London dispersion forces are weak intermolecular forces found in all atoms and molecules. However, it is an exclusive force binding the nonpolar molecules due to their lack of functional groups, avoiding competing interactions.
Even though London dispersion Forces are weak attractive forces, they glue many atoms and molecules together. Due to the combined, collective interactions, the London dispersion Force becomes substantial, which affects the physical states of atoms and molecules. It also raises the physical properties of the melting and boiling points.
Inert gases like He, Ne, and Ar or polyatomic gaseous atoms like CH4, SiH4, GeH4 or homonuclear diatomic gaseous molecules like H2, Cl2, F2, and I2 can condense to the liquid and the solid state due to London Dispersion Forces at low temperature and high pressure. Compression of gases allows commercial preparation of Liquefied petroleum gas (LPG), Ar, He, H2 cylinders, and laboratory-grade solid CO2. The gases can be stored and transported for industrial use.
The low temperature and high pressure required for the liquefaction of the atoms and molecules depends on their nature. The conditions are controlled by their critical constants- critical temperature, critical pressure, and critical volume. The liquefaction step is done under a vacuum to remove interfering air or other media that will prevent close contact between the atoms and molecules.
The extreme condition also lowers the kinetic energy and, therefore, the mobility of the atoms and the molecules. Reducing the kinetic energy of the atoms and the molecules is essential for the attractive London Dispersion Forces to glue them or to bring them closer.
For example, lowering the temperature is vital for the Cl2 gas condensation to the liquid state. However, its congener from the same column in the periodic table, Br2, is already a liquid at room temperature of 25 oC. The increase in molecular weight and subsequent increase in the number of electrons highly favors closer interaction of Br2 over smaller Cl2 molecules.
The London Dispersion forces are highly effective for larger, polarizable atoms like Br2 and I2. In contrast, smaller atoms like Cl2 require the application of extreme temperature and pressure conditions. A similar situation is seen for smaller CH4 compared to SiH4 and GeH4.
Vander Waal’s strength - CH4 < SiH4 < GeH4 since the size of C<Si<Ge.
Boiling Point- 112K <161K <183K
Van der Waals Strength: I2 > Br2 > Cl2 > F2
Melting point: 113.7 oC > -7.2 oC > -101.5 oC > -219.67 oC
In addition to the molecular weight, the molecular shape also affects the strength of the London Dispersion Force. The long, linear shape is favored over small, compact shapes. Linear chains of n-pentane have a higher boiling point of 36.2 oC over spherical neo-pentane at 9.5 oC.
The above excerpt supports the chapter Intermolecular Forces part of CurlyArrows' Introductory Organic Chemistry Course. Preview the Book.
Related Reading: London Dispersion Force
Role of atomic size in London Dispersion Force
Factors affecting the strength of London Dispersion Forces