Isomer shift and the size of Mössbauer atoms

Although the M\"ossbauer effect is by definition a solid-state effect, in most cases the free-atom (ion) electronic structure has been considered when discussing the properties of M\"ossbauer atoms embedded in crystals. It is argued that the use of free-atom (ion) wave functions in calculation of the electron contact density at the nuclei of M\"ossbauer atoms in solids can lead to numerical errors, making the calibration of the isomer shift (IS) unreliable. It is shown that the allowance for the finite size of atoms in crystals has a serious influence on the value of the electron contact density. This makes it possible to introduce a new IS scheme which directly correlates the isomer shifts to the size of the M\"ossbauer atom for various electronic configurations corresponding to the nature of its chemical bond in the crystal. As an example, the isomer shifts for several electron configurations of ¹¹⁹Sn atoms (ions) embedded in solids are discussed, including α-tin, β-tin, and their pressure dependence.