The electronic structures, bonding, stability, aromaticity, and spectroscopic properties of the endohedral metallofullerenes, M@C₂₀ (M = Pr^–, Pa^–, Nd, U, Pm⁺, Np⁺, Sm²⁺, Pu²⁺, Eu³⁺, Am³⁺, Gd⁴⁺, and Cm⁴⁺), have been investigated in a unified and systematic way using relativistic density functional theory (DFT) within the framework of zeroth-order regular approximation. The bare C₂₀ cage with D_3d point group transforms to highly symmetrical I_h structure through encapsulation of an f-block metal atom/ion with 6 valence electrons. The calculated values of HOMO–LUMO gap lie in the range of 2.5–4.9 eV (8.8–11.5 eV) at the B3LYP (HF) level. The stability of these metal encapsulated clusters can be attributed to the fulfillment of 26 valence electrons criteria corresponding to the fully occupied 2s2p1d atomic shells, where strong participation of the central metal atom orbitals in the a_g, t_1u, g_u, and h_g valence molecular orbitals have been observed.