atoms. These compounds typically include elements like fluorine, chlorine, bromine, or iodine bound to silicon. One of the significant properties of silicon halides is their hydrolyzable nature.

When silicon halides react with water, they undergo hydrolysis. This reaction produces silicic acid and releases hydrogen halides. For example, \(\text{SiCl}_4\) reacts with water to produce silicic acid, \(\text{H}_4\text{SiO}_4\), and hydrochloric acid, \(\text{HCl}\). This characteristic of silicon halides is crucial for various industrial applications and synthesis processes, where controlled hydrolysis is involved. Contrary to the incorrect statement, silicon halides do not resist hydrolysis, making the mentioned assertion about them not being hydrolyzed incorrect.