Atomic Electron Configurations and
Chemical Periodicity
We know the electronic structure of the hydrogen atom
states as determined by the quantum numbers n, l and m.
How does this apply to larger atoms?
i.e. multiple electron systems
How does the electron structure relate to the periodic
table ?
How does the electron structure relate to the chemical
properties of atoms ?
Electron Spin and Magnetism
Before we can talk about structure we need to learn a bit
about the magnetic properties of particles.
Recall that electron move the nucleus in orbits
corresponding to set angular momentum values
v
Recall, also that when electrons move
they generate a magnetic field, B.
This is analogous to electrical current
moving through a loop
B
Electron Spin and Magnetism
Electrons in orbit generate magnetic fields, therefore all materials are
magnetic. Is that so?
Imagine two electrons in the same orbit moving in opposite directions.
The magnetic fields cancel !!
Do electrons occur in pairs in orbitals!!!
Yes!
But not for this reason, since this is not physically correct.
v
B
v
Motion of electrons in their
orbitals is not responsible
for magnetism, even when
the electron is unpaired.
B
The net magnetic field
averages to zero.
Electron Spin and Magnetism
When a beam of atomic hydrogen is passed through a
non-uniform magnetic field is splits into two beams
This Magnetism
is not due to due
to orbital motion
Another source
of magnetism
From where?
Spin
Electron Spin and Magnetism
Electron spin is an inherent magnetism associated with
it, which has nothing to do with its translational motion.
The electron can the thought of as a little magnet
When an external magnetic field is applied the electron
will either align along or against the field.
B
Being aligned with the field is more stable
than against, therefore the up orientation is
slightly favored
Magnetic
field
More stable Less stable
UP (s=1/2) DOWN (s=-1/2)
The distribution of up to
down depends on strength of
the applied magnetic field.
Magnetic Materials
Diamagnetic Materials
Composed of atoms/molecules containing only paired electrons
They are repelled by an externally applied magnetic field.
Paramagnetic Materials
Composed of atoms/molecules with
unpaired electrons.
More electron electrons will align with
the field than against the externally
applied field.
The result is a net bulk magnetic field
parallel to the applied field, hence an
attractive force
Magnetic Materials
Ferromagnetic Materials – Have a permanent magnetic field
When two electron on separate
atoms are close, the field from
one will cause the other to align
with it as it would be more stable
The magnetic field from each atom
will add up, as long as the atoms
are correctly aligned to give a one
strong “bulk’ magnetic field. – ie.
Magnets
Pauli Exclusion Principle
Fermions - particles have spin ½.
electrons
protons
neutrons
“Fermions cannot occupy the same space and spin coordinates”
This means that no two electrons can have the same quantum numbers,
including the spin quantum numbers.
Therefore each orbital can only have 2 electrons since there are only two
spin states s =1/2 and -1/2.
Example: 1s orbital n = 1, l = 0, m = 0 and s = 1/2 or s = -1/2
1s Orbital
Atoms with more than one electron
Multi-electron wavefunctions are similar to those for the H atom
The ground state of such atoms requires that the lowest possible energy
wavefunctions be “occupied”
box diagram - a simple tool used to add or subtract electrons from the boxes
to represent the electron configuration of the element
Consider H, He, Li and Be
Hunds rule
Element # e’s
B
5
C
6
N
7
O
8
F
9
Ne
10
1s
2s
2p
Electrons
added to each
empty orbital
in parallel
When no new
orbitals are
available they
are paired
Maximize spin
Electron Configuration
A shorthand notation is commonly used to write out the electron configuration
of the atoms based on the number of electrons within each subshell
It consists of:
NUMBER
(shell i.d.)
