We describe the properties of neutron-rich nuclei around N = 28 and N = 20
in the shell model framework. The valence space includes the sd shell for p
rotons and pf shell for neutrons without any restrictions. Good agreement i
s found with the available data and the N = 28 shell closure is shown to be
persisting. Then we perform calculations in the N = 20 region where the va
lence space is enlarged to include intruder states. We are able to account
for the vanishing of the N = 20 neutron shell and the dominance of the intr
uders explains the collective features experimentally found in this region.