Question

Give one example of a compound having a linear molecular structure that has an overall dipole moment (is polar) and one example that does not have an overall dipole moment (is nonpolar). Do the same for molecules that have trigonal planar and tetrahedral molecular structures.

Short answer

Linear Polar: Hydrogen chloride (HCl) is an example of a linear polar compound due to the difference in electronegativities (H: 2.1, Cl: 3.0) forming an overall dipole. Linear Nonpolar: Carbon dioxide (CO2) is an example of a linear nonpolar compound, as the equal electronegativities of the oxygen atoms cause the dipoles to cancel each other out. Trigonal Planar Polar: Sulfur trioxide (SO3) is an example of a trigonal planar polar compound, with the difference in electronegativities between sulfur and oxygen creating an overall dipole moment. Trigonal Planar Nonpolar: Boron trifluoride (BF3) is an example of a trigonal planar nonpolar compound, as the dipoles cancel each other out due to the symmetry of the trigonal planar structure. Tetrahedral Polar: Ammonia (NH3) is an example of a tetrahedral polar compound, with the electronegativity difference between nitrogen and hydrogen resulting in an overall dipole moment. Tetrahedral Nonpolar: Methane (CH4) is an example of a tetrahedral nonpolar compound, as the molecule contains only nonpolar C-H bonds and the electronegativities of carbon and hydrogen are equal.

Step-by-step solution

Linear Polar Compound

An example of a linear polar compound is hydrogen chloride (HCl). In this molecule, hydrogen (H) has an electronegativity of 2.1 and chlorine (Cl) has an electronegativity of 3.0. Due to the difference in electronegativities, an overall dipole is formed, making the compound polar.

Linear Nonpolar Compound

An example of a linear nonpolar compound is carbon dioxide (CO2). In this molecule, the two oxygen (O) atoms have the same electronegativity (3.5), which makes the dipoles cancel each other out, resulting in a nonpolar compound.

Trigonal Planar Polar Compound

An example of a trigonal planar polar compound is boron trifluoride (BF3). In this molecule, although the bond between boron (B) and each fluorine (F) atom is polar, due to the symmetry of the trigonal planar structure, the dipoles cancel each other, and the molecule is considered nonpolar.

Trigonal Planar Nonpolar Compound

An example of a trigonal planar nonpolar compound is sulfur trioxide (SO3). In this molecule, the electronegativity difference between sulfur (S) and oxygen (O) results in an overall dipole moment, making the compound polar.

Tetrahedral Polar Compound

An example of a tetrahedral polar compound is methane (CH4). In this molecule, the four hydrogen (H) atoms have the same electronegativity (2.1) as carbon (C), and since the compound contains only nonpolar C-H bonds, the molecule is considered nonpolar.

Tetrahedral Nonpolar Compound

An example of a tetrahedral nonpolar compound is ammonia (NH3). In this molecule, the electronegativity difference between nitrogen (N) and hydrogen (H) creates a polar molecule, as there is an overall dipole moment.

Key concepts

Linear Molecular Structure

In a linear molecular structure, the atoms are arranged in a straight line. This structure is commonly seen in diatomic molecules but also in some triatomic molecules.
A classic example of a linear molecule is carbon dioxide (CO₂). The molecule is symmetrical: two oxygen atoms are positioned on either side of a central carbon atom. Because the dipole moments of the carbon-oxygen bonds are equal and opposite, they cancel each other out, resulting in a nonpolar molecule.
In contrast, a molecule like hydrogen chloride (HCl) is polar. The hydrogen and chlorine atoms differ significantly in electronegativity, causing a dipole moment that makes the molecule polar. Understanding these variations in molecular structure helps determine if a linear molecule is polar or nonpolar depending on the symmetry and electronegativity of atoms.

Trigonal Planar Structure

The trigonal planar structure features a central atom bonded to three other atoms, all in the same plane and separated by 120-degree angles.
Boron trifluoride (BF₃) is an excellent example of a nonpolar trigonal planar molecule. Although the B-F bonds are individually polar, symmetrical arrangement around the central boron results in the cancellation of the dipole moments, making the whole molecule nonpolar.
On the other hand, sulfur trioxide (SO₃) is another example where the electronegativity difference between sulfur and oxygen generates a polar trigonal planar molecule. Despite its planar appearance, the polar nature is a critical feature due to overall dipole formation.

Tetrahedral Structure

In a tetrahedral molecular structure, a central atom is surrounded by four atoms at the corners of a tetrahedron, forming angles of about 109.5 degrees between bonds.
Methane (CH₄) exemplifies a nonpolar tetrahedral molecule. It consists of nonpolar C-H bonds, and the symmetric shape ensures that the bond dipoles cancel out, resulting in no overall dipole moment.
Ammonia (NH₃), however, is a tetrahedral molecule that is polar. The nitrogen atom’s lone pair and the hydrogen atoms form an irregular pyramid shape, contributing to a net dipole moment and making it a polar molecule. The structure illustrates how the arrangement of atoms and lone pairs affects polarity.

Dipole Moment

Dipole moment is a vector quantity that provides insight into molecular polarity, where disparity in electronegativity among bonded atoms creates a separation of charges.
Molecules with a significant dipole moment are polar. For instance, water (H₂O) has a considerable dipole moment because the difference in electronegativity between hydrogen and oxygen is significant, and the bent shape leads to a net dipole.
In contrast, molecules like carbon dioxide (CO₂) have a net dipole moment of zero because the linear molecular structure allows equal but opposite bond dipoles to cancel out, thus classifying it as nonpolar. Understanding dipole moments helps predict molecular behavior in different environments.

Electronegativity

Electronegativity is the tendency of an atom to attract electrons in a chemical bond. It is a key factor in determining if a bond is covalent, and further, if the molecule is polar or nonpolar.
Atoms with higher electronegativity, like fluorine, oxygen, and nitrogen, tend to pull electrons toward themselves, creating a negative center and often resulting in polar bonds.
Differences in electronegativity between bonded atoms lead to bond polarity, characterized by a partial charge on each atom. For instance, in hydrogen chloride (HCl), chlorine's higher electronegativity compared to hydrogen creates a bond dipole with electrons gravitating towards chlorine. Consequently, electronegativity differences are fundamental in predicting and explaining molecular polarity.