Question

Formula of the following silicate anion is (1) ${\mathrm{SiO}}_4^4$ (2) ${\mathrm{Si}}_2{\mathrm{O}}_7^6$ (3) ${\mathrm{Si}}_2{\mathrm{O}}_8^{6-}$ (4) ${\mathrm{Si}}_3{\mathrm{O}}_9^{6-}$

Short answer

The correct formula is ${\text{Si}}_2{\text{O}}_7^{6-}$, which is option (2).

Step-by-step solution

Understand the Problem

The task is to identify the correct formula for the given silicate anion from the options provided.

Review Silicate Anions

Silicate anions are based on silicon and oxygen atoms. Common forms include orthosilicate ${\text{SiO}}_4^{4-}$, pyrosilicate ${\text{Si}}_2{\text{O}}_7^{6-}$, and more complex structures. Each form has a specific ratio and charge.

Analyze Each Option

Review each provided option: 1. Orthosilicate: ${\text{SiO}}_4^{4-}$ has a silicon to oxygen ratio of 1:4.2. Pyrosilicate: ${\text{Si}}_2{\text{O}}_7^{6-}$ has a silicon to oxygen ratio of 2:7.3. ${\text{Si}}_2{\text{O}}_8^{6-}$ is not a standard silicate anion structure.4. Cyclosilicate: ${\text{Si}}_3{\text{O}}_9^{6-}$ has a silicone to oxygen ratio of 3:9.

Identify the Correct Formula

Based on known structures:- Orthosilicate: ${\text{SiO}}_4^{4-}$.- Pyrosilicate: ${\text{Si}}_2{\text{O}}_7^{6-}$.- The correct formula matches the specific type of silicate anion.

Select the Correct Answer

Clearly, option (2) ${\text{Si}}_2{\text{O}}_7^{6-}$ matches the structure described as Pyrosilicate.

Key concepts

Orthosilicate

Orthosilicates are the simplest form of silicate anions. They have a formula of $${\text{SiO}}_4^{4-}$$. In this structure, a single silicon atom is surrounded by four oxygen atoms, forming a tetrahedral shape.
The term orthosilicate comes from the Greek word ortho, meaning 'straight' or 'correct,' indicating this is the most fundamental form.
Key points to remember about orthosilicates include:

• Orthosilicates are also called 'nesosilicates' as they consist of isolated tetrahedra.
• They have a silicon to oxygen ratio of 1:4.
• The tetrahedra do not share any oxygen atoms, making them isolated.

Orthosilicates are common in various minerals, such as olivine and zircon.
Understanding the fundamental orthosilicate structure is crucial for learning about more complex silicate anions.

Pyrosilicate

Pyrosilicates, also known as 'disilicates,' involve the combination of two orthosilicate tetrahedra.
The chemical formula for a pyrosilicate is $${\text{Si}}_2{\text{O}}_7^{6-}$$. In this structure, two tetrahedra share one oxygen atom, leading to a more complex anion.
Key characteristics of pyrosilicates include:

• They have a silicon to oxygen ratio of 2:7.
• In terms of structure, they consist of double tetrahedra units.
• The shared oxygen atom creates a bridge between the two silicon atoms.

Due to their more complex nature, pyrosilicates are seen in certain minerals but are less common than orthosilicates.
Understanding this form helps in recognizing the behavior of silicate structures that involve linked tetrahedral units.

Silicon to Oxygen Ratio

The silicon to oxygen ratio is an essential concept in understanding different types of silicate anions.
This ratio indicates the number of silicon atoms compared to oxygen atoms in a given anion.
Here are some important points about the silicon to oxygen ratio:

• In orthosilicate $${\text{SiO}}_4^{4-}$$, the ratio is 1:4, meaning one silicon atom is surrounded by four oxygen atoms.
• In pyrosilicate $${\text{Si}}_2{\text{O}}_7^{6-}$$, the ratio is 2:7, with two silicon atoms sharing an oxygen bridge and linking their tetrahedral structures.
• The formula can be used to determine whether a given anion is a standard structure or an uncommon one.

Understanding the silicon to oxygen ratio helps identify and classify the different silicate structures rapidly.
This is key for recognizing how silicate compounds are formed and how they behave chemically.