Transcript Atomic Structure Notes 1

Chem 59-250
The Electronic Structure of Atoms
Classical Hydrogen-like atoms:
Atomic Scale: 10-10 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)
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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)
ml = magnetic quantum number
ms = spin magnetic quantum number
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Energy levels and n
Spectral transitions predicted by:
 1
1
E  h  RH  2  2 
nh 
 nl
where:
nl < nh
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Bohr Model of the H atom
 Z2 
 1
En   R  2    R  2 
n 
n 
R 
2 2e 4
2
4

h
 0
2
R = 13.6 eV
radius(n) = n2a0
a0 = Bohr radius = 0.529 Å
 = reduced mass (nucleus and electron)
Z = nuclear charge (1 for H)
e = charge of electron
0 = permittivity of a vacuum
h = Plank’s constant
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n = principal  Energy, Size  SHELL
n = 1, 2, 3 …
n = 1 : ground state
n = 2 : first excited state
n = 3 : second excited state
 1
E n   13.6  2 
n 
E1 = -13.6 eV
E2 = -3.40 eV
E3 = -1.51 eV
E4 = -0.85 eV
E5 = -0.54 eV
•
•
•
E = 0 eV
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l = azimuthal, orbital angular momentum
 degeneracy, shape  SUBSHELL
l = 0, 1, 2 …n –1
degeneracy = 2 l + 1
l
0 1 2 3 4, 5, 6 …
type
s
p d f g, h, i …
degeneracy 1 3 5 7 9, 11, 13 …
s = “sharp”
p = “principal”
d = “diffuse”
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ml = magnetic  type, “orientation in space”  ORBITAL
ml = 0, ±1, ±2 …±l
l
# of orbitals in a subshell = 2 l + 1
ml
orbital
0
0 s
1
0 pz
±1 px
±1 py
2
0 d2z2-x2 -y2 = dz2
±1 dxz
±1 dyz
±2 dx2 –y2
1s orbital
±2 dxy
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Wave functions and Orbitals
 = wave function
2 = probability (electron) density
4r22 = radial distribution function
  E
H
 = Rnl(r)Ylml(,)
Rnl(r) – radial function
Ylml(,) – angular function
polar coordinates
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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
2 for d orbitals (except dz2)
A website demonstrating nodes for 2D wavefunctions can be found at:
http://www.kettering.edu/~drussell/Demos/MembraneCircle/Circle.html
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s orbitals
p orbitals
(the shading is backwards)
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d orbitals
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Summary of quantum numbers needed for H:
 Z2 
n  Energy of shell: E n   R  2 , size: radius(n) = n2a0
n 
 
l  type (degeneracy) of subshell: s(1), p(3), d(5), f(7) ….
ml  type and orientation of orbitals in a subshell
Energy level diagram:
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Many electron atoms
He, Z = 2
-
+
 Z2 
En   R  2 
n 
 22 
E1  R  2 
1 
Predict: E1 = -54.4 eV
Actual: E1 = -24.6 eV
Something is wrong with the Bohr Model!
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ms = spin magnetic  electron spin
ms = ±½
(-½ = ) (+½ = )
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
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Energy level diagram for He. Electron configuration: 1s2
Energy
0
1
2
l
3
2
1
n
H
N
paramagnetic – one (more) unpaired electrons
S
He
N
S
diamagnetic – all paired electrons
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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 - 
 Z *2 
E n   R  2 
n 
Z* is the effective nuclear charge
Z is the atomic number
 is the shielding or screening constant
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Helium , Z = 2
Predicted: E1 = -54.4 eV
Actual: E1 = -24.6 eV
-
+
 Z *2 
En   R  2 
 n 
 Z*2
- 24.6  -13.6 2 
 1 
Z* 
24.6  12
13.6
Z* = 1.34
1.34 = 2 - 
 = 0.66
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Lithium , Z = 3
Predicted: E2 = -30.6 eV
Actual: E2 = -5.4 eV
-
-
+
 Z *2 
En   R  2 
 n 
 Z *2 
-5.4  -13.6 2 
 2 
5.4  22
Z* 
13.6
-
Z* = 1.26
1.26 = 3 - 
 = 1.74
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We want to be able to predict  and Z*
Slater’s rules for the prediction of  for an electron:
1. Group electron configuration as follows:
(1s)(2s,2p)(3s,3p)(3d)(4s,4p)(4d)(4f)(5s,5p) etc.
2. 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-2 or lower group contributes 1.00
4. 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
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Example with a valence electron on oxygen: O, Z = 8
Electron configuration: 1s2 2s2 2p4
a) (1s2) (2s2 2p4)
Z* = Z - 
b)  = (2 * 0.85) + (5 * 0.35) = 3.45
1s
2s,2p
Z* = Z - 
Z* = 8 – 3.45 = 4.55
This electron is actually held with about 57% of the
force that one would expect for a +8 nucleus.
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Example with two electrons for nickel: Ni, Z = 28
Electron configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d8
(1s2) (2s2 2p6) (3s2 3p6) (3d8) (4s2)
Z* = Z - 
For a 3d electron:
 = (18 * 1.00) + (7 * 0.35) = 20.45
1s,2s,2p,3s,3p
Z* = Z - 
3d
Z* = 28 – 20.45 = 7.55
For a 4s electron:
 = (10 * 1.00) + (16 * 0.85) + (1 * 0.35) = 23.95
1s,2s,2p
Z* = Z - 
3s,3p,3d
4s
Z* = 28 – 23.95 = 4.05
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The basis of Slater’s rules for 
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
2. (n-1) s and p orbitals don’t completely shield n s and p orbitals
Chem 59-250 Periodicity of Effective Nuclear Charge
Z* on valence electrons
H
1.00
Li
1.30
Na
2.20
K
2.20
Rb
2.20
Cs
2.20
Be
1.95
Mg
2.85
Ca
2.85
Sr
2.85
Ba
2.85
B
2.60
Al
3.50
Ga
5.00
In
5.00
Tl
5.00
C
3.25
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
5.20
Cl
6.10
Br
7.60
I
7.60
At
7.60
He
1.65
Ne
5.85
Ar
6.75
Kr
8.25
Xe
8.25
Rn
8.25
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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 *2 
Hie  13.6 2 
 n 
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Shielding and Effective Nuclear Charge
Effective nuclear charge can be used to rationalize
properties such as the size of atoms and ions.
Cp*2Be
Cp*2B+
Be and B+ are isoelectronic (1s2 2s2) but very different
because of effective nuclear charge.