Difference electron density distribution

The different electron density distribution causes a considerable dipole moment in the ground state of pseudoazulenes (Section IV,D). Corresponding calculations have been carried out for various systems.54,60-62-63,1 17,170-171... 

The difference electron density distribution also finds an interesting application when the fully refined model of the crystal structure is used to compute a Fourier transformation. Although it may seem that such a Fourier map should result in zero electron density throughout the unit cell (since the differences in Eq. 2.135 are expected to approach zero), this is true only if electron shells of atoms in the crystal structure were not deformed. In reality, atoms do interact and form chemical bonds with their neighbors. This causes a redistribution of the electron density when compared to isolated atoms, for which atomic scattering functions are known. 

Two structures are in equilibrium if the structures are related by differing atomic connectivity (i.e., different sigma bonding pattern) and differing electron density distribution, as in the case of tautomers. 

The s-,p, d-,... orbitals have different shapes and different electron density distributions. For a given shell (same value of the principal quantum number n), s-electrons tend to be closer to the nucleus than p-electrons, which are closer than (/-electrons. We say that, other things equal, s-electrons are more penetrating than p-electrons, and p-electrons are more penetrating than (/-electrons. 

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