The nucleus formation process

The nucleation process involves conversion of a small volume of reactant into a stable particle of product and continued reaction (growth) 

Jacobs and Tompkins [28] consider the nucleation process in the general reaction 

The first-formed individual atoms (ions or molecules) of B cannot be regarded as a distinct and separate phase but initially, at least, are expected to tend to conform to the structure of, and retain their former positions with reference to, the reactant phase A. During the continued accumulation of atoms (ions or molecules) of B, the consequent increase in total deformation strain energy will lead to a transformation to the structure characteristic of the stable product, solid B. This is quantitatively expressed [28] as a change in free energy by 

Where the particle of B contains m molecules, AGB is the bulk free energy change per molecule, a is the shape factor (4irr2 for a spherical interface) and y is the strain energy per unit area of interface. For a spherical nucleus, where vm is the volume of product per molecule , 

When m mc, product assemblages are unstable and thermodynamic considerations predict that the product (B) will tend to revert to the reactant unless the local statistical fluctuations of energy are sufficient to achieve 

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