Forces and Potential Energy in Molecules Formation of Chemical Bonds

5 Forces and Potential Energy in Molecules Formation of Chemical Bonds 

What determines the stability of a chemical bond Why is the Hi molecule more stable than a pair of separated hydrogen atoms in the gas phase at normal temperatures and pressures But what is the meaning of more stable And how do bonds form spontaneously once the atoms are close enough together  

FIGURE 3.8 Coordinates for the hydrogen molecule. The nuclei are assumed stationary at fixed positions separated by the distance Rab- The distance of electron 1 from nuclei A and B is given by riA,riB the distance of electron 2 from nuclei A and B is given by r2A,i 2B, the distance between the electrons is given by 02. 

The first four terms in Equation 3.13 represent the attractions between the electrons and the nuclei, and all are negative. The last two terms represent the repulsions between the pair of electrons and the pair of protons, and both are positive. The value of V can be calculated for any configuration of the molecule. But just as in the case of the lithium atom in Section 3.3, this potential energy function does not give a simple pictorial explanation of the stability of the molecule, because there is no exact solution for the motions of four interacting particles. 

As explained in Section 3.2, the force between the protons is the negative of the slope of Veff, that is, the negative of its derivative with respect to Rab- For values of Rab larger than the minimum, the attractive forces tend to reduce Rab, for values of Rab smaller than the minimum, the repulsive forces tend to increase Rab-Thus, both the depth of the potential and the position of the minimum are 

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