Problem 5 - Biology

Question

Which of the following is a nonpolar molecule? (a) water, \(\mathrm{H}_{2} \mathrm{O}(\mathrm{b})\) ammonia, \(\mathrm{NH}_{3}(\mathrm{c})\) methane, \(\mathrm{CH}_{4}(\mathrm{d})\) ethane, \(\mathrm{C}_{2} \mathrm{H}_{6}\) (e) more than one of the preceding.

Step-by-Step Solution

Answer

Methane and ethane are nonpolar molecules.

Step 1: Identify Molecular Polarity

Determine whether each molecule is polar or nonpolar by analyzing the symmetry of the molecule and the electronegativity differences between the atoms. A nonpolar molecule typically has a symmetrical structure and equal sharing of electrons.

Step 2: Examine Water ( \mathrm{H}_{2} \mathrm{O} )

Water is a bent molecule due to its two lone pairs on oxygen, leading to a net dipole moment. Oxygen is more electronegative than hydrogen, creating an uneven distribution of electron density. Therefore, water is a polar molecule.

Step 3: Examine Ammonia ( \mathrm{NH}_{3} )

Ammonia has a trigonal pyramidal shape due to its lone pair on nitrogen. This shape causes a net dipole moment. Nitrogen is more electronegative than hydrogen, resulting in a polar molecule.

Step 4: Examine Methane ( \mathrm{CH}_{4} )

Methane has a tetrahedral geometry and is symmetrical with bonded hydrogen atoms evenly distributed around carbon, which has no lone pairs. There are no net dipole moments as the electronegativity difference is minor. Therefore, methane is a nonpolar molecule.

Step 5: Examine Ethane ( \mathrm{C}_{2} \mathrm{H}_{6} )

Ethane is composed of two carbon atoms singly bonded to each other and with hydrogen atoms. It has a symmetrical and linear structure with no significant electronegativity differences. It is a nonpolar molecule.

Step 6: Conclusion

Both methane and ethane have symmetrical structures leading to nonpolar characteristics. Therefore, more than one of the preceding molecules are nonpolar.

Key Concepts

Molecular Symmetry

Molecular symmetry plays a critical role in determining the polarity of a molecule. When a molecule is symmetrical, its shape allows for an even distribution of charge across the entire structure. This symmetry helps cancel out any potential dipole moments, resulting in a nonpolar molecule.

For instance, consider a molecule like methane ( CH_4 ). Methane has a tetrahedral shape, with hydrogen atoms symmetrically arranged around a central carbon atom. Due to this symmetrical structure, the individual bond dipoles cancel each other out, making methane nonpolar. Similarly, ethane ( C_2H_6 ) has a symmetrical configuration, contributing to its nonpolar nature.

Conversely, a lack of symmetry, as seen in water ( H_2O ) and ammonia ( NH_3 ), results in a polar molecule because the orientation of the lone pairs creates an uneven charge distribution.

Electronegativity

Electronegativity measures an atom's ability to attract shared electrons in a chemical bond. It's a key factor in determining molecular polarity. When atoms in a molecule have significantly different electronegativities, the electrons in their bonds are not shared equally.

In water ( H_2O ), oxygen is more electronegative than hydrogen. This difference pulls the shared electrons closer to the oxygen atom, creating partial charges at each end of the bond—oxygen being slightly negative ( δ− ) and hydrogen slightly positive ( δ+ ). As a result, water molecules have a net dipole moment and are considered polar.

Ammonia ( NH_3 ) experiences similar effects due to nitrogen's higher electronegativity compared to hydrogen. In contrast, molecules like methane ( CH_4 ) and ethane ( C_2H_6 ) involve atoms with similar electronegativities, leading to equal electron sharing and nonpolar characteristics.

Nonpolar Molecules

Nonpolar molecules are characterized by their lack of net dipole moments. This typically occurs when a molecule has a symmetrical shape and the atoms involved have comparable electronegativities. As a result, electrons are evenly distributed throughout the molecule.

Methane ( CH_4 ) and ethane ( C_2H_6 ) are examples of nonpolar molecules. Both exhibit symmetrical configurations; methane with its tetrahedral shape and ethane with a linear structure formed by carbon-carbon bonding. In both molecules, the electronegativity difference between the central atom(s) and hydrogen is negligible, allowing for equal sharing of electrons.

Because of their nonpolar nature, these molecules do not have an overall positive or negative side. This uniformity affects how they interact with other molecules, typically leading to lower solubility in water and affinity for nonpolar solvents.

Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges in a molecule. It is a vector quantity with both magnitude and direction. Dipole moments arise in molecules where there is an uneven distribution of electron density, usually due to differences in electronegativity and molecular geometry.

For example, water ( H_2O ) has a significant dipole moment. The bent shape and oxygen's higher electronegativity draw electron density away from the hydrogen atoms, giving water its polar nature. In ammonia ( NH_3 ), the trigonal pyramidal shape and difference in electronegativity between nitrogen and hydrogen lead to a net dipole moment.

On the other hand, compounds like methane ( CH_4 ) and ethane ( C_2H_6 ) have symmetrical shapes and similar electronegativities among their atoms. This configuration results in the cancellation of any potential dipoles, so these molecules have negligible dipole moments and are therefore nonpolar.