Question

(a) What is the characteristic geometry about silicon in all silicate minerals? (b) Metasilicic acid has the empirical formula \(\mathrm{H}_{2} \mathrm{SiO}_{3} .\) Which of the structures shown in Figure 22.32 would you expect metasilicic acid to have?

Short answer

(a) The characteristic geometry of silicon in all silicate minerals is a tetrahedron, formed by the silicon-oxygen tetrahedron \(\mathrm{[SiO}_{4}]^{4-}\). (b) Based on the empirical formula \(\mathrm{H}_{2}\mathrm{SiO}_{3}\), metasilicic acid is expected to have a structure with a single \(\mathrm{SiO}_{4}\) tetrahedron, where one of the oxygen atoms is protonated with a hydrogen atom to form a hydroxyl group (OH).

Step-by-step solution

Part (a): Identify the characteristic geometry about silicon in all silicate minerals

To understand the characteristic geometry of silicon in silicate minerals, we need to look at the composition and formation of silicates in general. The silicon-oxygen tetrahedron, [SiO4]^4-, is the basic building block of silicate minerals. It consists of a central silicon atom, which is coordinated to four surrounding oxygen atoms in the form of a tetrahedron. This is the characteristic geometry of silicon in silicate minerals. Now, let's move on to part (b) of the exercise.

Part (b): Determine the structure of metasilicic acid from the given empirical formula

Metasilicic acid has an empirical formula of H2SiO3. This means it has one silicon atom, two hydrogen atoms, and three oxygen atoms in each formula unit. We are asked to determine which of the structures shown in Figure 22.32 would best represent metasilicic acid. Unfortunately, we do not have access to Figure 22.32, but we can describe an appropriate structure based on the empirical formula. Since metasilicic acid has only one SiO4 tetrahedron due to its single silicon atom, it is reasonable to assume that one of the four oxygens in the SiO4 tetrahedron is protonated with a hydrogen atom (so it becomes a hydroxyl group, OH). This leaves three remaining oxygen atoms in the SiO4 tetrahedron, which corresponds to the structure of metasilicic acid, H2SiO3. In conclusion: 1. The characteristic geometry of silicon in all silicate minerals is a tetrahedron, formed by the silicon-oxygen tetrahedron [SiO4]^4-. 2. Based on the empirical formula H2SiO3, metasilicic acid is expected to have a structure with a single SiO4 tetrahedron, where one of the oxygen atoms is protonated with a hydrogen atom to form a hydroxyl group (OH).

Key concepts

Silicon-Oxygen Tetrahedron

At the heart of the structure of silicate minerals is the silicon-oxygen tetrahedron. This is the fundamental unit that defines the geometry about silicon in these minerals. A tetrahedron is a three-dimensional shape, comprising four triangular faces. Within the silicon-oxygen tetrahedron, one can imagine a silicon atom as the core. It is surrounded by four oxygen atoms, each positioned at a corner of the tetrahedron. The chemical formula of the silicon-oxygen tetrahedron is \(\mathrm{[SiO_4]^{4-}}\). This reflects a negatively charged group, with the silicon atom developing a covalent bond with each of these oxygen atoms.

• This tetrahedral arrangement is crucial as it forms the cornerstone of various silicate structures.
• It determines how these minerals might extend or connect in space.
• The versatility of these tetrahedral structures allows for the formation of numerous types of silicates with different properties.

Metasilicic Acid

The compound metasilicic acid, with the empirical formula \(\mathrm{H_2SiO_3}\), is an interesting addition to the family of silicates. It provides insight into the interaction between silicon, oxygen, and additional elements like hydrogen. In considering the structure of metasilicic acid, a key point is the presence of one silicon atom, accompanied by two hydrogen atoms and three oxygen atoms in each formula unit. Since Figure 22.32 is unavailable, we can infer its possible structure using basic chemical principles.

• The single silicon atom is expected to occupy the center of a tetrahedron, similar to the silicon-oxygen tetrahedron.
• Given the specific atoms involved, one of the oxygen atoms in this tetrahedron would be bonded to a hydrogen atom, forming a hydroxyl group (OH).
• This configuration aligns with the empirical formula, suggesting it fits the structure of metasilicic acid.

These adapted structures maintain tetrahedral attributes but display a unique arrangement due to the incorporation of hydrogen atoms.

Geometric Structure of Silicon

Silicon's geometric structure in materials like silicate minerals involves the consistent formation of tetrahedral shapes. This structuring is fundamental in defining the characteristics and behaviors of the minerals it composes. Silicon naturally prefers a tetrahedral coordination due to its size and bonding capabilities.

• Within these tetrahedral formations, each silicon atom forms stable covalent bonds with four oxygen atoms. This stable structure results from compatible atomic sizes and the efficient sharing of electrons.
• This tetrahedral geometry is not only essential for mineral stability but also for flexibility in extending into complex crystal network structures.
• The ability for these tetrahedra to link, sharing oxygen atoms, contributes to a wide variety of these complex silicate frameworks, defining much of Earth's crust.

Through this basic geometric arrangement, silicon enables a vast diversity of mineral forms, affecting both physical and chemical properties significantly.