Problem 15 - Chemistry: The Central Science

Question

Explain the following observations: (a) The highest fluoride compound formed by nitrogen is \(\mathrm{NF}_{3}\), whereas phosphorus readily forms \(\mathrm{PF}_{5} .\) (b) Although \(\mathrm{CO}\) is a well-known compound, SiO does not exist under ordinary conditions. (c) \(\mathrm{AsH}_{3}\) is a stronger reducing agent than \(\mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

Nitrogen forms NF3 due to its higher electronegativity and greater stability provided by the half-filled p-orbital, whereas phosphorus forms PF5 because it can accommodate an expanded octet. CO exists as a stable compound under ordinary conditions due to carbon and oxygen's similar electronegativities forming a thermodynamically stable double bond, while SiO doesn't exist due to silicon being less electronegative and unstable Si-O bond. AsH3 is a stronger reducing agent than NH3 as arsenic, being less electronegative than nitrogen, can more easily donate electrons to other substances.

Step 1: a) Explaining why nitrogen forms NF3 while phosphorus forms PF5

Nitrogen belongs to group 15 of the periodic table, and it has 5 valence electrons. Phosphorus is right below nitrogen and also has 5 valence electrons. However, nitrogen is more electronegative than phosphorus due to its smaller size and proximity to fluorine. Nitrogen has a half-filled p-orbital, which gives it extra stability, so it prefers to form only three covalent bonds to complete its octet and maintain stability. In the case of NF3, nitrogen shares 3 electrons with 3 fluorine atoms, forming a stable compound. Phosphorus, on the other hand, can expand its octet because it has vacant d-orbitals in the same principal quantum shell. This allows phosphorus to accommodate more than eight electrons and hence form PF5. So, nitrogen forms NF3 due to its higher electronegativity and greater stability provided by the half-filled p-orbital, while phosphorus forms PF5 because it can accommodate an expanded octet.

Step 2: b) Explaining why CO is a well-known compound, while SiO does not exist under ordinary conditions

Carbon and silicon both belong to group 14, but carbon is above silicon in the periodic table. Both elements have four valence electrons and can form four covalent bonds. Carbon is more electronegative than silicon due to its smaller size and greater electron affinity. Because carbon and oxygen have similar electronegativities, they form a thermodynamically stable double bond (CO) and exist as a stable compound under ordinary conditions. On the other hand, silicon is less electronegative than oxygen, leading to a higher polarity in the Si-O bond. Moreover, silicon readily forms tetrahedral structures due to its larger atomic size, and so the silicon-oxygen bond in a SiO molecule would be unstable due to steric factors. Thus, SiO does not exist under ordinary conditions, but more stable silicon-oxygen compounds exist, such as SiO2 (Quartz) where Si is bonded to four oxygen atoms.

Step 3: c) Explaining why AsH3 is a stronger reducing agent than NH3

While nitrogen and arsenic both belong to group 15, arsenic lies below nitrogen in the periodic table and has a lower electronegativity compared to nitrogen. Both elements can form hydrides with hydrogen (NH3 and AsH3). A reducing agent is a substance that donates electrons to another substance (in other words, it gets oxidized). Since arsenic is less electronegative than nitrogen, it has a weaker hold over the bonding electrons with hydrogen. This weaker bond makes it easier for arsenic to donate electrons compared to nitrogen. So, AsH3 is a stronger reducing agent than NH3 because arsenic, being less electronegative than nitrogen, can more easily donate electrons to other substances.

Key Concepts

Valence Electrons

Valence electrons are the electrons located in the outermost shell of an atom. They play a crucial role in determining how an element will react chemically with others.
For elements in the same group of the periodic table, like nitrogen and phosphorus, the number of valence electrons is the same.

• Nitrogen and phosphorus both have five valence electrons as they belong to group 15.
• These electrons are key players in forming chemical bonds.

In nitrogen's case, its valence electrons prefer to form three bonds due to the stability of achieving a filled octet. Phosphorus, with its ability to expand its octet, can use its valence electrons to form additional bonds, thanks to its available d-orbitals. Understanding valence electrons helps explain why different elements form different compounds.

Electronegativity

Electronegativity is a measure of how strongly an atom can attract and hold onto electrons within a chemical bond. It affects how atoms bond and the type of bonds they form.

• Nitrogen is more electronegative than phosphorus. This property influences its tendency to form three bonds in compounds like \(\mathrm{NF}_3\).
• Carbon, being more electronegative than silicon, forms stable compounds like carbon monoxide (CO) with oxygen.

Electronegativity plays a pivotal role in chemical reactivity and stability. Atoms with higher electronegativity pull electrons closer, resulting in stable, less reactive bonds, while lower electronegativity in elements like arsenic makes it easier to give up electrons, as seen in its reducing behavior.

Covalent Bonds

Covalent bonds are bonds formed when atoms share electrons. These bonds occur when atoms have similar electronegativities.

• In \(\mathrm{NF}_3\), nitrogen shares its three unpaired electrons with three fluorine atoms, forming strong covalent bonds.
• The sharing of electrons in \(\mathrm{CO}\) results in a stable double bond due to the closely matched electronegativities of carbon and oxygen.

Covalent bonds play a crucial role in determining the structure and properties of molecules. The strength and number of these bonds can significantly change how a compound behaves and reacts with other substances. Understanding covalent bonds provides insights into why certain compounds, like silicon monoxide (SiO), are not stable under ordinary conditions.

Reducing Agents

Reducing agents are substances that donate electrons in a chemical reaction, thus becoming oxidized themselves. This electron donation is foundational to many chemical processes.

• Arsenic, found in \(\mathrm{AsH}_3\), is a strong reducing agent because it can easily donate electrons.
• Its lower electronegativity compared to nitrogen means arsenic forms weaker bonds with hydrogen, facilitating electron donation.

Understanding reducing agents is essential for grasping how chemical transformations occur. These agents play a crucial role in driving reactions, especially in contexts like organic synthesis and oxidation-reduction processes. The efficiency of a reducing agent like \(\mathrm{AsH}_3\) highlights its potential applications and contrasts how it behaves compared to compounds with stronger intermolecular forces, such as ammonia (NH3).