©FBC/London/Lisk/24thFeb2013 ELECTRON ARRANGEMENTS IN MULTI-ELECTRON ATOMS AND THE ORIGIN OF ATOMIC SPECTRA F. B. CARLETON London N16 ©FBC/London/Lisk/24thFeb2013 ©FBC/London/Lisk/24thFeb2013 ABSTRACT The arrangement of multi-electron atomic systems in the rotating nuclear model of the atom is examined. It is shown that variations in potential energy, alone, are sufficient to account for the arrangement of well defined numbers of electrons into shells and that variation of the kinetic energy by displacement from the equatorial axis of rotation accommodates the repulsive forces within these shells. The spectra of helium, lithium and beryllium are consistent with the proposed structural arrangements of electrons into well defined shells and indicate that the position of these additional electrons are associated with a reduction in the shielding of the nuclear charge when the electron is in its respective ground state configuration. The shielding provided by the electrons in orbit around the nucleus can be predicted from classical considerations. INTRODUCTION ©FBC/London/Lisk/24thFeb2013 ©FBC/London/Lisk/24thFeb2013 The Mendeleevian periodicity of the elements has long been established. In 1869 his arrangement of the elements in order of increasing atomic weights and their assignment to groups of similar properties confirmed a long suspected 'natural' order of the elements. Gaps were left to accommodate new elements and predictions of their properties made. Relative atomic mass Eka Silicon (1871 prediction) 72 Germanium (discovered 1886) 72.32 Specific gravity 5.5 5.47 Specific heat 0.073 0.076 Atomic volume 13 13.22 Colour Dark grey Greyish white Specific gravity of Ge Cl 4 1.9 1.88 Specific gravity of Ge O 4.7 4.703 100 86 160 160 Property 2 Boiling point of Ge Cl 4 Boiling point of Ge ( C 2 H 5 ) 4 With the advent of the Bohr Periodic Table certain anomalies of Mendleev's Table were explained, the elements were now classified according to atomic number, electronic configuration - today. The ionisation potentials of the electrons of all the elements of the periodic table display a similar periodicity both within a given atomic system and between atomic systems. For example, the ionisation potentials of the electrons in the calcium atom, Z = 20, display the following pattern of behaviour. Electron number refers to the order in which the electrons are added to the atomic system to form the atom. Electrons 1 and 2 have ionisation potentials markedly greater than all the other electrons in this atom. There is a sharp transition to the ionisation potential of the third electron accompanied by a gradual successive diminution in the potentials of the following seven electrons. After electron number ten there is again a sharp transition, though reduced in magnitude, and electrons 11 to 18 again display a gradually decreasing ionisation ©FBC/London/Lisk/24thFeb2013 ©FBC/London/Lisk/24thFeb2013 potential pattern. Similar patterns, gradual transitions and sharp discontinuities, may be discerned in the ionisation potentials of all atoms and the discontinuities are observed at the same recurring intervals. When we consider the Ionisation potential (eV) 6000 5000 4000 3000 2000 1000 0 0 10 20 30 Electron number Figure 1. Ionisation potentials of the electrons in the calcium atom. ionisation potentials of the last electrons, i.e. the electron which renders the charge of each atom electrically neutral, an equally clear pattern exists. As we proceed across the periodic table ionisation potentials change gradually until we reach the potential associated with the inert gases. There is then a sharp discontinuity and the following element's ionisation potential is considerably reduced. In every case this element is metallic and readily loses it's last electron to form a positive ion - the alkali metals. ©FBC/London/Lisk/24thFeb2013 ©FBC/London/Lisk/24thFeb2013 30 He Ionisation potential (eV) Ne 20 Ar Kr 10 Li Na K Rb 0 0 10 20 30 40 50 Atomic number (Z) Figure 2. Ionisation potentials of the electron which renders an atom