  The shell corrections and shell gaps in nuclei are systematically studied
with the latest Weizs\"acker-Skyrme (WS4) mass model. We find that most of
asymmetric nuclei with (sub)-shell closures locate along the shell stability
line (SSL), $N=1.37Z+13.5$, which might be due to a strong correlation between
neutrons and protons near Fermi surface. The double magicity of nuclei
$^{46}$Si and $^{78}$Ni is predicted according to the corresponding shell gaps,
shell corrections and nuclear deformations. The unmeasured super-heavy nuclei
$^{296}$118 and $^{298}$120, with relatively large shell gaps and shell
corrections, also locate along the SSL, whereas the traditional magic nucleus
$^{298}$Fl evidently deviates from the line. The $\alpha$-decay energies of
super-heavy nuclei with $Z=113-126$ are simultaneously investigated by using
the WS4 model together with the radial basis function corrections. For
super-heavy nuclei with large shell corrections, the smallest $\alpha$-decay
energy for elements $Z=116$, 117 and 118 in their isotope chains locates at
$N=178$ rather than $184$.
