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 is nonpolar. b) NH3 is polar. c) SF4 is polar. d) XeF4 is nonpolar. e) CH3Br is polar. f) GaH3 is nonpolar.

Step-by-step solution

a) CCl4

1. Determine the shape: CCl4 molecule has 4 Cl atoms bonded to a central carbon atom, without any lone pairs. Therefore, the molecular shape is tetrahedral. 2. Assess the symmetry: All peripheral atoms are the same (chlorine), and there are no lone pairs on the central atom. Consequently, the molecule is symmetric, which means it's nonpolar.

b) NH3

1. Determine the shape: NH3 molecule has 3 hydrogen atoms bonded to a central nitrogen atom and has 1 lone pair. This glyph's molecular shape is a trigonal pyramid. 2. Assess the symmetry: The molecule has a lone pair and is thus asymmetrical, making it polar.

c) SF4

1. Determine the shape: SF4 molecule has 4 fluorine atoms bonded to a central sulfur atom and has 1 lone pair. The molecular shape is a seesaw. 2. Assess the symmetry: The molecule is not symmetrical, as it has one lone pair on the central atom. Therefore, the molecule is polar.

d) XeF4

1. Determine the shape: XeF4 molecule has 4 fluorine atoms bonded to a central xenon atom and 2 lone pairs. The molecular shape is square planar. 2. Assess the symmetry: All peripheral atoms are the same (fluorine), and the symmetry remains intact when considering lone pairs. Therefore, the molecule is symmetrical, making it nonpolar.

e) CH3Br

1. Determine the shape: CH3Br molecule has 3 hydrogen atoms and 1 bromine atom bonded to a central carbon atom. The molecular shape is tetrahedral. 2. Assess the symmetry: As the carbon atom is bonded to three hydrogen atoms and one bromine atom, the molecules are not symmetrical. Consequently, the molecule is polar.

f) GaH3

1. Determine the shape: GaH3 molecule has 3 hydrogen atoms bonded to a central gallium atom. The molecular shape is trigonal planar. 2. Assess the symmetry: All peripheral atoms are the same (hydrogen), so the molecule is symmetrical, making it nonpolar.

Key concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It significantly influences a molecule's physical and chemical properties.
When determining molecular geometry, it's essential to consider both the types of atoms involved and the presence of any electron lone pairs.
To begin with, each molecule's shape stems from how many atoms are attached to the central atom and whether there are any lone pairs. Here are some common geometries:

• Tetrahedral: Four atoms bonded to a central atom, as seen in \( ext{CCl}_4\), resulting in a symmetrical shape.
• Trigonal Pyramidal: Three atoms and one lone pair attached to the central atom, giving the molecule like \( ext{NH}_3\) an asymmetrical shape.
• Seesaw: Found in molecules with four attached atoms and one lone pair, like \( ext{SF}_4\).
• Square Planar: Involves four atoms and two lone pairs on the central atom, such as in \( ext{XeF}_4\).
• Trigonal Planar: Seen in molecules like \( ext{GaH}_3\) with three atoms and no lone pairs bonded to the central atom.

Understanding these geometries is crucial for predicting molecular polarity and behavior.

Symmetry in Molecules

Symmetry in molecules plays a pivotal role in determining whether a molecule is polar or nonpolar. Symmetrical molecules tend to distribute their charge evenly, often resulting in nonpolar molecules.
A symmetric molecule is one where any changes or rotations about its center result in an identical structure.
Key aspects of molecular symmetry include:

• Identical Peripheral Atoms: Molecules like \( ext{CCl}_4\) and \( ext{XeF}_4\), which have identical atoms arranged symmetrically around the central atom, typically are nonpolar.
• Lone Pairs on Central Atom: Lone pairs can disrupt symmetry. For instance, the lone pair in \( ext{NH}_3\) causes asymmetry, contributing to its polar nature.
• Even Charge Distribution: If the dipoles within a molecule cancel out due to symmetry, the molecule is nonpolar.
• Effect of Different Atoms: Molecules like \( ext{CH}_3 ext{Br}\) become asymmetrical due to the presence of different atoms, making them likely to be polar.

Symmetry helps determine the overall moment and hence the polarity of a molecule.

Polarity Prediction

Predicting the polarity of a molecule hinges on both its geometry and symmetry. A molecule is polar if it has distinct positive and negative ends due to unequal sharing of electrons
or asymmetrical arrangement.
Steps to predict molecule polarity include:

• Check Molecular Geometry: Identify the geometry to understand how atoms are spatially arranged.
• Examine Symmetry: Determine if the molecule is symmetrical or not by checking identical bonding or lone pairs.
• Electron Distribution: Polar molecules, like \( ext{NH}_3\) and \( ext{CH}_3 ext{Br}\), have uneven electron distributions leading to partial charges.
• Identify Lone Pairs: Lone pairs can shift the electron cloud, creating a dipole moment as in \( ext{SF}_4\).

Predicting polarity helps to forecast a molecule’s interaction with other substances and its solubility attributes.

Lewis Structures

Lewis structures serve as a critical tool for visualizing the electron arrangement in molecules. They portray how atoms are bonded together and where lone pairs reside.
Constructing accurate Lewis structures lays the groundwork for understanding molecular geometry and polarity.
Here’s how to construct a Lewis structure:

• Determine Total Valence Electrons: Add up all valence electrons from each atom in the molecule.
• Draw a Skeleton Structure: Connect atoms using single bonds, arranging them around the central atom.
• Assign Electrons: Use the remaining electrons to complete octets around each atom. Each bond counts as two electrons.
• Consider Formal Charges: Ensure that the arrangement gives the least formal charges and is overall neutral.
• Account for Lone Pairs: Place any remaining electrons as lone pairs, influencing the molecule's shape and polarity.

Lewis structures provide a fundamental perspective on how molecules interact and their subsequent properties.