candidate: In 1921 , while working with Niels Bohr ( who postulated that electrons moved in specific orbits ) , Rutherford theorized about the existence of neutrons , ( which he had christened in his 1920 Bakerian Lecture ) , which could somehow compensate for the repelling effect of the positive charges of protons by causing an attractive nuclear force and thus keep the nuclei from flying apart from the repulsion between protons .