electrically neutral versus atomic number (Z). Discussion The rotating nuclear atomic model conforms to all the constraints of classical physics. Stability of the lone ground state electron can be achieved only if there is no relative motion between the electron and the nucleus. Simultaneous changes of potential and kinetic energy conserve the total energy of the system. From the ground state, the lone electron must change its potential energy, alone, in order to absorb energy, the ground state kinetic energy of the lone electron is unique. A decrease in the kinetic energy of a lone ground state electron is prohibited by the properties of the simple harmonic oscillator model. The geometric position of the new permissible orbits are simple multiple integers of the ground state orbit and the associated potential energy change (kinetic energy is constant) produces electrical effects which vary as the square of these multiple integer changes of orbit. When a second electron is added to the lone electron system, the first electron becomes displaced from the equatorial plane of orbit and both electrons eventually assume identical orbits which are equally displaced north and south of the equatorial plane of orbit associated with the lone electron system. This ©FBC/London/Lisk/24thFeb2013 ©FBC/London/Lisk/24thFeb2013 displacement of orbit reduces the centrifugal forces acting on the electron. Stability of the two electron system is achieved by a change of kinetic energy of the electrons at constant potential energy. A change in the effective nuclear charge for a lone ground state electron is affected by the introduction of a second electron into the atomic system. Zeff = Z - σ (σ = shielding factor) (1) 0≤ σ ≤ 1 The second electron will experience a force of attraction, by the nucleus, this force of attraction will be less than that of the nucleus itself due to the presence of the first electron. Therefore, the value of, σ, is greater than zero. A value of zero would belie the existence of the charge, itself. If the value of the shielding factor were unity, the effective nuclear charge presented by an atomic system of known, Z, would be reduced to that of the element preceding it in the periodic table and the associated spectral properties of the two electron system would duplicate those of each preceding element with one electron. This is not the case, nor is it to be expected, since the electron is not an integral part of the nucleus. It has a negative charge with respect to the nucleus and its presence only partly reduces the overall effect of the atomic number, Z. The charge of any nuclear system minus some other constant charge (an electron) placed at a constant distance (r1) from the nucleus will present a total effective charge which is equal to the charge of the system less a constant amount of charge. All electrons are identical. This reasoning successfully accounts for the structure of all two electron helium like atomic systems, where the electrons are confined at the ground state radius of orbit. However, in general the arrangement and the effect of electron interactions must be different. Due to the presence of other electrons in an atom, any one electron does not experience the full effects of the nuclear charge, Z. At the electrons given location it will experience a shielded nuclear charge and the true charge, Z, is reduced to Zeff (the effective nuclear charge) by an amount called the shielding constant, σT . Zeff = n Z = Z - σT (2) In the rotating nuclear model of the atom, the charge Z, experienced by a lone electron is dependent on the position (i.e. the radial distance from the nucleus, as denoted by the ©FBC/London/Lisk/24thFeb2013 ©FBC/London/Lisk/24thFeb2013 value of, N) of the electron from the nucleus. The maximum charge that an electron in the Nth orbit may experience is Z/N. Thus, in the complete absence of other electrons, an electron in the Nth orbit is shielded from the full nuclear charge by virtue of its position Z- Z = σP N (3) An electrical shielding factor is defined as the