Question

Classify these bonds as ionic, polar covalent, or \(\mathrm{co}-\) valent, and give your reasons: (a) the CC bond in \(\mathrm{H}_{3} \mathrm{CCH}_{3},\) (b) the KI bond in \(\mathrm{KI},(\mathrm{c})\) the \(\mathrm{NB}\) bond in \(\mathrm{H}_{3} \mathrm{NBCl}_{3},\) (d) the \(\mathrm{ClO}\) bond in \(\mathrm{ClO}_{2}\).

Short answer

The CC bond in \(\mathrm{H}_3 \mathrm{CCH}_3\) is covalent. The KI bond in \(\mathrm{KI}\) is ionic. The NB bond in \(\mathrm{H}_3 \mathrm{NBCl}_3\) is polar covalent. The ClO bond in \(\mathrm{ClO}_{2}\) is covalent.

Step-by-step solution

Identify the CC bond in \(\mathrm{H}_3 \mathrm{CCH}_3\)

Both Carbon (C) atoms have the same electronegativity because they're the same element. Therefore, the electronegativity difference is zero, meaning the bond is covalent.

Identify the KI bond in \(\mathrm{KI}\)

Potassium (K) has a lower electronegativity than Iodine (I). The difference is 3.0 - 0.82 = 2.18, which is greater than 1.7, denoting an ionic bond.

Identify the NB bond in \(\mathrm{H}_3 \mathrm{NBCl}_3\)

Nitrogen (N) has an electronegativity of 3.0 and Boron (B) has an electronegativity of 2.0. When deducted, 3.0 - 2.0, the result is 1.0, which falls between 0.4 and 1.7, therefore, the bond is polar covalent.

Identify the ClO bond in \(\mathrm{ClO}_{2}\)

Chlorine (Cl) has an electronegativity of 3.16 and Oxygen (O) has an electronegativity of 3.44. The difference is 3.44 - 3.16 = 0.28, which is less than 0.4, therefore the bond is covalent.

Key concepts

Ionic Bond

An ionic bond is a type of chemical bonding that occurs when there is a complete transfer of electrons from one atom to another. This transfer causes one atom to become positively charged (cation) and the other negatively charged (anion), leading to a strong electrostatic attraction between them. Ionic bonds often form between metals and nonmetals, where metals have a tendency to lose electrons and nonmetals have a tendency to gain electrons.

For example, in a potassium iodide (KI) molecule, potassium (K) donates an electron to iodine (I), resulting in K⁺ and I⁻ ions. Due to the large electronegativity difference of 2.18 between potassium and iodine, the bond is classified as ionic. Compounds with ionic bonds, like KI, generally have high melting and boiling points and conduct electricity when dissolved in water.

Covalent Bond

A covalent bond is a type of chemical bond where atoms share one or more pairs of electrons. This bonding typically occurs between nonmetals with similar electronegativities. Since the atoms have similar tendencies to attract electrons, neither atom completely transfers electrons to the other; instead, they each contribute one or more electrons to be shared in a mutual valence electron cloud.

As an example, the carbon-carbon (CC) bond in ethane (H₃CCH₃) comprises two carbon atoms with equal electronegativity, leading to a pure covalent bond. Molecules containing covalent bonds demonstrate a range of physical properties, largely dependent on the strength and number of the covalent bonds and the molecular structure.

Polar Covalent Bond

Polar covalent bonding is an intermediate type of chemical bond between pure covalent and ionic bonding. In a polar covalent bond, the electrons are shared between two atoms, but they are not shared equally. This happens when two atoms have different electronegativities, causing an uneven distribution of charge. The more electronegative atom attracts the shared electrons more strongly, acquiring a partial negative charge (δ−), while the less electronegative atom becomes partially positive (δ+).

For instance, in the molecule of ammonia borane (H₃NBCl₃), the bond between nitrogen (N) and boron (B) shows a difference in electronegativity of 1.0, making it a polar covalent bond. Molecules with polar covalent bonds may have dipole moments, which can affect their physical properties and reactivities.

Electronegativity Difference

Electronegativity difference is a fundamental concept used to predict the type of bond between two atoms. It refers to the difference in electronegativity values of bonding atoms. Electronegativity is a measure of an atom's ability to attract and hold onto electrons within a bond. The larger the difference in electronegativity, the more polarized the electron distribution is within the bond.

In general, if the electronegativity difference is greater than 1.7, the bond is considered ionic; if it's between 0.4 and 1.7, the bond is polar covalent, and if it's less than 0.4, the bond is generally considered covalent. This is demonstrated in the ClO₂ molecule, where the electronegativity difference between chlorine (Cl) and oxygen (O) is 0.28, indicating a covalent bond. Understanding this concept is crucial for predicting molecular structure and properties.