Final answer:
Priority groups for determining R and S configuration are assigned based on the atomic number of the atoms directly attached to the chiral carbon or stereocenter.
Step-by-step explanation:
When determining R and S configuration, priority groups are based on atomic number. The priority is assigned based on the sequential order of the atoms directly attached to the chiral carbon or stereocenter. The atom with the highest atomic number gets the highest priority. For isotopes, the priority is given to the isotope with the higher mass number. It is essential to note that the priority does not depend on factors such as electron density, molecular weight, ionization energy, or electron affinity. Ionization energy and the size of atomic and ionic radii play crucial roles in other aspects of chemistry but not when assigning R and S configuration.