  We study modified theories of gravity of the f(R) type in Palatini formalism.
We first consider the stability of atoms when the Palatini gravitational
interaction is taken into account in the derivation of the non-relativistic
Schrodinger equation. We show that theories with infrared curvature corrections
are ruled out by the mere existence of atoms. In particular, we carry out fully
perturbative calculations that, for the first time, convincingly rule out the
1/R model of Carroll et al. in its Palatini version. We then study the Planck
scale corrected quadratic model f(R)=R+R^2/R_P and show that it can avoid the
big bang singularity for matter sources which satisfy all the energy
conditions. We comment on the mechanisms that cure this singularity and point
out that they are closely related to non-perturbative terms also present in the
atomic Hamiltonian of infrared corrected models.
