In this work, we generate synthons from a product by predicting the reaction center of the product. Typically, the reaction centers are defined as the chemical bonds of products that are newly constructed by the corresponding chemical reactions . This definition, however, only covers the reactions that change chemical bonds and thus is inapplicable for other reaction types. Therefore, we extend this definition to cover more reaction types -both chemical bonds and atoms can be recognized as reaction centers in our model. In particular, for the atoms and the bonds of Gp, we model their reactivity scores, denoted as s r = [s r n ] ∈ R |A|+|B| to predict reaction centers, where n ∈ A ∪ B indicates an atom or a bond. The reactivity score s r n is derived by passing the embedding pairs through a Multi-layer Perceptron (MLP), i.e.,