In this talk we study the self-adjointness of $H+V_{\lambda}$, where $H$ is the free Dirac operator in ${\mathbb R}^3$ and $V_{\lambda}$ is an electrostatic shell potential depending on a parameter $\lambda\in \mathbb{R}$. The potential is located on the boundary of a smooth domain in ${\mathbb R}^3$. We also study the spectral properties of $H+V_{\lambda}$. In particular, we see that there is an admissible range of $\lambda$'s for which the coupling $H+V_{\lambda}$ generates pure point spectrum in $(-m, m)$. Morerover, we give an isoperimetric-type inequality for this range of $\lambda$'s, that is, we want to determine how small can this range be under some constraint on the size of the domain and/or the boundary of the domain. We also see that the ball is the unique optimizer of this inequality.