  I explore weakly-collective singly-closed shell nuclei with high-j shells
where active valence neutrons and particle-particle correlations may be the
dominant collective degree of freedom. The combination of large and close-lying
proton and neutron pairing gaps extracted from experimental masses seems to
charaterize the origin of the weak collectivity observed in Ni and Sn
superfluids with $N\approx Z$. The trend of $E2$ transition strengths, i.e.,
$B(E2; 2^+_1\rightarrow 0^+_1)$ values, in these nuclei is predicted from
proton and neutron pairing-gap information. The agreement with the Ni isotopes
is excellent and recent experimental results support the trend in the Sn
isotopes. This work emphasizes the importance of atomic masses in elucidating
nuclear-structure properties. In particular, it indicates that many-body
microscopic properties such as nuclear collectivity could be directly inferred
from more macroscopic average properties such as atomic masses.
