Electrons probability maps

The Quantum-Mechanical Model The quantum-mechanical model for the atom describes electron orbitals, which are electron probability maps that show the relative probability of finding an electron in various places surrounding the atomic nucleus. Orbitals are specified with a number (n), called the principal quantum number, and a letter. The principal quantum number (n = 1,2,3. . . ) specifies the principal shell, and the letter (s, p, d, or f) specifies the subshell of the orbital. In the hydrogen atom, the energy of orbitals depends only on n. In multi-electron atoms, the energy ordering is Is 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s. 

One of the most intriguing recent examples of disordered structure is in tomato bushy stunt virus (Harrison et ah, 1978), where at least 33 N-terminal residues from subunit types A and B, and probably an additional 50 or 60 N-terminal residues from all three subunit types (as judged from the molecular weight), project into the central cavity of the virus particle and are completely invisible in the electron density map, as is the RNA inside. Neutron scattering (Chauvin et ah, 1978) shows an inner shell of protein separated from the main coat by a 30-A shell containing mainly RNA. The most likely presumption is that the N-terminal arms interact with the RNA, probably in a quite definite local conformation, but that they are flexibly hinged and can take up many different orientations relative to the 180 subunits forming the outer shell of the virus particle. The disorder of the arms is a necessary condition for their specific interaction with the RNA, which cannot pack with the icosahedral symmetry of the protein coat subunits. 

Even when this is done, electron density maps usually show, in the regions of low density, irregularities which do not appear to have any significance they are probably due to inaccuracies in the measurement of the intensities of the reflections, or to approximations in calculation. The positions of the atomic centres, however, are not in doubt. 

In agreement with the Heisenberg uncertainty principle, the model cannot specify the detailed electron motions. Instead, the square of the wave function represents the probability distribution of the electron in that orbital. This approach allows us to picture orbitals in terms of probability distributions, or electron density maps. 

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