Be  1s 2 s
1
2
1s1
2
1
2s1
3
LETTER
(subshell)
SUPERSCRIPT
(occupancy)
18
2
2s2
4
3s1
11
4s1
19
5s1
37
6s1
55
7s1
87
13
14
15
16
17
2p1
2p2
2p3
2p4
2p5
5
3s2
3
4
5
6
7
8
9
10
11
12
3d1
3d2
3d3
4s13d5
3d5
3d6
3d7
3d8
4s13d10
3d10
12
4s2
20
5s2
38
6s2
21
4d1
39
4d2
40
72
Ac-Lr
88
23
24
5s14d4
5s14d5
41
42
5d2
La-Lu
56
7s2
22
5d3
73
6d2
104
25
105
4d5
43
5d4
74
6d3
26
5d5
106
45
5d6
76
6d5
107
28
29
30
5s14d7 5s14d8 5s04d10 5s14d10
44
75
6d4
27
46
5d7
77
6d6
108
47
6s15d9
78
79
110
111
3p1
13
4d10
4p1
80
3p2
5p1
4p2
81
3p3
5p2
4p3
82
3p4
4p4
5p3
52
6p3
83
3p5
4p5
84
5p5
6p5
85
4f05d1
5f06d1
89
4f15d1
58
5f06d2
90
4f3
59
5f26d1
91
4f4
60
5f36d1
92
4f5
61
5f46d1
93
4f6
62
5f6
94
4f7
63
5f7
95
4f75d1
64
4f9
65
5f76d1
96
5f9
97
4f10
66
5f10
98
4f11
67
5f11
99
4f12
68
5f12
100
4f13
69
5f13
101
4f14
70
5f14
102
5p6
6p6
86

57
4p6
54
6d7
109
3p6
36
53
6p4
2p6
18
35
5p4
2
10
17
34
51
6p2
9
16
33
50
6p1
8
15
32
49
5d10
7
14
31
48
6s25d10
6
1s2
5d1
71
6d1
103
Electron Configuration
Element # e’s
1s
2s
2p
B
5
1s2 2s2 2p1
C
6
1s2 2s2 2p2
N
7
1s2 2s2 2p3
O
8
1s2 2s2 2p4
F
9
1s2 2s2 2p5
Ne
10
1s2 2s2 2p6
Aufbau order and Energy Levels
The sequence of subshells in the electron
configurations not the same as the energy
levels of H
The experimental sequence is known as
the aufbau order
It is a consequence of electron-electron
interactions have on the energies of the
wavefunctions in all multi-electron atoms
Levels within subshells are still
degenerate, the subshells in each shell
are no longer degenerate in each shell,
and differ in energy as s < p < d,
Some subshells can overlap the levels of a
different shell; thus, for example, in neutral
atoms 4s lies below 3d
Traditional aufbau sequence diagram
Instead of filling orbitals in order of increasing n, we should really be
filling them in order of increasing n + l
n is used as a ‘tiebreaker’
i.e the one with lowest n
first
Ex) Fluorine 9 e’s
1s2 2s2 2p5
Ex) Scandium 21 e’s
1s2 2s2 2p63s2 3p6 4s23d1
Ex) Strontium 38 e’s
1s2 2s2 2p63s2 3p6 4s23d10
4p6 5s2
Afbau sequence from Periodic Table
The periodic table can be used to determine the afbau order instead
As you increase the # electrons, the block structure indicates the sequence of
subshells
We can appreaciate that the origin of the periodic table is the electron configurations
of the elements
p block
1.008
H
2
1
6.939
9.012
Li
Be
3
22.990
4
24.312
Na
11
39.102
d block
Mg
12
40.08
K
Ca
19
85.47
20
87.62
3
44.956
4
47.90
Sc
21
88.905
5
6
7
8
9
50.942
51.996
54.938
55.847
58.933
V
Cr
Mn
Fe
Co
26
101.07
27
102.90
Ti
22
91.22
23
92.906
24
95.94
25
(98)
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
37
132.91
38
137.33
39
138.91
40
178.49
41
180.95
42
183.85
43
186.21
44
190.22
45
192.2
Cs
55
(223)
Fr
87
Ba
La
56
226.025
57
227.029
Ra
88
s block
Ac
89
Hf
Ta
W
Re
Os
Ir
72
(261)
73
(262)
74
(263)
75
(262)
76
(265)
77
(266)
Rf
Ha
Sg
Ns
Hs
Mt
10
11
12
65.37
14
15
16
17
10.811
12.011
14.007
15.999
18.998
B
C
N
O
F
Ne
5
26.982
6
