Question

Arrange the bonds in each of the following sets in order of increasing polarity: (a) \(\mathrm{C}-\mathrm{F}, \mathrm{O}-\mathrm{F}, \mathrm{Be}-\mathrm{F}\) (b) \(\mathrm{O}-\mathrm{Cl}, \mathrm{S}-\mathrm{Br}, \mathrm{C}-\mathrm{P}\) (c) \(\mathrm{C}-\mathrm{S}, \mathrm{B}-\mathrm{F}, \mathrm{N}-\mathrm{O}\)

Short answer

The sets of bonds in order of increasing polarity are: (a) O-F, C-F, Be-F (b) O-Cl, C-P, S-Br (c) C-S, N-O, B-F

Step-by-step solution

Find electronegativity values

Refer to a periodic table or electronegativity table to find the electronegativity values of the elements in the given bonds. Element | Electronegativity ---------|------------------- C | 2.55 F | 3.98 O | 3.44 Be | 1.57 Cl | 3.16 S | 2.58 Br | 2.96 P | 2.19 B | 2.04 N | 3.04

Calculate electronegativity differences

Subtract the smaller electronegativity value from the larger one for each bond to obtain the electronegativity difference. (a) C-F: \(3.98 - 2.55 = 1.43\) O-F: \(3.98 - 3.44 = 0.54\) Be-F: \(3.98 - 1.57 = 2.41\) (b) O-Cl: \(3.44 - 3.16 = 0.28\) S-Br: \(2.96 - 2.58 = 0.38\) C-P: \(2.55 - 2.19 = 0.36\) (c) C-S: \(2.58 - 2.55 = 0.03\) B-F: \(3.98 - 2.04 = 1.94\) N-O: \(3.44 - 3.04 = 0.40\)

Arrange bonds in order of increasing polarity

Rank the bonds within each set based on their electronegativity differences. (a) O-F: 0.54 C-F: 1.43 Be-F: 2.41 (b) O-Cl: 0.28 C-P: 0.36 S-Br: 0.38 (c) C-S: 0.03 N-O: 0.40 B-F: 1.94 The sets of bonds in order of increasing polarity are: (a) O-F, C-F, Be-F (b) O-Cl, C-P, S-Br (c) C-S, N-O, B-F

Key concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry that describes an atom's tendency to attract and hold onto electrons when forming a chemical bond. It's a numerical value that varies across different elements and plays a vital role in determining the type of bond—ionic, covalent, or polar covalent—that can form between atoms.

The most commonly used scale for electronegativity is the Pauling scale, devised by chemist Linus Pauling. On this scale, fluorine (F) is assigned the highest value of 3.98, reflecting its strong pull on electrons, while elements such as francium (Fr) have one of the lowest due to their weak attraction for electrons.

The concept of electronegativity can be used to predict bond polarity. When two atoms form a bond, the difference in their electronegativities will determine the distribution of electrons. If the difference is large, the bond is considered ionic with one atom donating and the other accepting electrons. If the difference is small, the bond is nonpolar covalent, meaning the electrons are shared equally. Between these extremes lies polar covalent bonds, where electrons are shared unequally, leading to partial charges on the atoms.

Bond Polarity Ranking

Bond polarity ranking involves comparing the polarity of different chemical bonds within a set. To accurately rank the polarity of bonds, one must first calculate the electronegativity difference between the bonded atoms. The greater the difference, the more polar the bond is.

When assessing polarity, it's crucial to recognize that a completely nonpolar bond would have an electronegativity difference of zero, as is nearly the case with identical atoms (like O₂ or N₂). For partially polar bonds, the differences in electronegativity will typically range from about 0.1 to 1.7. Beyond this range, if the electronegativity difference exceeds about 1.7, the bond is likely to be ionic rather than covalent.

In the exercise example given, we arrange the bonds from least to most polar based on their calculated electronegativity differences. However, it's important to understand that this difference only provides initial guidance. Other factors, such as bond length and molecular geometry, can also influence the actual bond polarity in a complex molecule.

Periodic Table

The periodic table is not just a tabular display of chemical elements; it's a powerful tool that reflects the systematic trends in properties like electronegativity. These trends can be observed in different directions of the table.

Electronegativity increases across a period from left to right due to the increased nuclear charge, which pulls electrons closer to the nucleus, and decreases down a group as atomic size increases, making the outer electrons farther from the nucleus and less tightly held.

This pattern is why elements on the right side of the periodic table, except for the noble gases, generally have higher electronegativities than those on the left. Thus, when looking at the periodic table to assess bond polarity, we can often make predictions about bond polarity just based on element positions. For instance, bonds between elements far apart in the periodic table (such as metals on the left and non-metals on the right) are likely to be more polar due to greater differences in electronegativity.