London Forces & Factors Affecting It.

What are London forces? Explain various factors affecting it.

Induced Dipole - Induced Dipole Forces or (Instantaneous Dipole) or (London Dispersion Forces)

Neither dipole - dipole nor dipole induced dipole forces can explain the fact that helium becomes a liquid at temperature below 4.2K. Non-polar gases like noble gases (He, Ne, Ar, Kr, Xe), methane, chlorine etc, becomes liquid at low temperature and high pressure.

A German physicist Fritz London in 1930 offered a simple explanation for these weak attractive forces between non-polar molecules.

In helium gas, the electrons of one atom influence the moving electrons of the other atom. Electrons repel each other and they tend to stay as far apart as possible. When the electrons of one atom move nearer to the electron of other atom, they are pushed away from each other. In this way a temporary dipole is created in the atom as shown in the Figure.

The result is that, at any moment, the electron density of the atom is no more symmetrical. It has more negative charge on one side than one the other. At that particular instant, the atom becomes a dipole. This is called instantaneous dipole. This instantaneous dipole then disturbs the electronic could of other molecule and forms induced dipole.

"The momentary force of attraction created between instantaneous dipole and the induced dipole is called Instantaneous dipole or induced dipole - induced dipole interaction or London forces."

It is a very short-lived attraction because the electrons keep moving. The movement of electrons cause the dipoles to vanish as quickly as they are formed. Anyhow, a moment later, the dipoles will appear in different orientation and again weak attractions are developed.

London force are present in all types of molecules wheather polar non-polar but they are very significant for non-polar molecules like Cl2, H2 and noble gases.

Polarizability 

"The distortion of electronic cloud of an atom or molecule is called polarizability."

Polarizability of atoms depend upon the size and atomic number. In a group of the periodic Table, size of atom increases and polarize ability increases. I2 has more polarizibility than Cl2 and Br2.

By increasing atomic number in a group, the polarizability increases.

Factors Affecting the London Dispersion Forces

(i) Boiling Points and Physical State of Noble Gases and Halogens 

London forces are weaker than dipole-dipole interactions. The strength of these forces depend upon the size of the electronic cloud of the atom or molecules. When the size of the atom or molecule is large then the dispersion becomes easy and these force become more prominent. The elements of the zero group in the periodic table are all mono-atomic gases. They don't make covalent bonds with other atoms because their outermost shells are complete. Their boiling points increase down in the group from helium to radon. The following graph shows the increase in their boiling points, Boiling points of noble gases are given in Table.

The atomic number increase down the group and the outermost electrons move away from the nuclei. The dispersion of the electronic clouds becomes more and more easy. So the polarize ability of these atoms go on increasing.

Polarizibiltyy is the quantitative measurement of the extent to which the electronic cloud can be polarized or distorted. This increased distortion of electrons creates stronger London forces and hence the boiling points are increases down the group.

Similarly, the boiling points of halogens in group VII-A also increase from fluorine to iodine. All the halogens are non-polardiatomic molecules, but there is a big difference in their physical states at room temperature. Fluorine is a gas and boils at -188.1°C. While iodine is solid at room temperature which boils at +184.4°C. The polarizability of iodine molecule is much greater than that of fluorine.

Table

Boiling Points of Halogens and Noble Gases
Group VII A	B.P (°C)	Zero Group	B.P (°C)
F2 Cl2 Br2 I2	-188.1 -34.6 58.8 184.4	He Ne Ar Kr Xe Rn	-268.6 -245.9 -185.7 -152.3 -107.1 -61.8

 (ii) Physical States and Boiling Points of Hydrocarbon Molecules

Another important factor that affects the strength of London forces is the number of atoms in a non-polar molecule. Greater the number of atoms in a molecule, greater is its polarizability. Let us discuss the boiling points of saturated hydrocarbons. These hydrocarbons have chain of C-atoms linked with hydrogen atoms. Compare the length of the chain for C2H6 and C6H14. They have the boling points -88.6°C and 68.7°C respectively. This means that the molecule with large chain length experiences stronger attractive forces. This reason is that longer molecules have more places along its length where they can be attracted to other molecules. It is very interesting to know that with the increasing molecular mass of these hydrocarbons, they change from gaseous to liquid and then finally become solids. The following Table gives the boiling points and the physical states of some hydrocarbons.