Recent studies of interaction cross sections, fragments momentum distributions combined with standard quantities such as mass and beta‐decay Q‐values reveal new dynamic change of shell structures. In particular, special attention is paid on neutron halo nuclei in the p‐sd shell. In halo nuclei and other neutron rich nuclei, the 2s1/2orbital shows irregular behavior. One such extreme case is the new neutron magic numberN=16. The global behavior of 2s1/2shell is discussed in relation to the interaction cross‐sections and fragment momentum distributions. Inconsistencies in understanding these data are pointed out. Peculiar changes of shells are discussed for19C and23O. © 2002 American Institute of Physics