Question

(a) How does a polar molecule differ from a nonpolar one? (b) Atoms \(\mathrm{X}\) and \(\mathrm{Y}\) have different electronegativities. Will the diatomic molecule \(\mathrm{X}-\mathrm{Y}\) necessarily be polar? Explain. (c) What factors affect the size of the dipole moment of a diatomic molecule?

Short answer

(a) A polar molecule has an uneven distribution of electrons due to the presence of polar covalent bonds, creating regions of positive and negative charges. In contrast, nonpolar molecules have an even distribution of electrons, and no net positive or negative charge. (b) A diatomic molecule X-Y with different electronegativities will be polar, as the uneven sharing of electrons creates a separation of charge and a polar covalent bond. (c) The dipole moment in a diatomic molecule depends on the electronegativity difference between the atoms and the bond length. A larger difference in electronegativity and a longer bond length generally result in a greater dipole moment.

Step-by-step solution

(a) Polar vs Nonpolar Molecules

A polar molecule is a molecule that has an uneven distribution of electrons within the molecule, resulting in partial charges on its atoms. In other words, the positively charged nuclei of the atoms do not fully cancel out the negatively charged electrons. This is due to the presence of polar covalent bonds, in which atoms with different electronegativities share electrons unevenly and create regions of positive and negative charges within the molecule. On the other hand, nonpolar molecules have an even distribution of electrons, and the atoms in the molecule have a similar electronegativity. This results in a symmetrical arrangement of charge within the molecule, and there are no areas of the molecule with a net positive or negative charge.

(b) Polarity of Diatomic molecule X-Y

If atoms X and Y have different electronegativities, then the electrons will not be equally shared within the molecule, since one atom attracts the electrons more strongly than the other. This uneven sharing of electrons creates a separation of charge, which results in a polar covalent bond. Consequently, a diatomic molecule X-Y with different electronegativities will be polar. In general, the greater the difference in electronegativity between the two atoms in a bond, the more polar the bond will be.

(c) Factors affecting the dipole moment of a diatomic molecule

The dipole moment of a diatomic molecule is a measure of the molecule's polarity, defined as the product of the magnitude of the charge separation and the distance between the charges. The factors affecting the dipole moment of a diatomic molecule include: 1. Electronegativity difference: A larger difference in electronegativity between the two atoms leads to a stronger separation of charges, and therefore a greater dipole moment. 2. Bond length: The distance between the two atoms in the bond also affects the dipole moment. A longer bond length generally results in a larger dipole moment since the charge separation is larger. 3. Molecular geometry: Although not applicable to a diatomic molecule, the molecular geometry for more complex molecules may affect the dipole moment. If there are multiple polar bonds in a molecule, the dipole moment of the molecule will depend on the angles between these bonds. In summary, the main factors affecting the dipole moment in a diatomic molecule are the electronegativity difference between the atoms and the bond length.

Key concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry that explains how strongly an atom attracts electrons towards itself when it forms a chemical bond. Different elements have different electronegativities, which can significantly impact the properties of a molecule. Some key points about electronegativity include:

• Electronegativity is often measured on a scale developed by Linus Pauling, where fluorine is the most electronegative element, with a value of 3.98, and cesium is one of the least, with about 0.7.
• When two atoms in a molecule have a significant difference in electronegativity, the more electronegative atom will attract the shared electrons more strongly, leading to an unequal distribution of electrons.
• This unequal sharing leads to the development of partial charges within the molecule, causing one end to become slightly positive (where the less electronegative atom is) and the other slightly negative (where the more electronegative atom is).
• If there is no or little difference in electronegativity, the result is a nonpolar covalent bond where electrons are shared equally.

Understanding electronegativity is crucial because it helps predict whether a molecule will be polar or nonpolar, influencing its chemical behavior and interactions with other molecules.

Dipole Moment

The dipole moment is a measure that quantifies the extent of polarity in a molecule. It results from the separation of positive and negative charges within the molecule. Some important points about dipole moments include:

• The dipole moment (\( \mu \)) is calculated using the formula \( \mu = q \times d \), where \( q \) is the magnitude of the charges, and \( d \) is the distance between them.
• A larger dipole moment indicates a greater difference in charge and a longer distance between these charges, meaning the molecule is more polar.
• For diatomic molecules, the dipole moment is influenced primarily by the difference in electronegativity and the bond length between the two atoms.
• Polar molecules, having high dipole moments, tend to have stronger intermolecular forces like hydrogen bonding or dipole-dipole attractions, affecting the molecule's physical properties such as boiling and melting points.

In summary, the dipole moment serves as an indicator of the polarity of a molecule, which directly impacts the molecule's physical and chemical behaviors.

Polar and Nonpolar Molecules

Polar and nonpolar molecules differ mainly in how their electrons are distributed, which influences their interactions with each other and with other substances.

• **Polar Molecules:** These have an uneven distribution of electron density, leading to regions of partial positive and negative charges. This unevenness is often due to differences in electronegativity among the atoms in the molecule.
• The presence of a permanent dipole moment characterizes polar molecules, meaning they have one part that is more positively charged and another that is more negatively charged.
• Water (\( \text{H}_2\text{O} \)) is a classic example of a polar molecule, with its bent shape and significant electronegativity difference between oxygen and hydrogen.
• **Nonpolar Molecules:** These typically have a symmetrical distribution of electrons, resulting in no overall dipole moment. The charges are balanced, and there is an even distribution across the molecule.
• Molecules like methane (\( \text{CH}_4 \)) or diatomic nitrogen (\( \text{N}_2 \)) are nonpolar because of their symmetrical shape and similar electronegativity values.
• Understanding the polarity of molecules is crucial for predicting how substances will behave in chemical reactions and in different environments, such as solubility in water or oil.

Recognizing the differences between polar and nonpolar molecules helps in understanding a wide range of chemical and physical properties of substances.