Problem 16 - Chemistry The Central Science In Si Units

Question

Which of the following statements are true? (a) Si can form an ion with six fluorine atoms, \(\mathrm{SiF}_{6}^{2-}\), whereas carbon cannot. (b) Si can form three stable compounds containing two Si atoms each, \(\mathrm{Si}_{2} \mathrm{H}_{2}, \mathrm{Si}_{2} \mathrm{H}_{4},\) and \(\mathrm{Si}_{2} \mathrm{H}_{6}\) (c) In \(\mathrm{HNO}_{3}\) and \(\mathrm{H}_{3} \mathrm{PO}_{4}\) the central atoms, \(\mathrm{N}\) and \(\mathrm{P}\), have different oxidation states. (d) \(\mathrm{S}\) is more electronegative than Se.

Step-by-Step Solution

Answer

Statements (a) and (d) are true; Statements (b) and (c) are false.

Step 1: Understanding Ion Formation

Silicon (Si) can expand its octet and easily bond with six fluorine atoms, forming \(\mathrm{SiF}_{6}^{2-}\). Carbon, on the other hand, cannot extend its octet beyond four bonds due to lack of d-orbitals, hence cannot form \(\mathrm{CF}_{6}^{2-}\). Thus, the statement (a) is true.

Step 2: Evaluating Silicon Compounds

Silicon can indeed form multiple hydrides. Known stable silicon hydrides including those that contain two silicon atoms are \(\mathrm{Si}_{2} \mathrm{H}_{6}\) (disilane). \(\mathrm{Si}_{2} \mathrm{H}_{2}\) and \(\mathrm{Si}_{2} \mathrm{H}_{4}\) are not stable. Consequently, statement (b) is false.

Step 3: Analyzing Oxidation States

In \(\mathrm{HNO}_{3}\), nitrogen has an oxidation state of +5, while in \(\mathrm{H}_{3} \mathrm{PO}_{4}\), phosphorus has an oxidation state of +5 as well. Hence, statement (c) is false because their oxidation states are the same.

Step 4: Electronegativity Comparison

Sulfur (S) is indeed more electronegative than Selenium (Se) due to its position higher up in the same group in the periodic table. Therefore, statement (d) is true.

Key Concepts

Ion Formation

Ion formation is a fundamental concept in chemistry that explains how atoms gain or lose electrons to form charged particles called ions. When atoms bond together, they often do so to achieve a more stable electronic configuration, often resembling that of noble gases. Silicon ( Si) is an interesting element because it can form complex ions by expanding its octet. This means that unlike carbon, which cannot exceed four bonds, silicon can form ions like SiF_{6}^{2-}, where it bonds with six fluorine atoms. This is possible because silicon has empty d-orbitals that allow it to accommodate more electrons, expanding beyond the typical octet rule.

• Octet Expansion: Silicon has the capability to expand its octet by using its d-orbitals, allowing the formation of ions with more than eight electrons.
• Lack of d-Orbitals in Carbon: Carbon cannot expand beyond four bonds because it does not have available d-orbitals.

In summary, the ability of silicon to form six-coordinate complex ions exemplifies the unique behavior of elements with accessible d-orbitals.

Oxidation States

Oxidation states, also known as oxidation numbers, are assigned to atoms within compounds to indicate the distribution of electrons among them. This concept helps in understanding electron transfer during chemical reactions. In acids like HNO_{3} and H_{3}PO_{4}, oxidation states provide insight into the chemical bonding and reactivity of different elements.

• Nitrogen and Phosphorus: In both HNO_{3} and H_{3}PO_{4}, the central atoms are nitrogen and phosphorus, respectively. Both these atoms exhibit an oxidation state of +5 in these compounds.
• Determining Oxidation States: This involves a set of rules, such as assigning a -2 oxidation state to oxygen, except in peroxides, and adjusting the sum to equal the charge of the molecule.

Despite being in different compounds, nitrogen and phosphorus have the same oxidation state of +5, indicating a similar level of electron sharing in their respective compounds.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons in a chemical bond. It's a key concept that helps predict how atoms will interact in a molecule. For example, sulfur (S) is more electronegative than selenium (Se).

• Periodic Table Trend: Electronegativity typically increases across a period and decreases down a group in the periodic table.
• Sulfur vs. Selenium: Sulfur is located above selenium in the same group, thus it has a higher electronegativity, meaning it more effectively attracts electrons in bonds.

Understanding electronegativity is crucial for predicting the nature of bonds and the behavior of molecules, including whether a bond is likely to be ionic or covalent.

Silicon Compounds

Silicon exhibits versatile bonding, leading to the formation of a variety of compounds, some of which include silicon hydrides. However, not all conceivable silicon hydrides are stable.

• Stable Silicon Hydrides: Si_{2}H_{6}, also known as disilane, is a stable compound with two silicon atoms.
• Unstable Variants: Other hydrides like Si_{2}H_{2} and Si_{2}H_{4} are not stable because they do not exist under normal conditions due to their high reactivity and the lack of sufficient bonding stability.

Silicon's ability to form a range of compounds reflects its position and electronic structure in the periodic table, enabling robust industrial applications, but emphasizing the necessity to recognize which derivatives are stable and utilizable.