Problem 68 - Chemistry The Science in Context

Question

All the group 15 elements form compounds with the generic formula \(\mathrm{H}_{3} \mathrm{E}(\mathrm{E}=\mathrm{N}, \mathrm{P}, \mathrm{As}, \mathrm{Sb}, \text { and } \mathrm{Bi}) .\) Which compound is the most polar? Which compound do you predict to have the smallest \(\mathrm{H}-\mathrm{E}-\mathrm{H}\) bond angle?

Step-by-Step Solution

Answer

Answer: The most polar compound is H₃N (ammonia), and the compound with the smallest H-E-H bond angle is H₃Bi (bismuthine).

Step 1: Determine the electronegativity values

To predict the most polar compound among the group 15 elements, we need to determine the electronegativity values of the elements. Electronegativity is the ability of an atom to attract bonding electrons. From the periodic table, the electronegativity values for the group 15 elements are: N: 3.04 P: 2.19 As: 2.18 Sb: 2.05 Bi: 2.02

Step 2: Analyze the bond polarity

For a bond to be polar, there must be a difference in electronegativity between the two atoms. The polarity of a bond can be determined using the electronegativity difference between the atoms. Among the group 15 elements, nitrogen has the highest electronegativity value, and when combined with hydrogen, a significant difference in electronegativity is observed. Therefore, the most polar compound will be H₃N, also known as ammonia.

Step 3: Determine the molecular geometries

To predict the smallest H-E-H bond angle among the group 15 elements, we have to analyze the molecular geometries of these compounds. The group 15 elements possess 5 valence electrons. When the element forms a compound with three hydrogen atoms, it will have one lone pair and three bonding pairs around the central atom. This generates a tetrahedral electronic geometry with a trigonal pyramidal molecular geometry.

Step 4: Analyze the bond angles based on molecular geometries and atomic sizes

As the compounds have a trigonal pyramidal molecular geometry, the bond angle between H-E-H atoms will generally decrease as we go down the group 15 elements, due to the atomic size increase in accumulating the lone pair electrons. This leads to increased electron repulsion, causing the H-E-H bond angle to compress. So, based on the analysis of atomic sizes and molecular geometries, we can predict that H₃Bi, which has the largest atomic size among the group 15 elements, will have the smallest H-E-H bond angle. The final answer for this problem is: the most polar compound is H₃N (ammonia), and the compound with the smallest H-E-H bond angle is H₃Bi (bismuthine).

Key Concepts

Electronegativity

Electronegativity refers to how strongly an atom can attract electrons in a chemical bond. It's like a tug-of-war, where different atoms have different pulling strengths. In the group 15 elements, which include nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi), nitrogen has the highest electronegativity at 3.04. This high value means nitrogen attracts electrons more strongly than the other elements in the group.

When examining molecular polarity, the difference in electronegativity between bonded atoms is crucial. A greater difference usually results in a more polar bond. In our context, ammonia ( H_3N ), where nitrogen bonds with hydrogen, has a high polarity due to nitrogen's strong electronegativity compared to hydrogen's relatively low value (2.20). This makes ammonia the most polar compound among those formed with group 15 elements.

Molecular Geometry

Molecular geometry describes the three-dimensional arrangement of atoms in a molecule. For group 15 elements, when they form compounds like H_3E , they exhibit a trigonal pyramidal geometry. Here's why:

• Each central element (E) in group 15 has five valence electrons.
• When it forms a compound with three hydrogen atoms, it uses three electrons for bonding.
• The remaining two electrons form a lone pair.

The presence of this lone pair causes the molecule to shape into a trigonal pyramidal structure rather than a flat triangle, because the lone pair exerts a repulsive force that compresses the bond angles.

Molecular geometry influences bond angles. Typically, the H-E-H bond angle in these compounds is about 107° in ammonia. However, the angle decreases with larger central atoms like bismuth due to increased electron cloud size and repulsions, making H_3Bi have the smallest angle.

Group 15 Elements

Group 15 elements, sometimes called the "nitrogen group," include nitrogen, phosphorus, arsenic, antimony, and bismuth. These elements are located in the same vertical column of the periodic table and share some common properties.

A key characteristic of these elements is the presence of five valence electrons. This configuration is significant because it explains their ability to form three single bonds with hydrogen in H_3E compounds.

As we move down the group from nitrogen to bismuth, we notice trends:

• Electronegativity decreases.
• Atomic size increases.
• Bond angles in their H_3E compounds decrease.

This makes the study of group 15 elements particularly interesting, as they display a variety of behaviors while still maintaining some chemical consistency. Understanding these trends helps predict the properties of their compounds, such as the polarity and bond angles we analyzed with H_3N and H_3Bi .