difference between the total shielding factor, σΤ, and the positional shielding factor σP, so that, σΤ − σP = σE ∴ Ζ − n N Z = N σE (5) Z - σ E N (6) nZ = F.R. = ( 1 - nN N since (4) 1 N 2 n = ( 2 - ) = σ Ε NZ (6a) n = F.R. N Z - N σE NZ ) (7) When an electron is removed to some considerable distance from an atomic system of nuclear charge, Z, with x electrons in orbit around the nucleus, the effective charge in the far electric field exerted by such an atomic system on the distant electron must accurately approach, Z - x, by the laws of classical physics. When a second electron is in the far electric field, approaching a lone electron hydrogen like atomic system (NσΕ = x = 1), n = ( Z - 1 ) NZ (8) ©FBC/London/Lisk/24thFeb2013 ©FBC/London/Lisk/24thFeb2013 For the helium atom, (Z = 2), the values of, n, derived from empirical spectra (Atomic Energy Levels) are given in Table 1 and Figure 3 where they are plotted against the theoretical values derived, from equation (8), for each corresponding value of, N. Equation (8) accurately predicts the spectra of not only the helium atom, it has been fitted with equal success to the two electron helium like spectra of lithium and beryllium. It should be noted that the approach to the equilibrium position for a particular electron leads to increasing deviation from the simple model proposed, compare numerical values in Table 1. The explanation of this apparent deviation from the model has its origins in the measurement of the electrons associated electrical energy and the measurement scale employed. The treatment of this aberration is addressed in another paper. 0.3 (Z-1)/NZ 0.2 0.1 0.0 0.0 0.1 0.2 0.3 n Figure 3. Theoretical spectrum of the second electron in a helium atom compared with empirical spectrum, expressed in terms of displacement, n. The addition of a third electron to the two electron system will change, yet again, the energy of the atomic system. Since simultaneous change of potential and kinetic energy is an energy conservative process for the basic rotating nuclear atomic model, either the kinetic energy alone or the potential energy alone of the two electron system must ©FBC/London/Lisk/24thFeb2013 ©FBC/London/Lisk/24thFeb2013 undergo change, initially. The values of, N, derived for isoelectronic series containing three to ten electrons are shown in Figure 4, see equation 6. 12 10 nZ(3) nZ(4) nZ 8 nZ(5) nZ(6) 6 nZ(7) 4 nZ(8) nZ(9) 2 0 nZ(10) 0 10 20 30 40 Z Figure 4. The relationship between electron numbers 3 to 10 (bracketed number) and the parameter N. 8 6 nZ(11) nZ(12) nZ nZ(13) nZ(14) 4 nZ(15) nZ(16) nZ(17) 2 0 nZ(18) 10 20 30 40 Z Figure 5. The relationship between electron numbers 11 to 18 (bracketed number) and the parameter N. ©FBC/London/Lisk/24thFeb2013 ©FBC/London/Lisk/24thFeb2013 For nuclear systems containing three to ten electrons the value of N is 2. When more than ten electrons are present about the atomic nucleus there is an abrupt change in the gradient (1/N) of the relationship and the assignment of electrons 11 to 18, to a distance N = 3, from the nucleus is indicated. For this reason the two electron groupings are shown separately. In other words there are eight electrons accommodated in the second orbit (N = 2) about the atomic nucleus. The transitions between the various possible radii, denoted by the value of N, occur by alteration of the potential energy at constant kinetic energy. Whereas, at a constant radius the linear velocity of the electrons can change to accommodate repulsive forces acting on the electron or to accommodate the addition of more electrons to the atomic system. The values of the parameter, σΕ, determined from the Figures 4 and 5, for electron numbers 3 to 18 are given in Figure 6. 6 Electron shielding factor 5 4 3 2 1 0 0 10 20 Electron number Figure 6. Shielding factor, σΕ, as a function of electron number. The spectrum of the third electron in the lithium atom (Z = 3, x = 2) is given in Table 2 and Figure 7, for all values of N > 2. In this example stability of the electron which renders the atom electrically neutral is obtained when this electron resides in the second ©FBC/London/Lisk/24thFeb2013 ©FBC/London/Lisk/24thFeb2013 orbit about the nucleus, N =2, in accordance with the foregoing analysis, see Figure 4. 