Which of the following substances exhibit ONLY dispersion forces?
(a) Xe
(b) O2
(c) OCl2
(d) BCl3
The correct answer and explanation is :
Let’s solve it carefully:
First, dispersion forces (also called London dispersion forces) are the weakest type of intermolecular forces and are present in all molecules, but they are the only type of intermolecular force in nonpolar molecules.
Thus, to answer this question, we need to check:
- Is the substance nonpolar?
- If yes, it will have only dispersion forces.
- If not (i.e., if it’s polar), it will have dipole-dipole forces along with dispersion.
Now, let’s go through the options one by one:
(a) Xe (Xenon)
- Xe is a noble gas atom, monoatomic.
- No polar bonds, no permanent dipole.
- Interactions are only via dispersion forces.
✅ Xe has only dispersion forces.
(b) O₂ (Oxygen gas)
- O₂ is a diatomic molecule with two identical oxygen atoms.
- It is nonpolar (equal sharing of electrons).
- So, only dispersion forces exist between O₂ molecules.
✅ O₂ has only dispersion forces.
(c) OCl₂ (Dichlorine monoxide)
- OCl₂ is a bent molecule because oxygen forms bonds with two Cl atoms and has lone pairs, similar to the structure of H₂O.
- Thus, OCl₂ is a polar molecule.
- Polar molecules exhibit dipole-dipole interactions in addition to dispersion forces.
❌ OCl₂ has dipole-dipole and dispersion forces (not only dispersion).
(d) BCl₃ (Boron trichloride)
- BCl₃ is a trigonal planar molecule with symmetrical distribution of chlorine atoms around boron.
- Even though B-Cl bonds are polar individually, the overall molecule is nonpolar due to symmetry (the dipoles cancel out).
- Therefore, only dispersion forces act between BCl₃ molecules.
✅ BCl₃ has only dispersion forces.
Final Answer:
✅ (a) Xe, (b) O₂, and (d) BCl₃ exhibit only dispersion forces.
❌ (c) OCl₂ does not.
300-word Explanation:
Dispersion forces, also called London dispersion forces, are the weakest intermolecular forces arising from momentary dipoles created by the movement of electrons in atoms and molecules. Every molecule has dispersion forces, but only nonpolar molecules rely solely on them for intermolecular interactions.
For a molecule to have only dispersion forces, it must be nonpolar. Polar molecules have permanent dipoles leading to additional dipole-dipole interactions. Therefore, to determine if a substance exhibits only dispersion forces, we must assess whether it is nonpolar.
Xe is a noble gas atom, existing as isolated atoms with no permanent dipole. The only attractive force between Xe atoms is dispersion forces.
O₂ is a diatomic molecule with two identical atoms sharing electrons equally, making it nonpolar. Thus, O₂ molecules are attracted only through dispersion forces.
OCl₂, on the other hand, has a bent molecular geometry due to lone pairs on the oxygen atom, similar to water (H₂O). This asymmetry causes a net dipole moment, making OCl₂ polar. Therefore, OCl₂ experiences dipole-dipole forces in addition to dispersion forces.
BCl₃ has a trigonal planar structure where the three B–Cl bonds are symmetrically arranged around the boron atom. Although individual B–Cl bonds are polar, the molecular symmetry causes their dipoles to cancel, making BCl₃ nonpolar overall. Hence, BCl₃ experiences only dispersion forces.
In conclusion, Xe, O₂, and BCl₃ are nonpolar and exhibit only dispersion forces, whereas OCl₂ is polar and exhibits both dipole-dipole and dispersion forces.