  A systematics of the atomic nuclei in the frame of the nucleon number $A = Z
+ N$ and the proton-neutron difference $F = Z - N$ is considered. The
classification scheme is provided by means of the non-compact algebra
$sp(4,R)$. In this scheme the nuclei are ordered into isobaric multiplets, for
which $A={fix}$, as well as in F-multiplets, for which $F={fix}$. The
dependence of the mass excess $\Delta$, the first exited states $E_{2^+}$ and
the ratio $R_2=E_{4^+}/E_{2^+}$ on the nucleon number $A$ is empirically
investigated within the $F$-multiplets. Appropriate filters are used to study
the properties of the mass excess. Many structural effects are observed. The
mirror symmetry is clearly indicated for the energy levels of the nuclei with
the same value of $A$ and opposite $F$-values.
