Problem 38 - Introductory Chemistry Atoms First

Question

Which of each pair is most polar? Explain. (a) Chlorine and phosphorus trichloride (b) Carbon disulfide and sulfur difluoride (c) Nitrogen trifluoride and phosphorus trifluoride (d) Silicon tetrabromide and hydrogen cyanide \((\mathrm{HCN})\) (e) Nitrogen trichloride and oxygen dichloride

Step-by-Step Solution

Answer

(a) Phosphorus trichloride is more polar due to its polar P-Cl bonds. (b) Sulfur difluoride is more polar because of its polar S-F bonds. (c) Nitrogen trifluoride is more polar due to greater electronegativity difference. (d) Hydrogen cyanide is more polar because of its electronegativity difference. (e) Oxygen dichloride is more polar because of larger electronegativity difference.

Step 1: (a) Chlorine vs. Phosphorus Trichloride

To compare the polarity of these two compounds, we need to consider the electronegativity difference between the atoms involved. Chlorine (Cl) is a diatomic element, so there is no electronegativity difference between the two chlorine atoms. Therefore, chlorine is a nonpolar molecule. Phosphorus trichloride (PCl3) is a polar molecule due to the three polar P-Cl bonds. Hence, phosphorus trichloride is more polar than chlorine.

Step 2: (b) Carbon Disulfide vs. Sulfur Difluoride

Carbon disulfide (CS2) has a linear molecular geometry with sulfur atoms having an electronegativity of 2.58 and carbon with 2.55. The small electronegativity difference between carbon and sulfur atoms makes the bonds in CS2 almost nonpolar. Sulfur difluoride (SF2) contains two polar S-F bonds due to the high electronegativity of fluorine (3.98). Consequently, sulfur difluoride is more polar than carbon disulfide.

Step 3: (c) Nitrogen Trifluoride vs. Phosphorus Trifluoride

Both nitrogen trifluoride (NF3) and phosphorus trifluoride (PF3) contain polar bonds due to the electronegativity difference between nitrogen/phosphorus and fluorine. However, NF3 has a greater electronegativity difference between nitrogen (3.04) and fluorine (3.98) compared to the difference between phosphorus (2.19) and fluorine. Therefore, nitrogen trifluoride is more polar than phosphorus trifluoride.

Step 4: (d) Silicon Tetrabromide vs. Hydrogen Cyanide

Silicon tetrabromide (SiBr4) is a nonpolar molecule with a tetrahedral molecular geometry. The electronegativity difference between silicon (1.90) and bromine (2.96) is small, leading to nonpolar bonds in SiBr4. Hydrogen cyanide (HCN) has a significant electronegativity difference between hydrogen (2.20) and nitrogen (3.04), making HCN a polar molecule. Thus, hydrogen cyanide is more polar than silicon tetrabromide.

Step 5: (e) Nitrogen Trichloride vs. Oxygen Dichloride

Nitrogen trichloride (NCl3) is a polar molecule with nitrogen-chlorine polar bonds due to the electronegativity difference of nitrogen (3.04) and chlorine (3.16). Oxygen dichloride (OCl2) also contains polar oxygen-chlorine bonds because of the electronegativity difference between oxygen (3.44) and chlorine. However, the difference in electronegativities between oxygen and chlorine atoms is greater than that in nitrogen trichloride, making oxygen dichloride more polar than nitrogen trichloride.

Key Concepts

Electronegativity

Electronegativity is the ability of an atom to attract electrons in a chemical bond. It plays a crucial role in determining the polarity of molecules. Different atoms have varying electronegativities, which affects how they share electrons with other atoms.
For example, in the exercise, fluorine has a high electronegativity of 3.98, making it a strong attractor of electrons. This high electronegativity leads to more polar bonds when fluorine is bonded with less electronegative elements like sulfur or phosphorus.

• Electronegativity differences cause polar or nonpolar bonds.
• A large difference results in polar bonds where electrons are unequally shared.
• If differences are minimal, the bond tends to be nonpolar, with electrons shared more equally.

These differences impact chemical bonding and contribute to the molecular polarity, a topic essential in chemistry.

Polar Bonds

Polar bonds occur when there is a significant difference in electronegativity between the two bonded atoms, resulting in an uneven distribution of electron density. In such bonds, electrons are drawn more towards the more electronegative atom.
In the given exercise, sulfur difluoride (SF2) and phosphorus trifluoride (PF3) both have polar bonds due to the significant electronegativity difference between sulfur/phosphorus and fluorine. This creates a partial negative charge on the fluorine atoms.

• Polar bonds lead to dipoles – partial charges within the molecule.
• The greater the electronegativity difference, the stronger the dipole moments.
• Polar bonds are key in determining the overall molecular polarity.

Understanding polar bonds is crucial for predicting molecular behavior in chemical reactions.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. It influences molecular polarity by affecting how the polar bonds are oriented.
For example, carbon disulfide (CS2) is linear, which means any polarities in the molecule cancel each other out, making it nonpolar despite individual bond polarities.

• In polar molecules, the sum of dipoles does not cancel out.
• SiBr4 is a tetrahedral molecule, but its symmetry results in nonpolarity due to equal distribution of charges around the central atom.
• HCN has a linear geometry, creating a dipole moment along its bond axes because of its polarity.

Understanding molecular geometry helps predict whether a molecule will have an overall dipole moment, contributing to its behavior and interactions.

Chemical Bonding

Chemical bonding involves the interactions between atoms to form molecules. Understanding bonding types is crucial for determining molecular properties, including polarity.
In the provided exercises, atoms interact to form covalent bonds, where electrons are shared between atoms. The nature of these bonds affects whether the resulting molecules are polar or nonpolar.

• Covalent bonds can be polar or nonpolar.
• Polarity of individual bonds contributes to the overall molecular polarity.
• In nonpolar molecules, like SiBr4, the symmetrical arrangement of polar bonds results in no net dipole moment.
• Other molecules, such as HCN, have polar bonds, leading to an asymmetrical distribution of charge and a net dipole moment.

Comprehending chemical bonding provides a basis for understanding molecular structure and properties, bridging to topics such as molecular geometry and polar bonds.