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 \(22.34\) would you expect metasilicic acid to have? 22.76 Speculate as to why carbon forms carbonate rather than silicate analogs.

Short answer

(a) In silicate minerals, silicon exhibits a tetrahedral arrangement, with each silicon atom bonded to four oxygen atoms at the corners of a regular tetrahedron, forming the basic building block \(\mathrm{SiO}_{4}^{4-}\). (b) Metasilicic acid may consist of a single silicon atom bonded to three oxygen atoms, with two of these oxygen atoms also bonded to two hydrogen atoms. However, one needs to consult a reference showing Figure 22.34 or further information on the molecular structure to confirm this. Carbon prefers to form carbonates rather than silicate analogs due to the difference in electronegativity and size between carbon and silicon atoms. The stronger C-O bonds and smaller size of carbon enable it to form double bonds with oxygen, stabilizing the carbonate structure, while silicon prefers to form network silicate structures instead.

Step-by-step solution

(a) Silicon's characteristic geometry in silicate minerals

In silicate minerals, silicon atoms are bonded to oxygen atoms to form a tetrahedral arrangement, meaning that each silicon atom is bonded to four oxygen atoms at the corners of a regular tetrahedron. This arrangement forms the basic building block for all silicate minerals and can be represented as \(\mathrm{SiO}_{4}^{4-}\).

(b) Structure of metasilicic acid

Knowing the empirical formula of metasilicic acid, \(\mathrm{H}_{2}\mathrm{SiO}_{3}\), we can determine its structure. Since the \(\mathrm{SiO}_{4}^{4-}\) tetrahedral unit is the basic building block in silicate minerals and considering the given formula, we can hypothesize that the structure of metasilicic acid may consist of a single silicon atom bonded to three oxygen atoms, with two of these oxygen atoms also bonded to two hydrogen atoms. To confirm this, one would need to consult a reference showing Figure 22.34 or further information on the molecular structure provided.

22.76 Carbon forming carbonate rather than silicate analogs

Carbon's preference for forming carbonates rather than silicate analogs can be attributed to the difference in electronegativity and size between the carbon and silicon atoms. Carbon has an electronegativity of 2.55, while silicon has an electronegativity of 1.9. The difference in electronegativity between carbon and oxygen (3.44) is more significant than between silicon and oxygen, leading to stronger C-O bonds in carbonates when compared to Si-O bonds in silicate analogs. Additionally, the smaller size of carbon compared to silicon enables carbon to form double bonds with oxygen, stabilizing the carbonate structure. On the other hand, silicon, due to its larger size, cannot efficiently form double bonds with oxygen, preferring to form the network silicate structures instead.

Key concepts

Silicon Tetrahedral Geometry

Silicon tetrahedral geometry is a fascinating topic within the field of chemistry. In silicate minerals, the silicon atom occupies a central position, surrounded by four oxygen atoms. This arrangement forms a shape known as a tetrahedron, which is a three-dimensional structure with four triangular faces. Each oxygen atom resides at one corner of this regular tetrahedron.

This specific geometry is important because it repeats throughout various silicate minerals, establishing a consistent and foundational structure. You can think of this tetrahedron as a building block—much like a LEGO piece—that combines in various ways to form the wide assortment of silicate minerals we encounter on Earth. With the formula \(\mathrm{SiO}_{4}^{4-}\), silicon bonds with oxygen to form a stable and resilient structure ideal for the mineral kingdom.

When these tetrahedral units link together, they create diverse silicate structures, ranging from single chains to complex frameworks. This vast variability in structure results in the extensive diversity of silicate minerals that populate the Earth's crust.

Metasilicic Acid Structure

The structure of metasilicic acid is an interesting study due to its unique configuration. Metasilicic acid has the empirical formula \(\mathrm{H}_{2}\mathrm{SiO}_{3}\) and this places it within the realm of silicic acids, related to silicate minerals. Understanding its structure is essential for grasping its chemical behavior and properties.

In metasilicic acid, the silicon atom is bonded to three oxygen atoms. Among these, two oxygen atoms are linked each to a hydrogen atom, forming hydroxyl groups. This specific formation indicates a modification from the typical \(\mathrm{SiO}_{4}^{4-}\) unit. This framework suggests that the acid is poised to form silicic bonds while retaining characteristics that distinguish it from simple silicates.

This structure affects the specific properties and uses of metasilicic acid, frequently acting as an intermediate in forming more complex silicate structures. In applications involving silicic acids, understanding the arrangement is pivotal to anticipate its reactivity and potential role in larger systems.

Carbon vs Silicon Bonding

The comparison between carbon and silicon bonding unveils their unique traits and why each element prefers to form certain compounds. Carbon and silicon, despite both belonging to Group 14 of the periodic table, differ significantly in their bonding capabilities.

One major difference lies in their electronegativity and atomic size. Carbon's electronegativity, at 2.55, is higher than that of silicon, which stands at 1.9. This difference leads to stronger carbon-oxygen bonds than silicon-oxygen bonds, which is why carbon readily forms carbonate compounds. Additionally, the smaller atomic size of carbon allows it to form stable double bonds with oxygen. This ability helps stabilize carbonates due to the resonance and rigidity of double-bond configurations.

Conversely, silicon's larger atomic size precludes the formation of effective double bonds with oxygen, steering it towards creating extended network structures like silicates. These network structures are robust and contribute to the durable nature of silicate minerals. Understanding this distinction helps explain the different chemistry and utility of carbon and silicon compounds across various fields.