hardness of molecular surface (investigated by using the helium atom probe) is proposed as its descriptor. As an example, a homological series of the first-row hydrides has been studied. The molecular surface ("molecular shape") is defined as an isosurface of the valence repulsion energy that is related to the Pauli exclusion principle. Interestingly, the amplitude of the surface-heavy atom distance is almost the same for all of the molecules, except the methane molecule, for which it is larger by about 33%. The Pauli hardness of a point on the isosurface is defined as the first derivative of the valence repulsion energy in the direction normal to the isosurface. Higher-order derivatives correspond to the nonlinear effects (hyperhardnesses). It turned out that the molecular surfaces of these molecules are convex and the Pauli hardness of a molecule varies within about 20% as a function of position on the molecular surface. The quantity also changes by about 30% among the molecules of the series. The molecule with the greatest Pauli hardness in the series is hydrogen fluoride, and the maximum Pauli hardness increases almost linearly with the atomic number of the heavy atom in the homological series studied. The minimum Pauli hardness behaves in a different way: it is the largest for the hydrogen fluoride and then decreases for the water and ammonia, while the methane molecule represents a remarkable exception showing considerable increase of this quantity. As a result, the methane molecule exhibits the smallest, while the ammonia molecule the largest, hardness anisotropy among the first-row hydrides.