Question

Predict whether each of the following molecules is polar or nonpolar: (a) \(\mathrm{CCl}_{4}\), (b) \(\mathrm{NH}_{3}\), (c) \(\mathrm{SF}_{4}\), (d) \(\mathrm{XeF}_{4}\), (e) \(\mathrm{CH}_{3} \mathrm{Br}\), (f) \(\mathrm{GaH}_{3}\).

Short answer

(a) CCl4: nonpolar (tetrahedral geometry, symmetric charge distribution) (b) NH3: polar (trigonal pyramidal geometry, net dipole towards N) (c) SF4: polar (see-saw geometry, unequal charge distribution) (d) XeF4: nonpolar (square planar geometry, symmetric charge distribution) (e) CH3Br: polar (tetrahedral geometry, asymmetrical charge distribution) (f) GaH3: polar (trigonal planar geometry, unequal charge distribution)

Step-by-step solution

(a) CCl4 molecule:

Find the molecular geometry and analyze the charge distribution. In CCl4, carbon forms four single bonds with four chlorine atoms. 1. Determine geometry: The molecule has tetrahedral geometry, which occurs when a central atom is bonded to four other atoms. 2. Check for charge distribution: The C-Cl bonds have some electronegativity difference, but the molecule is symmetrically shaped. Thus, any difference in charge will be evenly distributed, and no net dipole will exist. Conclusion: CCl4 is a nonpolar molecule.

(b) NH3 molecule:

Determine the molecular geometry and analyze the charge distribution. In NH3, nitrogen forms three single bonds with three hydrogen atoms and has one lone pair. 1. Determine geometry: The molecule has trigonal pyramidal geometry, which occurs when a central atom is bonded to three other atoms and has one lone pair. 2. Check for charge distribution: Nitrogen is more electronegative than hydrogen, leading to a net dipole towards the nitrogen atom. Conclusion: NH3 is a polar molecule.

(c) SF4 molecule:

Determine the molecular geometry and analyze the charge distribution. In SF4, sulfur forms four bonds with four fluorine atoms and has one lone pair. 1. Determine geometry: The molecule has see-saw geometry, which occurs when a central atom is bonded to four other atoms (two of the same type and two of another type), and has one lone pair. 2. Check for charge distribution: Because of the lone pair and difference in electronegativity between sulfur and fluorine, there is an unequal distribution of charge in the molecule. Conclusion: SF4 is a polar molecule.

(d) XeF4 molecule:

Determine the molecular geometry and analyze the charge distribution. In XeF4, Xenon forms four bonds with four fluorine atoms and has two lone pairs. 1. Determine geometry: The molecule has square planar geometry, which occurs when a central atom is bonded to four other atoms (same type) and has two lone pairs. 2. Check for charge distribution: Although xenon is less electronegative than fluorine, the symmetric shape of the molecule nullifies any differences in charge distribution. Conclusion: XeF4 is a nonpolar molecule.

(e) CH3Br molecule:

Determine the molecular geometry and analyze the charge distribution. In CH3Br, carbon forms three single bonds with three hydrogen atoms and one single bond with a bromine atom. 1. Determine geometry: The molecule has tetrahedral geometry, which occurs when a central atom is bonded to four other atoms. 2. Check for charge distribution: The electronegativity difference between carbon and bromine creates an asymmetrical charge distribution. Conclusion: CH3Br is a polar molecule.

(f) GaH3 molecule:

Determine the molecular geometry and analyze the charge distribution. In GaH3, gallium forms three single bonds with three hydrogen atoms. 1. Determine geometry: The molecule has trigonal planar geometry, which occurs when a central atom is bonded to three other atoms (same type), and no lone pairs are present on the central atom. 2. Check for charge distribution: Because of the difference in electronegativity between gallium and hydrogen, there is an unequal charge distribution. Conclusion: GaH3 is a polar molecule.

Key concepts

Molecular Geometry

In molecular geometry, understanding the 3D arrangement of atoms in a molecule is crucial. Knowing the molecular geometry helps predict the behavior and reactivity of the molecule. The VSEPR (Valence Shell Electron Pair Repulsion) theory assists in predicting these shapes. According to VSEPR, electron pairs surrounding a central atom will position themselves as far apart as possible. This positioning minimizes repulsion, determining the molecule's shape. Common molecular geometries include:

• Tetrahedral - Four atoms bonded to a central atom, forming a three-dimensional shape with bond angles of approximately 109.5°.
• Trigonal Planar - Three atoms around a central atom in a flat arrangement with 120° angles.
• Trigonal Pyramidal - Like trigonal planar but with a lone pair on the central atom, giving a three-sided pyramid shape.
• See-Saw - Four atoms in a t-shape around a central atom with a lone pair, creating an asymmetric shape.
• Square Planar - Four atoms in a square around the central atom, typically with two lone pairs.

Electronegativity

Electronegativity refers to the tendency of an atom to attract bonding electrons towards itself. It's a significant factor in determining how electrons are distributed in a molecule. Consider the electronegativity values of elements: the greater the difference, the more polar the bond. For example, chlorine is more electronegative than carbon, resulting in the electronegative distribution in a CCl bond. Points to remember:

• Highly electronegative elements (like fluorine, oxygen, and nitrogen) attract electrons stronger than others.
• An electronegativity difference results in partial positive and negative charges, leading to a dipole moment.
• If atom A is more electronegative than atom B, in a bond between A and B, electron density is pulled toward A.

Symmetry in Molecules

Symmetry plays a vital role in determining the overall polarity of a molecule. A molecule can have polar bonds but still be nonpolar if its shape is symmetrical, canceling out any charges. Imagine pulling a rope attached to a central post at equal distances in opposite directions; the forces balance out. This balance is similar to how symmetrical molecules can nullify polarity. Key considerations:

• A symmetrical shape (e.g., tetrahedral, linear, or square planar), where identical atoms are equally spaced around the central atom, can lead to nonpolarity, even with polar bonds.
• Asymmetrical molecules cannot cancel out dipole moments, often resulting in overall molecular polarity.
• Assessing molecular shape allows prediction of whether molecular symmetry will neutralize or emphasize polarity.

Dipole Moments

Dipole moments indicate the separation of charge in a molecule, a crucial factor in understanding molecular polarity. A dipole moment (\(\mu\)) is a vector quantity, having both magnitude and direction.Molecules with polar bonds may have dipole moments, but the overall shape can determine if these dipoles cancel out or add up.Important points to understand:

• \(\mu\) arises from differences in electronegativity, where electrons are pulled towards the more electronegative atom.
• For a molecule with several polar bonds, check if the dipoles add up into a net dipole moment or if they cancel out due to molecular symmetry.
• Nonpolar molecules have zero net dipole moments, as individual dipole moments cancel each other out due to symmetry, while polar molecules have net non-zero dipole moments.