28.086
7
30.974
8
32.064
9
35.453
10
39.948
Al
Si
13
69.72
14
72.59
P
15
74.922
S
16
78.96
63.546
Cu
Zn
Ga
Ge
As
Se
Br
Kr
28
106.4
29
107.87
30
112.40
31
114.82
32
118.69
33
121.75
34
127.60
35
126.90
36
131.30
Pd
Ag
Cd
In
Sn
Sb
46
195.09
47
196.97
48
200.59
49
204.38
50
207.19
51
208.98
Pt
78
new
Au
79
Hg
Tl
Pb
Bi
Te
I
Xe
52
(209)
53
(210)
54
(222)
Po
81
82
83
84
85
174.97
109
110
111
140.12
140.91
144.24
(145)
150.36
151.97
157.25
158.93
162.50
164.93
167.26
168.93
173.04
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
58
232.04
59
231.04
60
238.03
61
237.05
65
(247)
66
(251)
Pa
91
U
92
Np
93
Pu
94
Am
95
Cm
96
Rn
86
new
108
64
(247)
At
80
107
Th
Ar
18
83.80
Ni
106
90
Cl
17
79.904
2
20.183
58.71
105
63
(243)
He
13
104
62
(244)
4.003
Bk
97
Cf
98
67
(252)
Es
99
68
(257)
Fm
100
69
(258)
Md
101
70
(259)
No
102
Lu
71
(260)
Lr
103
f block
A more detailed look at the block structure
1
2
3
2
3
3
4
5
6
4
5
6
7
4
5
4
5
6
Electron Configurations
Electron configurations for the larger elements are lengthy to write out.
noble gas notation - the symbol for a noble gas is used as an
abbreviation for its electrons.
The core electrons are represented by the noble gas followed by
configuration of the valence electrons.
Ex)
Ne has an electron configuration of 1s22s22p6.
For Na, we can write either 1s22s22p63s1 or [Ne]3s1
Ex) Sr 38
Kr 36
1s2 2s2 2p63s2 3p6 4s23d10 4p6 5s2
1s2 2s2 2p63s2 3p6 4s23d104p6
[Kr] 5s2
Valence e’s
Core e’s
Exercises
Identify the elements with the following electron configurations.
N
a) 1s22s22p3
b) 1s22s22p63s23p64s23d7 Co
c) [Ne]3s23p3 P
d) [Kr]5s24d5 Tc
How many core and valence electrons do these atoms have?
2
5
a) ____core,
____valence
18
9
b) ____core,
____valence
10
5
c) ____core,
____valence
36
7
d) ____core,
____valence
Exceptions to the aufbau order
Exceptions to Afbau order result of:
Full shell stability
Half Shell stability
Stability of higher spin state
1
18
1s1
2
1
2s1
3
13
2s2
2p1
4
3s1
11
4s1
19
5s1
37
6s1
55
7s1
87
Cr
3s2
3
12
4s2
20
5s2
38
6s2
4
3d1
21
39
4d2
40
72
88
3d3
5s14d4
5s14d5
41
42
6d2
104
105
4d5
5d4
106
3d6
5d5
107
3d7
10
11
3d8
4s13d10
28
29
45
5d6
46
5d7
77
6d6
108
47
6s15d9
79
110
111
57
5f06d1
89
3p1
4p1
5p1
80
4p2
81
4p3
5p2
51
6p2
82
3p4
4p4
52
6p3
83
3p5
4p5
84
3p6
4p6
36
5p5
53
6p4
2p6
18
35
5p4
1s2
2
10
17
34
5p3
2p5
9
16
33
50
6p1
3p3
17
2p4
8
15
32
49
5d10
3p2
16
2p3
7
14
31
4d10
2p2
6
13
15
5p6
54
6p5
85
6p6
86
6d7
109

4f05d1
3d10
48
6s15d10
78
12
30
5s14d7 5s14d8 5s04d10 5s14d10
76
6d5
9
27
44
75
6d4
8
26
43
74
6d3
3d5
25
5d3
73
7
4s13d5
24
5
Cu
6
23
5d2
La-Lu
Ac-Lr
5
3d2
22
4d1
56
7s2
14
Developed by Prof. R. T. Boeré (updated January, 1999)
4f15d1
58
5f06d2
90
4f3
59
5f26d1
91
4f4
60
5f36d1
92
4f5
61
5f46d1
93
4f6
62
5f6
94
4f7
63
5f7
95
4f75d1
64
4f9
65
5f76d1
96
66
5f9
97
4f10
5f10
98
4f11
67
5f11
99
4f12
68
5f12
100
4f13
69
5f13
101
4f14
70
5f14
102
5d1
71
6d1
103
Valence Revisited
Electron configurations for fourth row from Gallium and beyond.
