Why do liquids usually have greater viscosity than gases?

In the study of fluid mechanics, viscosity is defined as a measure of internal friction that arises when adjacent layers of a fluid slide over one another. While both liquids and gases exhibit this property, liquids are typically about 100 times more viscous than gases.12

The fundamental difference lies in the molecular structure and the nature of intermolecular forces. In gases, molecules are spaced far apart and move almost independently, with viscosity resulting from the transfer of momentum as molecules move between sliding layers.32

In contrast, molecules in liquids are in close proximity, held together by significant short-range attractive forces. Because these molecules are essentially in contact, the internal friction—and thus the resistance to flow—is substantially higher than in the sparse, independent molecular environment of a gas. Furthermore, the temperature dependence of these states is distinct: as temperature increases, the viscosity of liquids drops rapidly as intermolecular cohesion is overcome, whereas the viscosity of gases increases because momentum transfer becomes more efficient at higher molecular velocities.21345