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London Dispersion Forces

The atoms are combined to form molecule. In a molecule, atoms are bonded with chemical bonds. Chemical bonds are formed by sharing electrons between atoms. On the basis of sharing of electrons between atoms, chemical bonds can be classified in different types such as ionic, covalent, metallic and coordination bonds. 

Ionic bonds are formed by formation of cation and anions. An atom forms cation after lose of electron and such ions have positive charge. If an atom accepts electrons, it results the formation of anion which has negative charge. Cation and anion attract each other to form ionic bond. So we can say that ionic bonds are electrostatic force of attraction between oppositely charged ions. For example; NaCl is an ionic compound in which Na+ and Cl- combine to form ionic compound; sodium chloride. Covalent bonds are formed by equal sharing of electrons between bonded atoms. All atoms tend to complete the octet configuration that provides stability to them. 

The sharing of electrons helps to get the octet configuration to both bonded atoms. Covalent bonds are usually formed between two non-metals. They can e polar or non-polar in nature. The polarity of covalent bonds depends on the electro negativity of both bonded atoms. We know that metals have tendency to lose electrons and form metal cations. These free mobile electrons remain in the metallic lattice. The electrostatic force of attraction between metal ions and free mobile electrons is called as metallic bond.  The unique physical properties of metals such as malleability, ductility etc are due to this metallic bond only.

Coordination bonds are basically a type of covalent bond which is formed by un-equal sharing of electrons between two atoms. Here one atom acts as acceptor and other acts as donor.  These chemical bonds are formed between atoms to form molecules. There are several attraction forces between molecules like dipole-dipole interaction, dipole- induce dipole interaction, Vander Wall interaction, Hydrogen bonding, London dispersion forces etc. 

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London Dispersion Forces Definition

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So we can say that covalent bond, ionic bond, coordination bond are intra-molecular force of attraction which form within a molecule.  The forces of attraction between molecules which hold them together are called as inter-molecular force of attraction. These forces are weaker than inter-molecular forces. These forces are responsible for the liquids, solids and solutions state of any compound. Some common types of inter-molecular forces are London dispersion, dipole-dipole, Hydrogen bonding and ion-ion force. The order of strength of these inter-molecular forces is given below.

London dispersion < dipole-dipole < H-bonding < Ion-ion

So we can say that London dispersion forces are weakest inter-molecular force. London dispersion forces can be defined as a temporary attractive force due to formation of temporary dipoles in a non-polar molecule.  When the electrons in two adjacent atoms displace in such a way that atoms get some temporary dipoles, they attract each other through London dispersion force.  These inter-molecular forces occur between non-polar substances. Due to these forces, they can condense to liquids and or freeze into solids at low temperature.

London Dispersion Forces Example

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The un-equal distribution of electrons about the nucleus in an atom can induce some dipole in the atom. When another atom or molecule comes in contact with this induce dipole, it can be distorted that leads to an electrostatic attraction between either atoms or molecules. 

London Dispersion Forces
If these atoms or molecules touch each other, dispersion forces are present between any them. For example London dispersion forces between two chlorine molecules are shown below. 
London Dispersion Forces
Here both chlorine atoms are bonded through covalent bond which forms by equal sharing of valence electrons between two chlorine atoms. The force of attraction between two chlorine molecules is London dispersion force here which is due to un-equal distribution of electron density in the molecule. 

London Dispersion Forces Formula

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The tendency of molecules to form charge separation or induce dipole is called as polarizability.  The interaction between two dipoles can be expressed as its strength which is denoted as μ. The strength is directly proportional to the strength of the electric field (E).

$\mu = \alpha \times E$

$\mu$ = Induced dipole moment
$\alpha$ = Polarizability 
E = Electric field

The interaction energy can be calculated with the help of London dispersion force formula.

$V_{11}$ = - $\frac{3 \alpha_{2} I }{4 r^{6}}$
This formula is for the potential energy between two identical atoms or molecules. The formula was modified by German physicist, Fritz London for two un-identical atoms or molecules as given below.

$V_{12}$ = - $\frac{3 I_{1}I_{2} a^{'}_{1} a^{'}_{2}}{2 I_{1}+ I_{2} r^{6}}$

•    I = Ionization energy 
•    Α = Polarizability
•    r = Distance between molecules

London Dispersion Forces vs Van der Waals

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In general all the intermolecular forces of attractions between molecules are called as Van der Waals forces. Van der Waals forces can be classified as weak London dispersion Forces and stronger dipole-dipole forces. Both of these forces are due to momentarily dipole formation. The displacement of electrons causes a non-polar molecule to be a polar molecule. 

The capability of a molecule to become polar is called as polarizability of molecule. As we move from top to bottom in a group of periodic table, the polarizability increases whereas it increases from right to left within periods. As the polarity in the molecule increases, the melting and boiling points also increase as more heat is needed to break the bonds.

So we can say that as the mass increases, the number of electrons increases, and melting and boiling points also increase. Long chain molecules exhibits strong London dispersion forces because more displacement can be possible in such molecules.

London Dispersion Forces vs Dipole Dipole

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Both London dispersion forces and dipole-dipole interactions are types of Van der Wall forces. London dispersion forces are weaker than dipole-dipole forces as they are because of momentarily dipoles. The dipole –dipole interactions are due to interaction of partially positively charged part of a molecule with the partially negatively charged part of the neighbor molecule. So we can say that theses interactions are in polar molecules such as water, hydrochloric acid etc. 

Dipole Dipole Attraction

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