Ex) Ga
At. No. = 31 1s2 2s2 2p63s2 3p6 4s23d10 4p1
Ex) Ar
At. No. = 18
1s22s22p63s23p6.
Then for Ga we should write: [Ar] 4s23d104p1
What is the valence for Ga?
It has 3d10 electron that belong to the 3 shell not the 4 shell, hence it is
strictly speaking not part of the valence if it is complete and should be
considered as part of the core.
Therefore: [Ar]3d10 is the core
and
4s24p1 is the valence
How about Thallium 81?
Electron configurations of ions
Electron configurations of ions can be determined from that of
the neutral atom, i.e. electron configurations predict ions
Oxide forming from oxygen:
O = 1s22s22p4
O2- = 1s22s22p6
Ne = 1s22s22p6
Same electron configuration as neon
Magnesium cation from magnesium:
Mg = 1s22s22p63s2
Mg2+ = 1s22s22p6
Ne = 1s22s22p6
Same electron configuration as neon
This rationalizes the kinds of stable ions that are formed for certain elements
Electron configurations of ions
Thus, cation electron configuration is obtained by removing electrons in
the reverse Aufbau sequence
Anion electron configurations are obtained by adding electrons in the
usual Aufbau sequence
Ions try to achieve:
(1) the closest noble gas configuration
(2) a pseudo noble gas configuration (closed d or f subshell)
(3) a noble gas configuration for everything except d or f electrons
Cations always have their electron configurations in the sequence of the H.
E.C of Element
1. Li+
1s22s1
2. Br-
[Ar]4s23d104p5
Core
1s 2  [ He]
[Ar]3d10
Valence
2s1 = 1 e4s24p5 = 7 e-
Nearest stable Core
1s 2  [ He]
[Ar]4s23d104p6=[Kr]
Electron configurations of ions
E.C of Element
Nearest stable Core
Valence
1s22s22p2
1s 2  [ He]
2s22p2
2. P3-
[Ne]3s23p3
1s 2 2s 2 2 p6 3s 2 3 p6  [ Ar ]
3p3
3. Ga3+
[Ar]3d104s24p1
1s 2 2s 2 2 p 6 3s 2 3 p3 3d 10
4s24p1
4. Sn2+
[Kr]5s24d105p2
[ Kr ]5s 2 4d 10
5p2
5. Sn4+
[Kr]5s24d105p2
1.
C4+
[ Kr ]4d
10
5s25p2
Exercise
Which of the following ions are likely to form? For those
which are not what ion would you expect to form from that
element?
a) O2b) Mg6-
c) Cl+
d) Ca+
e) Pb4+
f) Ga3+
a) 8
b) 12
O = 1s22s22p4
10 O2- =1s22s22p6 = Ne
Mg = 1s22s22p6 3s2
18 Mg6- = 1s22s22p6 3s2 3p6 = Ar
c) 17
Cl = 1s22s22p6 3s23p5
16 Cl1+ = 1s22s22p6 3s23p4
d) 20
Ca = 1s22s22p6 3s23p6 4s2
19 Ca1+ = 1s22s22p6 3s23p6 4s1
e) 82
Pb = [Xe]6s24f145d106p2
78 Pb4+ = [Xe]4f145d10
Ga = [Ar]4s23d104p1
28
f ) 31
Ga3+ = [Ar]3d10
Order of Energy Levels in Ions
“Aufbau” energy levels:
s below d
“Aufbau” energy levels
In Cations
Electrons more
strongly bound as e-n
interaction are
increased
Energy levels in
Anions
Electrons less strongly
bound as e-n
interaction are
decreased