Argon ground state electronic configuration, 18 19

Figure 5.12 Radial wavefunctions P /r) with n = 3 for neutral chlorine (Cl I), singly-ionized argon (Aril), and doubly-ionized potassium (Kill) which demonstrate the collapse of the 3d orbital (all these systems have the electron configuration ls22s22p63s23p5 in the ground state while the results apply to the. .. 3s23p4( D)3d 2S states). Note that the horizontal scales are slightly different, but the distance from the origin to (3s + 3p)/2 is the same in all three cases. It is the average position 3s and , i.e., the 3d orbital is clearly uncollapsed. The opposite is true for Kill where the 3d orbital has clearly collapsed. The 3d orbital of Aril falls between these two cases. From [SHa83].

The build-up of the third period, from sodium to argon, is an echo of what happened in the second first the one 3s orbital is filled, and then the three 3p orbitals. As the number of electrons in an atom reaches 15 or 20, it is frequently the practice to explicitly include only those electrons added in the building up beyond the last preceding noble-gas element. The configuration of that noble gas is then represented by its chemical symbol enclosed in brackets. The ground-state configuration of silicon, for example, is written [Ne]3s 3p using this system. 

In scattering experiments on argon gas using a He beam, the appearance of Ar ions in electronically excited states can be observed, at certain appearance potentials. From a theoretical point of view, this means that potential surfaces of He + Ar must cross those which dissociate into excited argon ions plus helium atom in its ground state. The calculated curves are seen in Fig. 21. The various configurations that correlate with the entrance channel Ar( S) +... 

Our first many-electron example is argon. Argon is a noble gas with the closed shell configuration ls 2s 2p 3s 3p , so its ground-state is spherical. In Fig. 6.5 we plot the electron density for this atom as a function of the distance to the nucleus. The function n r) decays monotonically, with very little structure, and is therefore not a very elucidative quantity to behold. However, if we choose to represent r n(r), we can clearly identify the shell structure of the atom Three maxima, corresponding to the center of the three shells,... 

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