Problem 37 - Chemistry The Molecular Science

Question

For each pair of bonds, indicate the more polar bond and use \(\delta+\) or \(\delta-\) to show the partial charge on each atom. (a) \(\mathrm{C}-\mathrm{O}\) and \(\mathrm{C}-\mathrm{N}\) (b) \(\mathrm{B}-\mathrm{O}\) and \(\mathrm{P}-\mathrm{S}\) (c) \(\mathrm{P}-\mathrm{H}\) and \(\mathrm{P}-\mathrm{N}\) (d) \(\mathrm{B}-\mathrm{H}\) and \(\mathrm{B}-\mathrm{Cl}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{C}-\mathrm{O}\); (b) \(\mathrm{B}-\mathrm{O}\); (c) \(\mathrm{P}-\mathrm{N}\); (d) \(\mathrm{B}-\mathrm{Cl}\).

Step 1: Compare Electronegativity for Pair (a)

The electronegativity of oxygen (O) is greater than that of nitrogen (N). Therefore, in the bonds \( \mathrm{C}-\mathrm{O} \) and \( \mathrm{C}-\mathrm{N} \), the \( \mathrm{C}-\mathrm{O} \) bond is more polar. In the \( \mathrm{C}-\mathrm{O} \) bond, oxygen (O) is more electronegative, so it has a partial negative charge \( \delta- \) and carbon (C) has a partial positive charge \( \delta+ \).

Step 2: Compare Electronegativity for Pair (b)

Oxygen (O) is more electronegative than both boron (B) and phosphorus (P). Therefore, in the bonds \( \mathrm{B}-\mathrm{O} \) and \( \mathrm{P}-\mathrm{S} \), the \( \mathrm{B}-\mathrm{O} \) bond is more polar. Here, oxygen holds the partial negative charge \( \delta- \) and boron holds the partial positive charge \( \delta+ \).

Step 3: Compare Electronegativity for Pair (c)

Nitrogen (N) is more electronegative than hydrogen (H), which makes the \( \mathrm{P}-\mathrm{N} \) bond more polar compared to the \( \mathrm{P}-\mathrm{H} \) bond. In the \( \mathrm{P}-\mathrm{N} \) bond, nitrogen takes the partial negative charge \( \delta- \), and phosphorous takes the partial positive charge \( \delta+ \).

Step 4: Compare Electronegativity for Pair (d)

Chlorine (Cl) is more electronegative than hydrogen (H). Consequently, in the \( \mathrm{B}-\mathrm{H} \) and \( \mathrm{B}-\mathrm{Cl} \) bonds, the \( \mathrm{B}-\mathrm{Cl} \) bond is more polar. Chlorine will have the partial negative charge \( \delta- \), while boron will have the partial positive charge \( \delta+ \).

Key Concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry. It measures an atom's ability to attract and hold electrons. Think of it as a popularity contest for electrons among atoms! The higher the electronegativity, the more an atom wants to grab those electrons.
For instance, in the molecule C-O, oxygen has a higher electronegativity compared to carbon. This causes a partial charge distribution where oxygen attracts the electrons more strongly, leaving it partially negative (\( \delta- \)) and carbon partially positive (\( \delta+ \)).
Understanding how electronegativity influences chemical bonds helps predict the behavior of molecules in different chemical reactions.
Here are a few key points about electronegativity:

• The periodic table trends: Electronegativity increases across a period from left to right and decreases down a group.
• Fluorine (F) is the most electronegative element.
• This concept is crucial for determining bond polarity.

Partial Charges

Partial charges occur in a molecule when atoms with different electronegativities form a bond. In such polar bonds, the more electronegative atom pulls the electrons closer. This results in a partial negative charge (\( \delta- \)), while the less electronegative atom becomes partially positive (\( \delta+ \)).
This charge difference is not full, like the charge in ions, but it significantly impacts the molecule's characteristics. For example, in the B-O bond, oxygen's high electronegativity leaves boron with a partial positive charge.
Recognizing these partial charges in bonds helps us understand molecule polarity, which in turn affects properties like solubility and reactivity.
Here are some highlights about partial charges:

• They arise from differences in electronegativity between bonded atoms.
• These charges are represented as \( \delta+ \) and \( \delta- \) to indicate the direction of electron pull.
• Partial charges can determine the interaction strength between molecules.

Chemical Bonding

Chemical bonding is the process by which atoms combine to form molecules. It's the glue holding atoms together. There are different types of bonds, including ionic, covalent, and metallic bonds.
Polar covalent bonds arise when there's unequal sharing of electrons due to a difference in electronegativity between the atoms. This results in partial charges. For instance, in a P-N bond, nitrogen's higher electronegativity makes it slightly negative.
Understanding bonding helps us predict and explain molecular structure and behavior. Key points about chemical bonding include:

• Ionic bonds result from the transfer of electrons, leading to full positive and negative charges.
• Covalent bonds involve sharing electrons between atoms.
• Polar covalent bonds feature partial charges and create dipoles within molecules.

Polar Bonds

Polar bonds occur when there's unequal sharing of bonding electrons. Due to the difference in electronegativity, one atom pulls electrons more closely toward itself. These polar bonds are a crucial aspect of molecular chemistry and affect molecule's properties.
In the B-Cl bond, chlorine is more electronegative, making it partially negative, while boron becomes partially positive. Polar bonds are responsible for creating molecules with dipoles, which means one end of the molecule is positively charged, and the other is negatively charged.
Here are some foundational points on polar bonds:

• Not all bonds are polar; some are nonpolar with equal electron sharing.
• Polar bonds lead to polar molecules, which have distinct physical and chemical properties.
• They play a significant role in the solubility of compounds in water and influence intermolecular forces.