Question

Classify the following bonds as covalent, polar covalent, or ionic, and explain: (a) the \(\mathrm{CC}\) bond in \(\mathrm{H}_{3} \mathrm{CCH}_{3},\) (b) the KI bond in KI, (c) the \(\mathrm{NB}\) bond in \(\mathrm{H}_{3} \mathrm{NBCl}_{3}\), (d) the CF bond in \(\mathrm{CF}_{4}\).

Short answer

CC: nonpolar covalent, KI: ionic, NB: polar covalent, CF: polar covalent.

Step-by-step solution

Classifying the CC bond in H3CCH3

In the molecule H3CCH3, the bond between carbon atoms is a CC bond. Since both atoms are carbon with identical electronegativity values, the electrons are shared equally between the two carbon atoms, resulting in a nonpolar covalent bond. Thus, the CC bond in H3CCH3 is classified as nonpolar covalent.

Classifying the KI bond in KI

The KI bond involves a potassium (K) atom and an iodine (I) atom. Potassium has a low electronegativity while iodine has a much higher electronegativity. This significant difference (greater than 1.7 on the Pauling scale) in electronegativity values leads to the potassium atom losing an electron to the iodine atom, resulting in an ionic bond. Therefore, the KI bond in KI is classified as ionic.

Classifying the NB bond in H3NBCl3

In the molecule H3NBCl3, the bond between nitrogen (N) and boron (B) involves a modest difference in electronegativity. Nitrogen is more electronegative than boron, meaning it attracts the shared electrons more strongly, but not enough to completely transfer electrons. The electrons are unequally shared, indicating a polar covalent bond. Thus, the NB bond in H3NBCl3 is classified as polar covalent.

Classifying the CF bond in CF4

In CF4, the carbon (C) atom forms bonds with fluorine (F) atoms. Fluorine is highly electronegative compared to carbon, resulting in an unequal sharing of electrons. This substantial electronegativity difference results in polar covalent bonds. Therefore, each CF bond in CF4 is classified as polar covalent.

Key concepts

Covalent Bond

A covalent bond occurs when two atoms share electrons to achieve a stable electron configuration. This bonding type is primarily found between nonmetal atoms.
The electrons are shared equally between the two atoms if they hold similar electronegativities.
This results in a strong connection that holds the atoms together tightly.

• Unlike ionic bonds, covalent bonds do not completely transfer electrons.
• This bond is often seen in molecules like \( ext{H}_3 ext{CCH}_3\), where the \( ext{CC}\) bond is nonpolar because both carbon atoms have the same electronegativity.
• A truly nonpolar covalent bond will form if the electronegativity difference is less than 0.5.

In summary, covalent bonds are the result of equal sharing of electrons, leading to a stable molecule with strong atomic connections.

Polar Covalent Bond

Polar covalent bonds are similar to covalent bonds, but with a crucial difference: the electrons are shared unequally between two atoms.
This typically happens when the atoms have different electronegativity values, but not to the extent required for an ionic bond.

• The more electronegative atom attracts the shared electrons closer to itself, making one end of the molecule slightly negative and the other slightly positive.
• For example, in \( ext{CF}_4\), the carbon-fluorine bond is polar covalent because fluorine pulls the shared electrons more strongly than carbon, due to its higher electronegativity.
• The same applies to the \( ext{NB}\) bond in \( ext{H}_3 ext{NBCl}_3\), where nitrogen is more electronegative than boron, resulting in a polar covalent bond.
• This type of bond usually has an electronegativity difference between 0.5 to 1.7.

Therefore, polar covalent bonds have an uneven electron distribution, creating slight charges or dipoles within the molecule.

Ionic Bond

Ionic bonds occur when one atom transfers one or more electrons to another atom, leading to the formation of ions. These ions are positively and negatively charged and are strongly attracted to each other due to electrostatic forces.
Unlike covalent bonds, ionic bonds involve a complete transfer of electrons rather than sharing.

• This bond typically forms between metals and nonmetals, where the metal loses electrons and becomes a cation, while the nonmetal gains those electrons, becoming an anion.
• An example is the bond in \( ext{KI}\). Potassium, with lower electronegativity, transfers an electron to iodine, which has a higher electronegativity, resulting in an ionic bond.
• The electronegativity difference in ionic bonds is usually greater than 1.7 on the Pauling scale.

In essence, ionic bonds involve a complete electron transfer, connecting atoms through strong ionic attractions and forming compounds with distinct properties, such as high melting points and electrical conductivity.