Question

Which of the following bonds are polar: (a) \(\mathrm{B}-\mathrm{F},\) (b) \(\mathrm{Cl}-\mathrm{Cl}\), $$ \text { (c) } \mathrm{Se}-\mathrm{O} $$ (d) \(\mathrm{H}-\mathrm{I} ?\) Which is the more electronegative atom in each polar bond?

Short answer

The \(\mathrm{B}-\mathrm{F}\) and \(\mathrm{Se}-\mathrm{O}\) bonds are polar, with fluorine and oxygen being the more electronegative atoms, respectively. The \(\mathrm{Cl}-\mathrm{Cl}\) and \(\mathrm{H}-\mathrm{I}\) bonds are nonpolar.

Step-by-step solution

Analyze the B-F bond

First, let's analyze the electronegativity difference between boron (B) and fluorine (F). Looking at the periodic table, we can see that B has an electronegativity value of 2.04 while F has a significant higher value of 3.98. The difference between these two values is 3.98 - 2.04 = 1.94. Since this difference is significant (usually, a difference of greater than 0.5 is considered significant), the \(\mathrm{B}-\mathrm{F}\) bond is polar. The more electronegative atom in this bond is fluorine (F).

Analyze the Cl-Cl bond

For the \(\mathrm{Cl}-\mathrm{Cl}\) bond, we see that both atoms are chlorine (Cl), meaning there is no electronegativity difference between the two atoms. Therefore, the bond is nonpolar.

Analyze the Se-O bond

The electronegativity difference between selenium (Se) and oxygen (O) needs to be figured out. From the periodic table, we find that Se has an electronegativity value of 2.55 and O has a value of 3.44. The difference between these values is 3.44 - 2.55 = 0.89. Since this difference is significant (greater than 0.5), the \(\mathrm{Se}-\mathrm{O}\) bond is polar. The more electronegative atom in this bond is oxygen (O).

Analyze the H-I bond

Lastly, we need to analyze the \(\mathrm{H}-\mathrm{I}\) bond. Looking at the periodic table, we find that hydrogen (H) has an electronegativity value of 2.20 and iodine (I) has a value of 2.66. The difference between these values is 2.66 - 2.20 = 0.46. This difference is less than 0.5, so the \(\mathrm{H}-\mathrm{I}\) bond is considered nonpolar. In conclusion: - The \(\mathrm{B}-\mathrm{F}\) bond is polar, with fluorine being the more electronegative atom. - The \(\mathrm{Cl}-\mathrm{Cl}\) bond is nonpolar. - The \(\mathrm{Se}-\mathrm{O}\) bond is polar, with oxygen being the more electronegative atom. - The \(\mathrm{H}-\mathrm{I}\) bond is nonpolar.

Key concepts

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons within a chemical bond. In essence, it shows how 'hungry' an atom is for electrons. Atoms with high electronegativity values, such as fluorine (F) with an electronegativity of 3.98, are very good at pulling electrons towards themselves. In contrast, atoms with lower values, like boron (B) whose electronegativity is 2.04, don't have as much pull.

When atoms with different electronegativities form a bond, the electrons are not shared equally. This uneven sharing leads to a dipole moment, or a separation of charges, causing the bond to be polar. On the flip side, when two atoms with identical or very similar electronegativity values bond, such as in chlorine (Cl-Cl), the electrons are shared evenly, resulting in a nonpolar bond.

Chemical Bonding

Chemical bonds are the glue that holds atoms together in molecules. They arise from the attractions between the positively charged nuclei of atoms and the negatively charged electrons shared between them. The three primary types of chemical bonds are ionic, covalent, and metallic bonds.

Covalent bonds are formed when atoms share electrons. If the sharing is equal due to similar electronegativity (like Cl-Cl), the bond is nonpolar covalent. If there is an unequal sharing of electrons because of a significant difference in electronegativity (like B-F or Se-O), the bond is polar covalent. Ionic bonds, on the other hand, involve the complete transfer of electrons from one atom to another, forming ions. This often occurs between metals and nonmetals with a very high difference in electronegativity.

Molecular Polarity

Molecular polarity is about symmetry and the distribution of electrons across a molecule. If the arrangement of atoms and the distribution of electrons are symmetrical, the molecule is often nonpolar because the dipoles cancel each other out. An example is the oxygen molecule (O2), where the electronegativity is the same on both sides, leading to a balanced and symmetrical distribution.

In contrast, molecules like water (H2O) are polar because the shape of the molecule means the distribution of electrons is asymmetrical. The oxygen atom, being more electronegative, pulls the shared electrons closer, creating a partial negative charge on that end, while the hydrogen atoms carry a partial positive charge. It's these partial charges that can lead to intermolecular attractions like hydrogen bonds, which are crucial for properties like water's high surface tension.