Final answer:
Molecules in NH3(l) are held together by dipole-dipole intermolecular forces caused by the unequal charge distribution in the molecule, with nitrogen having a lone pair of electrons.
Step-by-step explanation:
The molecules in a sample of NH3(l) are held together by intermolecular forces due to unequal charge distribution within the molecule. These forces arise because NH3 has a polar molecule structure, where the nitrogen atom has a lone pair of electrons that does not bond with surrounding hydrogen atoms.
The dipole-dipole interactions are a result of the electrostatic attractions between the positively charged hydrogen atoms and the negatively charged region of the nitrogen atom. This makes option (4) the correct answer, as these intermolecular forces are not caused by ions, electrons alone, or different numbers of neutrons, but by how the electrons are distributed across the molecule creating poles.