Neon isoelectronic series

Table 2 Correlation energies of the neon isoelectronic series Comparison ofE 2 tth the...

Table 2 Correlation energies of the neon isoelectronic series Comparison ofE 2 1- (4-6), with the corresponding second order Moller-Plesset (MP2) [30], RLDA [16] andPW91-GGA [8] results. In addition, AHF 1- is compared With the difference IaE, Eq. (4.9), between RHF [60] and x-only ROPM [16]...

Calculations of IIq(O) are very sensitive to the basis set. The venerable Clementi-Roetti wavefunctions [234], often considered to be of Hartree-Fock quality, get the sign of IIq(O) wrong for the sihcon atom. Purely numerical, basis-set-free, calculations [232,235] have been performed to establish Hartree-Fock limits for the MacLaurin expansion coefficients of IIo(p). The effects of electron correlation on IIo(O), and in a few cases IIq(O), have been examined for the helium atom [236], the hydride anion [236], the isoelectronic series of the lithium [237], beryllium [238], and neon [239] atoms, the second-period atoms from boron to fluorine [127], the atoms from helium to neon [240], and the neon and argon atoms [241]. Electron correlation has only moderate effects on IIo(O). 

Within an isoelectronic series, these factors have striking results. Within the dashed outline in Figure 8.21, the ions are isoelectronic with neon. Period 2 anions are much larger than Period 3 cations because the same number of electrons are attracted by an increasing nuclear charge. The pattern is... 

Molecules Isoelectronic with Neon.—Another set of molecules whose structure is typical of many standard chemical environments is the set of ten-electron first row hydrides Ne, HF, H20, NH3, and CH4. On p. 281 we saw how the outer electrons of the neon atom could be described either as being in the configuration (2s)2(2p)6 or, alternatively, as occupying four tetrahedral orbitals %2, %s, and y4 orientated relative to one another in a tetrahedral manner, the orientation of the tetrahedron in space being arbitrary. The electronic structures of the other molecules of the series can now be discussed in terms of this basic system if we imagine unit positive charges to be removed successively from the nucleus. 

Consider the following series of substances methane (CH4), ammonia (NH3), water (H2O), hydrofluoric acid (HF), and neon (Ne). These substances are all isoelectronic, i.e. they all have 10 electrons (hence also 10 protons — since the molecules themselves are electro-neutral). 

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