Question

Which should have the largest molecular dipole moment: \(\mathrm{H}_{2}, \mathrm{CO}_{2}, \mathrm{CH}_{3} \mathrm{~F}\), or \(\mathrm{CH}_{7} \mathrm{I}\) ?

Short answer

Among the given molecules H2, CO2, CH3F, and CH3I, CH3F should have the largest molecular dipole moment due to the largest difference in electronegativity between C and F atoms and the noncanceling dipoles in its tetrahedral geometry.

Step-by-step solution

1. Analyze and Draw the Geometry of H2

Hydrogen molecule (H2) is a diatomic molecule consisting of two H atoms bonded together. In this homonuclear diatomic molecule, both atoms have the same electronegativity, so there is no dipole moment.

2. Analyze and Draw the Geometry of CO2

Carbon dioxide molecule (CO2) is a linear molecule with a central carbon atom bonded with two oxygen atoms. The difference in electronegativity between C and O atoms produces two polar bonds and dipoles, but in a linear and symmetrical configuration, the bond dipoles cancel each other, leading to an overall dipole moment equal to zero.

3. Analyze and Draw the Geometry of CH3F

Methyl fluoride (CH3F) is a tetrahedral molecule with a central carbon atom bonded to three H atoms and one F atom. In this case, the difference in electronegativity between C and F produces a polar bond, and due to the tetrahedral geometry of the molecule, the dipoles do not cancel each other out. Consequently, CH3F has a molecular dipole moment.

4. Analyze and Draw the Geometry of CH3I

Methyl iodide (CH3I) is also a tetrahedral molecule with a central carbon atom bonded to three H atoms and one I atom. Here, the difference in electronegativity between C and I generates a polar bond. Since the molecular geometry is also tetrahedral, the bond dipoles do not cancel out, and CH3I has a molecular dipole moment.

5. Compare Molecular Dipole Moments

To determine which molecule has the largest molecular dipole moment, we should compare the dipole moments of the two tetrahedral molecules: CH3F and CH3I. The difference in electronegativity between C and F is larger than the difference between C and I, which implies that the molecular dipole moment in CH3F is larger than that in CH3I.

6. Conclusion

Among the given molecules H2, CO2, CH3F, and CH3I, CH3F should have the largest molecular dipole moment.

Key concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. This concept is crucial because the geometry determines how dipoles within the molecule interact and whether they cancel each other out. For example, in a linear molecule like carbon dioxide (CO2), which has a central carbon atom bonded symmetrically to two oxygen atoms, the dipoles created by the difference in electronegativity between carbon and oxygen cancel each other out. As a result, CO2 has no net dipole moment. On the other hand, in tetrahedral molecules like methyl fluoride (CH3F) and methyl iodide (CH3I), the geometry is such that the dipoles do not cancel. This results in a net molecular dipole moment, which is a vector sum of the individual bond dipoles.

Electronegativity

Electronegativity is the tendency of an atom to attract electrons towards itself when it forms a compound. This property is fundamental in determining the nature of bonds between atoms. In our context, electronegativity differences between atoms like carbon and fluorine or carbon and iodine lead to polar bonds. For instance, fluorine is much more electronegative than carbon, resulting in a highly polar bond in CH3F. This, combined with the molecular geometry, results in a significant net dipole moment. In contrast, the small electronegativity difference between hydrogen atoms in H2 means no polar bond forms, and thus no dipole moment arises in the molecule.

Polar Bonds

Polar bonds occur when two atoms in a molecule have a significant difference in electronegativity. This causes an uneven distribution of electrons and creates a dipole moment. In cases like CH3F and CH3I, the more electronegative atoms (F and I, respectively) attract shared electrons more strongly than carbon. This creates a partial positive charge on carbon and partial negative charge on fluorine or iodine. The strength and direction of these polar bonds' dipoles are determined by the electronegativity difference and the molecular geometry, both of which play crucial roles in determining the overall molecular dipole moment.

Tetrahedral Molecules

Tetrahedral molecules have a geometry where a central atom is bonded to four other atoms, positioned at the corners of a tetrahedron. This geometry, which can be seen in molecules like CH3F and CH3I, plays a critical role in determining the molecular dipole moment. In tetrahedral structures, bond angles of approximately 109.5° can lead to a situation where polar bonds do not cancel out, unlike linear or symmetrical geometries. In CH3F, the electronegativity difference between carbon and the more electronegative fluorine creates a strong dipole, which, combined with the geometry, results in a large net dipole moment. Meanwhile, CH3I, though similar in structure, has a smaller net dipole moment due to the lesser electronegativity difference between carbon and iodine compared to carbon and fluorine.