هادی پارسا

Using classical definition of isothermal compressibility in thermodynamic systems, a new molecular property is introduced for diatomic molecules, which is called “Molecular Compressibility”. For both homo-nuclear and hetero-nuclear molecules, this quantity has the simple form as: Which R and k are bond length and force constant, respectively. Whereas r quantity is the atomic radius in homo-nuclear molecules, in hetero-nuclear ones this parameter is a cross-sectional radius in a point between two atoms which the molecular compressibility is derived and is defined in four different ways: A) a position in which the levers rule is obeyed. B) A point in which the sum of the exerted forces are zero. C) A point in which the cross-sectional radius is the mean of the atomic radii and D) a point in which the cross-sectional radius is the same as a radius of one of the heteroatom in a molecule. All of the obtained compressibilities for some selected hydrides have linear relation with each other, which show that the defined molecular compressibliy is independent of the position with which it derived. On the other hand in both homo- and hetero-atomic molecules, the calculated compressibilities have a good linear relationship with the calculated polarizabilities of molecules. On the basis of this linear relation and the minimum polarizability principle, the molecular compressibility may be introduced as a criterion of the stability of molecules.