A chemical reaction happend in a simultaneous process that is start of breaking of one bond and forming of another bond. This two process happens concurrently. Simply to say one bond is breaking and another bond is forming. With an advancement of this process there found an intermediate state where there have highest energy and form a structure of odd compound . This is not a real molecule. It's next step is the final product. Here leaving atom is adjunct to the central atom with paetial bond and new atom is also that.
