Hydrogen bond strengths typically are in the range 4 - 46 kJ/mol, much less than the strengths of typical covalent bonds. These result in much higher boiling points than are observed for substances in which London dispersion forces dominate, as illustrated for the covalent hydrides of elements of groups 1417 in Figure \(\PageIndex{5}\). The most significant force in this substance is dipole-dipole interaction. Similarly, solids melt when the molecules acquire enough thermal energy to overcome the intermolecular forces that lock them into place in the solid. Nitrogen tribromide is slightly polar in nature. In the case of liquids, molecular attractions give rise to viscosity, a resistance to flow. Comparing the two alcohols (containing -OH groups), both boiling points are high because of the additional hydrogen bonding due to the hydrogen attached directly to the oxygen - but they are not the same. In truth, there are forces of attraction between the particles, but in a gas the kinetic energy is so high that these cannot effectively bring the particles together. Why do strong intermolecular forces produce such anomalously high boiling points and other unusual properties, such as high enthalpies of vaporization and high melting points? When we consider the boiling points of molecules, we usually expect molecules with larger molar masses to have higher normal boiling points than molecules with smaller molar masses. (X and Y may be the same or different elements.). The properties of liquids are intermediate between those of gases and solids, but are more similar to solids. Solids have stronger intermolecular forces, making them rigid, with essentially no tendency to flow. determine the dominant intermolecular forces (IMFs) of organic compounds. Arrange each series of substances in order of increasing boiling point. It should therefore have a very small (but nonzero) dipole moment and a very low boiling point. (see Interactions Between Molecules With Permanent Dipoles). We will concentrate on the forces between molecules in molecular substances, which are called intermolecular forces. The three compounds have essentially the same molar mass (5860 g/mol), so we must look at differences in polarity to predict the strength of the intermolecular dipoledipole interactions and thus the boiling points of the compounds. Three obvious consequences of Equations \(\ref{Col}\) and \(\ref{Force}\) are: To complicate matters, molecules and atoms have a distribution \(\rho(\vec{r})\) that result from the 3D distribution of charges (both nuclei and especially electrons).