Final answer:
Out of the given substances, hydrogen gas (H2) is the one that only exhibits London dispersion forces, as it is a non-polar molecule without a permanent dipole.
Step-by-step explanation:
The question asks which of the following substances only has London dispersion forces: H2 (hydrogen gas), HCl (hydrogen chloride), H2O (water), and NH3 (ammonia).
- H2 (hydrogen gas) only has London dispersion forces because it is a non-polar molecule, meaning it has no permanent dipole.
- HCl (hydrogen chloride) has dipole-dipole interactions because it is a polar molecule, though it also has London dispersion forces.
- H2O (water) has strong hydrogen bonding due to the presence of hydrogen attached to the highly electronegative oxygen atom. Hydrogen bonds are much stronger than London dispersion forces.
- NH3 (ammonia) also has hydrogen bonding because it has hydrogen atoms attached to a highly electronegative nitrogen atom.
Therefore, the correct answer is H2 (a), which only has London dispersion forces.