The elements with atomic numbers 61, 17, and 20 are:
Atomic number 61: Promethium (Pm) Electronic configuration: [Xe] 4f^5 6s^2
Atomic number 17: Chlorine (Cl) Electronic configuration: [Ne] 3s^2 3p^5
Atomic number 20: Calcium (Ca) Electronic configuration: [Ar] 4s^2
Please note that the electronic configurations provided are the ground state configurations for each element. The notation [Xe] represents the electron configuration of xenon (Xe) prior to the valence shell.