  We propose a many-ion optical atomic clock based on three-dimensional Coulomb
crystals of order one thousand Sn$^{2+}$ ions confined in a linear RF Paul
trap. Sn$^{2+}$ has a unique combination of features that is not available in
previously considered ions: a $^1$S$_0$ $\leftrightarrow$ $^3$P$_0$ clock
transition between two states with zero electronic and nuclear angular momentum
(I = J = F = 0) making it immune to nonscalar perturbations, a negative
differential polarizability making it possible to operate the trap in a manner
such that the two dominant shifts for three-dimensional ion crystals cancel
each other, and a laser-accessible transition suitable for direct laser cooling
and state readout. We present calculations of the differential polarizability,
other relevant atomic properties, and the motion of ions in large Coulomb
crystals, in order to estimate the achievable accuracy and precision of
Sn$^{2+}$ Coulomb-crystal clocks.
