P460 - atoms1 Multi-Electron Atoms - Energy Levels and the Periodic Table can’t solve S.E. exactly --> use approximations Hartree theory (central field.

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Presentation transcript:

P460 - atoms1 Multi-Electron Atoms - Energy Levels and the Periodic Table can’t solve S.E. exactly --> use approximations Hartree theory (central field model) for n-electron at0ms. Need an antisymmetric wave function (1,2,3 are positions; i,j,k…are quantum states) while it is properly antisymmetric for any 1 j practically only need to worry about valence effects

P460 - atoms2 Schrod. Eq. For multi-electron have kinetic energy term for all electrons and (nominally) a complete potential energy: Simplify if look at 1 electron and sum over all the others --> ~spherically symmetric potential as filled subshells have no (theta,phi) dependence. Central- Field Model. Assume:

P460 - atoms3 Multi-electron Prescription “ignore” e-e potential….that is include this in a guess at the effective Z energy levels can depend on both n and L wavefunctions are essentially hydrogen-like 1guess at effective Z: Z e (r) 2solve (numerically) wave equation 3fill energy levels using Pauli principle 4use wavefunctions to calculate electrons’ average radii (radial distribution) 5redetermine Z e (r) 6go back to step 2

P460 - atoms4 First guess Look at electron proability P(r). Electrons will fill up “shells” (Bohr-like) assume all electrons for given n are at the same r with a V n for each n. Know number of electrons for each n. One can then smooth out this distribution to give a continuous Z(r) P(r) radius N=1 n=2 n=3 ZnZn radius Z-2 Z-8 Z-18

P460 - atoms5 Argon Z=18 First shell n=1 1S Z1 ~ 18-2 = 16 Second shell n=2 2S,2P Z2 ~ = 8 Third shell n=3 3S,3P Z3 ~ /4 = 4 for the outermost shell…good first guess is to assume half the electrons are “screening”. Will depend on where you are in the Periodic Table Use this to guess energy levels. ZnZn radius Z(r)

P460 - atoms6 Atomic Energy Levels After some iterations, get generalizations Afor the innermost shell: BOutermost shell atoms grow very slowly in size. The energy levels of the outer/valence electrons are all in the eV range

P460 - atoms7 Comment on X-ray Spectra If one removes an electron from an inner orbit of a high Z material, many transitions occur as other electrons “fall” as photon emitted, obey “large” energies so emit x-ray photons. Or need x- ray photons (or large temperature) to knock one of the electrons out can use the proportion of ionized atoms to measure temperature: Fe+ vs Fe++ vs Fe+++ vs Fe++++ study of photon energy gives effective Z for inner shells