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Tuesday, 7 February 2017

States of Matter I


Some Important Points and Terms of the Chapter

1. Intermolecular forces are the forces of attraction and repulsion between interacting particles (atoms and molecules). This term does not include the electrostatic forces that exist between the two oppositely charged ions and the forces that hold atoms of a molecule together i.e., covalent bonds. Attractive intermolecular forces are known as van der Waals forces, in honour of Dutch scientist Johannes van der Waals (1837-1923).

2. Dipole - Dipole Forces: Dipole-dipole forces act between the molecules possessing permanent dipole. Ends of the dipoles possess partial charges and these charges are shown by Greek letter delta (δ
3. Dipole -Induced Dipole Forces: This type of attractive forces operate between the polar molecules having permanent dipole and the molecules lacking permanent dipole. Permanent dipole of the polar molecule induces dipole on the electrically neutral molecule by deforming its electronic cloud.Thus an induced dipole is developed in the other molecule

4. Dispersion Forces or London Forces: Atoms and non polar molecules are electrically symmetrical and have no dipole moment because their electronic charge cloud is symmetrically distributed. But a dipole may develop momentarily even in such atoms and molecules. This can be understood as follows. Suppose we have two atoms _A‘ and _B‘  in the close vicinity of each other. It may so happen that momentarily electronic charge distribution in one of the atoms, say _A‘, becomes unsymmetrical i.e., the charge cloud is more on one side than the other. This results in the development of instantaneous dipole on the atom _A‘ for a very short time. This instantaneous or transient dipole distorts the electron density of the other atom _B‘, which is close to it and as a consequence a dipole is induced in the atom _B‘. The temporary dipoles of atom _A‘ and _B‘ attract each other. Similarly temporary dipoles are induced in molecules also. This force of attraction was first proposed by the German physicist Fritz London, and for this reason force of attraction between two temporary dipoles is known as London force. Another name for this force is dispersion force.

5. Hydrogen bond: Hydrogen bond is represented by a dotted line (– – –) while a solid line represents the covalent bond. Thus, hydrogen bond can be defined as the attractive force which binds hydrogen atom of one molecule with the electro negative atom (F, O or N) of another molecule this is special case of dipole-dipole interaction.

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