0.12 (Z-2)/NZ 0.10 0.08 0.06 0.04 0.02 0.02 0.04 0.06 0.08 0.10 0.12 n Figure 7. Theoretical spectrum of the third electron in a lithium atom compared with empirical spectrum, expressed in terms of displacement, n. Conclusions In the introduction, the calcium atom was employed to illustrate the regularities and patterns displayed by the ionisation potentials of the electrons in a given atomic system. Comparisons between atomic systems were made by considering the patterns displayed by the electron, which renders the atom electrically neutral. In this paper, periodicity has been demonstrated by considering the same numbered electrons (isoelectronic series) in various atoms from Z = 3 to Z = 18, where "same numbered electron" refers to the number in the order in which electrons are added about the nucleus to construct the atom. This procedure establishes the existence of shells at clearly defined distances (integer multiples, N, of the ground state orbit) with characteristic numbers of electrons allocated to each of the shells. The origin of spectra has been examined by making comparison with the spectra of helium, lithium and beryllium. It has been shown that these spectra are in accordance with the predictions of classical physics when the effective electric ©FBC/London/Lisk/24thFeb2013 ©FBC/London/Lisk/24thFeb2013 field is considered to be the resultant of the difference between the number of positive charges and negative charges, as experienced by a single electron, in the far electric field. A full explanation of the electrons stability as it approaches its position of maximum stability is discussed elsewhere. The reasons for the pronounced increase in stability on the final transition, N + 1 to N, is related to the treatment of the associated electrical energy that contributes to a particular ground state configuration and is beyond the scope of this paper. REFERENCES ©FBC/London/Lisk/24thFeb2013 ©FBC/London/Lisk/24thFeb2013 1. CRC Handbook of Chemistry and Physics, 59th Edition, CRC Press Inc., (1978-79). 2. Atomic Energy Levels, As Derived From The Analyses of Optical Spectra., C. E. Moore, Circular of the National Bureau of Standards 467,Washington, D. C., 1947 3. F. B. Carleton, Ph. D. thesis, QUB, 1972. ©FBC/London/Lisk/24thFeb2013 ©FBC/London/Lisk/24thFeb2013 Table 1 I.P. 1= 54.40 -1 1/λ (cm. ) Designation L 2P 3P 4P 5P 6P 7P 8P 9P 10 11P 12P 13P 14P 15P 16P 17P 18P 19P 20P 198305.00 171129.14 186203.62 191486.95 193936.75 195269.17 196073.41 196595.56 196953.95 197210.41 197400.18 197544.56 197656.95 197746.15 197818.12 197877.04 197925.87 197966.80 198001.44 198031.02 Table 2 1/λ (cm. -1) 43487.19 30925.38 36469.55 39015.56 40390.84 41217.35 41751.63 42118.27 42379.16 42569.10 42719.14 L = Limit N 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1po 1po 1po 1po 1po 1po 1po 1po 1po 1po 1po 1po 1po 1po 1po 1po 1po 1po I.P. 1= 122.419 N L 3P 4P 5P 6P 7P 8P 9P 10P 11P 12P 2 3 4 5 6 7 8 9 10 11 12 I.P. (eV) x 5.390 1.556 .8695 .5540 .3835 .2811 .2148 .1694 .1371 .1135 .0949 2 1/2 m0 vx 2 = 1/2 m0 v1 vx v1 = 2 n (Z-1)/NZ .67216 .24883 .16604 .12463 .09976 .08316 .07130 .06240 .05548 .04993 .04540 .04162 .03842 .03568 .03330 .03122 .02939 .02775 .02629 .02498 .5000 .2500 .1667 .1250 .1000 .0833 .0714 .0625 .0555 .0500 .0454 .0416 .0384 .0357 .0333 .0312 .0294 .0277 .0263 .0250 L = Limit Designation 2p o 2p o 2p o 2p o 2p o 2p o 2p o 2p o 2p o 2p o 2p o I.P. (eV) x 24.585 3.3690 1.5001 .84514 .54142 .37624 .27653 .21179 .16736 .13557 .11204 .09414 .08021 .06915 .06023 .05292 .04687 .04179 .03750 .03383 I.P. x I.P. 1 = I. P. x v0 Z1 (Z-2)/NZ .2098 .1127 .0842 .0672 .0559 .0479 .0418 .0372 .0334 .0304 .0278 .1666 .1111 .0833 .0666 .0555 .0476 .0416 .0370 .0333 .0303 .0277 (4 ) I.P. 1 v0 Z x n = n (5) ©FBC/London/Lisk/24thFeb2013
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