Question

Indicate the major type of attractive force \(-(1)\) ionic bonds, (2) dipole-dipole attractions, (3) hydrogen bonds, (4) dispersion forces-that occurs between particles of the following substances: a. \(\mathrm{NH}_{3}\) b. HI c. \(\mathrm{Br}_{2}\) d. \(\mathrm{Cs}_{2} \mathrm{O}\)

Short answer

(a) Hydrogen bonds; (b) Dipole-dipole interactions; (c) Dispersion forces; (d) Ionic bonds.

Step-by-step solution

- Identify the type of molecule or compound

Identify whether the substance is ionic or molecular. Ionic compounds consist of metals and non-metals, while molecular compounds consist of non-metals only.

- Determine if molecule is polar or nonpolar

For molecular compounds, determine if the molecule is polar or nonpolar. Nonpolar molecules will primarily experience dispersion forces, while polar molecules will experience dipole-dipole interactions or hydrogen bonds.

- Identify specific intermolecular forces

Check if hydrogen bonds are possible (molecules containing N-H, O-H, or F-H bonds). If not, determine if dipole-dipole attractions are present (polar molecules). If neither, use dispersion forces for nonpolar molecules.

Application to (a) \mathrm{NH}_{3}

\(\mathrm{NH}_{3}\) is a molecular compound consisting of nitrogen and hydrogen. Since it has N-H bonds, it can form hydrogen bonds which are strong dipole-dipole attractions.

Application to (b) HI

HI is a molecular compound consisting of hydrogen and iodine. It is a polar molecule, so the main intermolecular force is dipole-dipole interactions.

Application to (c) \mathrm{Br}_{2}

\(\mathrm{Br}_{2}\) is a diatomic molecule consisting of two bromine atoms and is nonpolar. Therefore, the main intermolecular force is dispersion forces.

Application to (d) \mathrm{Cs}_{2} \mathrm{O}

\(\mathrm{Cs}_{2} \mathrm{O}\) is an ionic compound consisting of cesium (a metal) and oxygen (a non-metal), so the attractive force is ionic bonds.

Key concepts

Ionic Bonds

Ionic bonds are strong attractive forces between oppositely charged ions. These bonds typically form between metals and non-metals. When a metal atom loses one or more electrons to become a positively charged ion (cation), and a non-metal atom gains those electrons to become a negatively charged ion (anion), an ionic bond is established. Example: In the compound \(\text{Cs}_{2}\text{O}\), cesium (Cs) atoms lose electrons to form \(Cs^{+}\) ions, while oxygen (O) gains electrons to form \(O^{2-}\) ions. The attraction between these oppositely charged ions forms a strong ionic bond.

Dipole-Dipole Attractions

Dipole-dipole attractions occur between polar molecules. This type of intermolecular force arises from the positive end of one polar molecule attracting the negative end of another polar molecule. Dipole-dipole attractions are stronger than dispersion forces but weaker than hydrogen bonds and ionic bonds. Example: In hydrogen iodide (HI), the molecule is polar because iodine (I) is more electronegative than hydrogen (H), creating a dipole. Therefore, the primary intermolecular force in HI is dipole-dipole interactions.

Hydrogen Bonds

Hydrogen bonds are special types of dipole-dipole attractions. They occur when hydrogen is covalently bonded to a highly electronegative atom like nitrogen (N), oxygen (O), or fluorine (F), creating a strong dipole. The hydrogen bond forms between the hydrogen atom of one molecule and the electronegative atom of another molecule. Example: In ammonia (NH₃), nitrogen is bonded to hydrogen, and the presence of N-H bonds allows for the formation of hydrogen bonds. These hydrogen bonds are responsible for the relatively high boiling point of NH₃ compared to other molecules of similar size.

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are the weakest type of intermolecular forces. They occur between nonpolar molecules due to the momentary distribution of electrons that creates temporary dipoles. These temporary dipoles induce similar dipoles in neighboring molecules, resulting in a weak, short-lived attractive force. Example: In bromine (Br₂), which is a nonpolar diatomic molecule, the primary intermolecular force is dispersion forces. Despite being weak, dispersion forces increase with the size of the molecules and their electron clouds.