Fig The force between two molecules (blue curve) is zero at a separation r = r o, where the potential energy (red curve) is a minimum. The force is attractive when the separation is greater than r and repulsive when the separation is less than r o.

Fig Schematic representation of the cubic crystal structure of sodium chloride.

Fig Elastic collision of a molecule with an idealized container wall. The velocity component parallel to the wall does not change; the component perpendicular to the wall reverses direction. The speed v does not change.

Fig A molecule moving toward the wall with speed v x collides with the area A during the time interval dt only if it is within a distance v x dt of the wall at the beginning of the interval. All such molecules are contained within a volume A v x dt.

Fig In a time dt a molecule with a radius r will collide with any other molecule within a cylindrical volume of radius 2r and length v dt.

Fig When an amount of heat dQ is added to (a) a constant volume of monatomic ideal gas molecules, (b) the total translational kinetic energy increases by dK tr = dQ, and the temperature increases by dT = dQ / nCv.