Problem 41 - Chemistry The Central Science

Question

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

Step-by-Step Solution

Answer

The polar bonds are (a) B-F and (c) Se-O. In the B-F bond, F is the more electronegative atom; in the Se-O bond, O is the more electronegative atom. The Cl-Cl and H-I bonds are non-polar.

Step 1: Identify the electronegativity values

Before we compare the electronegative differences, we must know the electronegativity values for each atom in the bonds. Here are the electronegativity values for B, F, Cl, Se, O, H, and I: - B (Boron): 2.04 - F (Fluorine): 3.98 - Cl (Chlorine): 3.16 - Se (Selenium): 2.55 - O (Oxygen): 3.44 - H (Hydrogen): 2.20 - I (Iodine): 2.66 With these electronegativity values, we can now compare them for each bond.

Step 2: Determine the polarity of the B-F bond

The electronegativity difference between B and F is as follows: \(3.98 - 2.04 = 1.94\) Since the electronegativity difference is greater than 0.5, the bond is considered polar. In this polar bond, F is more electronegative than B.

Step 3: Determine the polarity of the Cl-Cl bond

Since the Cl-Cl bond is made of two identical atoms, their electronegativity difference will be zero: \(3.16 - 3.16 = 0\) With an electronegativity difference of 0, the bond is considered non-polar.

Step 4: Determine the polarity of the Se-O bond

We will now check the electronegativity difference between Se and O: \(3.44 - 2.55 = 0.89\) With an electronegativity difference greater than 0.5, the bond is considered polar. In this case, the more electronegative atom is O.

Step 5: Determine the polarity of the H-I bond

Lastly, we will calculate the electronegativity difference between H and I: \(2.66 - 2.20 = 0.46\) The electronegativity difference is less than 0.5, so the bond is considered non-polar.

Step 6: Conclusion

Among the given bonds, (a) B-F and (c) Se-O are polar. In the B-F bond, F is the more electronegative atom, whereas in the Se-O bond, O is the more electronegative atom. The Cl-Cl and H-I bonds are non-polar.

Key Concepts

Electronegativity

Understanding the concept of electronegativity is fundamental when examining the nature of chemical bonds between atoms. Electronegativity is a measure of an atom's ability to attract and hold onto electrons when it is part of a compound. The higher the electronegativity value, the greater the atom's power to attract electrons towards itself.

Atoms are not created equal in this respect; some, like Fluorine (F) with an electronegativity of 3.98, are very greedy for electrons, while others, such as Boron (B) with an electronegativity of 2.04, have a much weaker pull. The difference in electronegativity values between two bonded atoms can determine the type of bond they'll form:

• If the difference is less than 0.5, the bond is generally considered non-polar covalent.
• If it is between 0.5 and 1.7, we're looking at a polar covalent bond.
• A difference greater than 1.7 usually means the bond is ionic.

For example, in our exercise, B-F has a large electronegativity difference (1.94), indicating a strong polarity and thus, a polar bond with Fluorine being the more electronegative atom.

Chemical Bonding

Chemical bonds are the glue that holds atoms together in molecules. The type of bond formed between atoms depends on their electronegativity differences, as we've seen in our exercise. There are three primary types of chemical bonds:

• Covalent bonds where atoms share electron pairs.
• Ionic bonds where one atom donates electrons to another, creating oppositely charged ions.
• Metallic bonds which occur between metal atoms sharing a 'sea' of electrons.

In a covalent bond, if the electronegativities are identical or very similar as in the Cl-Cl bond (electronegativities both 3.16), the electrons are shared equally, leading to a non-polar covalent bond. However, if there's a significant difference, like in the Se-O bond (0.89 difference), the sharing is uneven, and the bond is polar covalent, with the more electronegative atom pulling the electrons closer to itself.

Molecular Polarity

The polarity of a molecule arises from the sum of all the individual bond polarities and the molecule's shape. When a molecule has polar bonds, like B-F and Se-O as seen in our exercise, we can't automatically assume the whole molecule is polar. Instead, we must also consider the molecule's three-dimensional structure.

If the polar bonds are symmetrically arranged, their polarities can cancel out, resulting in a non-polar molecule. However, if there is an asymmetry in the arrangement, the molecule will be polar because the bond dipoles do not cancel out.

Molecular Dipole Moment

Additionally, the molecular polarity can be quantified by a molecular dipole moment, a mathematical measurement of the overall polarity of a molecule. In the case of Se-O, if the molecule were asymmetric, Oxygen, being more electronegative, would create a dipole where the negative charge is concentrated towards the oxygen atom, with a partial positive charge on the Selenium atom, resulting in a polar molecule overall.