Question

Are the bonds in each of the following substances ionic, nonpolar covalent, or polar covalent? Arrange the substances with polar covalent bonds in order of increasing bond polarity: (a) KCl (b) \(\mathrm{P}_{4}\) (c) BF \(_{3}\) (d) \(\mathrm{SO}_{2}\) (e) \(\mathrm{Br}_{2}\) (f) \(\mathrm{NO}_{2}\)

Short answer

(a) ionic, (b) nonpolar covalent, (c) polar covalent, (d) polar covalent, (e) nonpolar covalent, (f) polar covalent. Order of increasing bond polarity: \( \text{NO}_2 < \text{SO}_2 < \text{BF}_3 \).

Step-by-step solution

- Determine the bond types

To identify whether the bonds are ionic, nonpolar covalent, or polar covalent, compare the electronegativity differences between the atoms in each molecule.(a) KCl: The electronegativity difference between K (potassium) and Cl (chlorine) is large (> 2.0), indicating an ionic bond.(b) \(\text{P}_4\): Bonds between identical atoms (P-P) have no electronegativity difference, indicating nonpolar covalent bonds.(c) \(\text{BF}_3\): The electronegativity difference between B (boron) and F (fluorine) is moderate (between 0.5 and 2.0), indicating polar covalent bonds.(d) \(\text{SO}_2\): The electronegativity difference between S (sulfur) and O (oxygen) is moderate, indicating polar covalent bonds.(e) \(\text{Br}_2\): Bonds between identical atoms (Br-Br) have no electronegativity difference, indicating nonpolar covalent bonds.(f) \(\text{NO}_2\): The electronegativity difference between N (nitrogen) and O (oxygen) is moderate, indicating polar covalent bonds.

- Arrange polar covalent bonds in order of increasing bond polarity

Bond polarity is related to the difference in electronegativity between atoms. Larger differences indicate more polarity:(c) \(\text{BF}_3\): Electronegativity difference is between B (2.04) and F (3.98) = 1.94 (d) \(\text{SO}_2\): Electronegativity difference is between S (2.58) and O (3.44) = 0.96(f) \(\text{NO}_2\): Electronegativity difference is between N (3.04) and O (3.44) = 0.40Therefore, the order from least polar to most polar is: \( \text{NO}_2 < \text{SO}_2 < \text{BF}_3 \)

Key concepts

ionic bonds

Ionic bonds form between atoms when there is a large electronegativity difference. This typically occurs between metals and nonmetals. The metal atom loses electrons to become a cation, while the nonmetal gains electrons to become an anion. The resulting electrostatic attraction between the positively charged cation and the negatively charged anion creates the ionic bond.
Examples of ionic bonds include sodium chloride (NaCl) and potassium chloride (KCl). In the case of KCl, potassium (K) has a very low electronegativity and easily loses an electron, while chlorine (Cl) has a high electronegativity and readily gains an electron. The difference in their electronegativities is greater than 2.0, leading to the formation of an ionic bond.

nonpolar covalent bonds

Nonpolar covalent bonds occur when two atoms share electrons equally. This typically happens between atoms with similar or identical electronegativities. In these bonds, there is no significant charge separation, and the electron density is evenly distributed between the atoms.
An excellent example is the molecule \(\text{P}_4\), where phosphorus atoms bond with each other. Since all the atoms have the same electronegativity, the electrons are shared equally, resulting in nonpolar covalent bonds.
Another example is diatomic molecules like \(\text{Br}_2\), where the bond between the two bromine atoms is nonpolar, as both atoms have the same electronegativity.

polar covalent bonds

Polar covalent bonds form between atoms when there is a moderate difference in their electronegativities. This causes an unequal sharing of electrons, resulting in a slight charge separation within the molecule.
Consider \(\text{BF}_3\), \(\text{SO}_2\), and \(\text{NO}_2\) as examples. In \(\text{BF}_3\), boron (B) and fluorine (F) have an electronegativity difference of 1.94, leading to polar covalent bonds. Similarly, in \(\text{SO}_2\), sulfur (S) and oxygen (O) have a difference of 0.96, creating polar covalent bonds. Lastly, in \(\text{NO}_2\), nitrogen (N) and oxygen (O) differ by 0.40 in electronegativity, also resulting in polar covalent bonds.
These differences in electronegativity cause the electrons to be more attracted to the more electronegative atom, creating partial positive and negative charges (\(\text{δ+}\) and \(\text{δ-}\)) within the molecule.

electronegativity difference

Electronegativity is a measure of an atom's ability to attract electrons toward itself in a chemical bond. The difference in electronegativity between two bonded atoms determines the type of bond they form:

• If the difference is greater than 2.0, the bond is typically ionic.
• If the difference is between 0.5 and 2.0, the bond is generally polar covalent.
• If the difference is less than 0.5, the bond is usually nonpolar covalent.

Understanding these differences helps predict the properties of the molecules. For instance, polar molecules tend to have higher boiling points and solubility in water compared to nonpolar molecules due to the dipole-dipole interactions. When arranging substances with polar covalent bonds in order of increasing bond polarity, we consider their electronegativity differences. Hence, from least to most polar: \(\text{NO}_2 < \text{SO}_2 < \text{BF}_3\).