Charge radii are sensitive observables for identifying nuclear structure phenomena across the nuclear chart. In particular, a local decrease in charge radii along an isotopic chain is closely associated with shell effects. In this work, the systematic evolution of charge radii for the isotopic chains with \(Z=8\) , 10, 12, 14, and 18 is investigated within the relativistic Hartree–Bogoliubov model. An ansatz for neutron–proton correlations around the Fermi surface is adopted to describe the anomalous behavior of nuclear charge radii. Our results show that neutron–proton pairing corrections around the Fermi surface lead to pronounced increases in the charge radii at \(N=8\) , 20, and 28, indicating an enhancement of the corresponding shell closures. The reproduction of the charge radius at \(N=14\) in Mg isotopes is sensitive to the treatment of pairing correlations. The BCS approach overestimates the shell effect, whereas the Bogoliubov quasiparticle transformation, which predicts stronger pairing correlations near the proton Fermi surface, yields better agreement with the experimental data. The deviations between theoretical and experimental values for 25 even–even nuclei show that the neutron–proton pairing correction improves the charge radii calculated with meson-exchange interactions. However, no comparable improvement is obtained for density-dependent effective interactions.
