The Electronic Structure of Atoms

Transcription

1 The Electronic Structure of Atoms Classical Hydrogen-like atoms: Atomic Scale: m or 1 Å + - Proton mass : Electron mass 1836 : 1 Problems with classical interpretation: - Should not be stable (electron should spiral into nucleus) - Atomic spectra wrong (discrete lines instead of continuum)

2 Quantum Chemical Interpretation: e - are best described by wave functions and quantum numbers. Quantum means not continuous. Quantum numbers for electrons: n = principal quantum number l = azimuthal quantum number (orbital angular momentum) m l = magnetic quantum number m s = spin magnetic quantum number

3 Energy levels and n Spectral transitions predicted by: 1 1 E = hυ = RH n n l h where: n l < n h

4 Bohr Model of the H atom Z En = R R n = 1 n R = 4 πμe 4 ( πε ) 0 h R = 13.6 ev radius(n) = n a 0 a 0 = Bohr radius = 0.59 Å μ = reduced mass (nucleus and electron) Z = nuclear charge (1 for H) e = charge of electron ε 0 = permittivity of a vacuum h = Plank s constant

5 n = principal Energy, Size SHELL n = 1,, 3 n = 1 : ground state n = : first excited state n = 3 : second excited state E n = n E 1 = ev E = ev E 3 = ev E 4 = ev E 5 = ev E = 0 ev

6 l = azimuthal, orbital angular momentum degeneracy, shape SUBSHELL l = 0, 1, n 1 degeneracy = l + 1 l , 5, 6 type s p d f g, h, i degeneracy , 11, 13 s = sharp p = principal d = diffuse

7 m l = magnetic type, orientation in space ORBITAL m l = 0, ±1, ± ±l # of orbitals in a subshell = l + 1 l m l 0 0 s 1 0 p z ±1 ±1 0 d z -x -y = d z ±1 orbital p x p y d xz ±1 d yz 1s orbital ± d x y ± d xy

8 Wave functions and Orbitals Ψ = wave function Ψ = probability (electron) density 4πr Ψ = radial distribution function $HΨ = EΨ Ψ = R nl (r)y lml (θ,φ) R nl (r) radial function Y lml (θ,φ) angular function polar coordinates

9 Nodes: surfaces where there is 0 probability of finding an electron Number of nodes = n -1 Number of radial nodes = n - l 1 Number of angular nodes = l 0 for s orbitals 1 for p orbitals for d orbitals (except d z ) A website demonstrating nodes for D wavefunctions can be found at:

10 s orbitals p orbitals (the shading is backwards)

11 d orbitals

12 Summary of quantum numbers needed for H: Z En = R n n Energy of shell:, size: radius(n) = n a 0 l type (degeneracy) of subshell: s(1), p(3), d(5), f(7). m l type and orientation of orbitals in a subshell Energy level diagram:

13 Many electron atoms He, Z = Z En = R n E 1 = 1 R Predict: E 1 = ev Actual: E 1 = -4.6 ev Something is wrong with the Bohr Model!

14 m s = spin magnetic electron spin m s = ±½ (-½ = α) (+½ = β) Pauli exclusion principle: Each electron must have a unique set of quantum numbers. Two electrons in the same orbital must have opposite spins. Electron spin is a purely quantum mechanical concept. H N S

15 Energy level diagram for He. Electron configuration: 1s 0 1 l Energy 3 1 n H N S paramagnetic one (more) unpaired electrons He N S diamagnetic all paired electrons

16 Effective Nuclear Charge, Z* The presence of other electrons around a nucleus screens an electron from the full charge of the nucleus. We can approximate the energy of the electrons by modifying the Bohr equation to account for the lower effective nuclear charge: Z* = Z - σ Z En = R n Z* is the effective nuclear charge Z is the atomic number σ is the shielding or screening constant *

17 Helium, Z = Predicted: E 1 = ev Z En = R n * Actual: E 1 = -4.6 ev Z* = Z* = Z* = = - σ σ = 0.66

18 Lithium, Z = 3 Predicted: E = ev Z En = R n * Actual: E = -5.4 ev Z* -5.4 = Z* = Z* = = 3 - σ σ =

19 We want to be able to predict σ and Z* Slater s rules for the prediction of σ for an electron: 1. Group electron configuration as follows: (1s)(s,p)(3s,3p)(3d)(4s,4p)(4d)(4f)(5s,5p) etc.. Electrons to the right (in higher subshells and shells) of an electron do not shield it. 3. If the electron of interest is an ns or np electron: a) each other electron in the same group contributes 0.35 (0.30 for 1s) b) each electron in an n-1 group contributes 0.85 c) each electron in an n- or lower group contributes If the electron of interest is an nd or nf electron: a) each other electron in the same group contributes 0.35 b) each electron in a lower group (to the left) contributes 1.00

20 Example with a valence electron on oxygen: O, Z = 8 Electron configuration: 1s s p 4 a) (1s ) (s p 4 ) Z* = Z - σ b) σ = ( * 0.85) + (5 * 0.35) = 3.45 Z* = Z - σ 1s s,p Z* = = 4.55 This electron is actually held with about 57% of the force that one would expect for a +8 nucleus.

21 Example with two electrons for nickel: Ni, Z = 8 Electron configuration: 1s s p 6 3s 3p 6 4s 3d 8 (1s ) (s p 6 ) (3s 3p 6 ) (3d 8 ) (4s ) For a 3d electron: σ = (18 * 1.00) + (7 * 0.35) = s,s,p,3s,3p 3d Z* = Z - σ Z* = Z - σ Z* = = 7.55 For a 4s electron: σ = (10 * 1.00) + (16 * 0.85) + (1 * 0.35) = s,s,p 3s,3p,3d 4s Z* = Z - σ Z* = = 4.05

22 The basis of Slater s rules for σ s and p orbitals have better penetration to the nucleus than d (or f) orbitals for any given value of n i.e. there is a greater probability of s and p electrons being near the nucleus This means: 1. ns and np orbitals completely shield nd orbitals. (n-1) s and p orbitals don t completely shield n s and p orbitals

23 Periodicity of Effective Nuclear Charge Z* on valence electrons H 1.00 Li 1.30 Na.0 K.0 Rb.0 Cs.0 Be 1.95 Mg.85 Ca.85 Sr.85 Ba.85 B.60 Al 3.50 Ga 5.00 In 5.00 Tl 5.00 C 3.5 Si 4.15 Ge 5.65 Sn 5.65 Pb 5.65 N 3.90 P 4.80 As 6.30 Sb 6.30 Bi 6.30 O 4.55 S 5.45 Se 6.95 Te 6.95 Po 6.95 F 5.0 Cl 6.10 Br 7.60 I 7.60 At 7.60 He 1.65 Ne 5.85 Ar 6.75 Kr 8.5 Xe 8.5 Rn 8.5

24 Shielding and Effective Nuclear Charge The energy of valence electrons in an atom/ion changes with the loss of addition of an electron. Slater s rules are only approximate and can give poor predictions. For example: They ignore the differences in penetration between s and p orbitals. Real s and p orbitals do not have the same energy. They assume that all electrons in lower shells shield outer electrons equally effectively. Z* can be used to estimate ionization energy: Z* ΔHie = 13.6 n

25 Shielding and Effective Nuclear Charge Effective nuclear charge can be used to rationalize properties such as the size of atoms and ions. Cp* Be Cp* B + Be and B + are isoelectronic (1s s ) but very different because of effective nuclear charge.