Electronic Configurations
Many-electrons atoms
• Dealing with many-electron atoms becomes quickly very
complicated:
– Electrons interact with nucleus (+Ze)
– Electrons interact with each other, but they also induce
screening effects
 Complex potential interactions
• It is impossible to solve analytically the Schrödinger
equation for many-electron atoms.
– Need help of (powerful) computers
• But: it is possible to understand some chemical
properties of the elements by looking in quantum
mechanics.
The Pauli Exclusion Principle
• 1925 – Wolfgang Pauli 1900-1958
formulates the exclusion principle:
“No two electrons in an atom may have the
same set of quantum numbers (n,l,ml,ms)”
The Pauli exclusion principle applies to all half-integer spin particles (known as
“Fermions”). This is very important, since protons and neutrons are fermions too
(Nuclear Physics)
One more observation:
The electrons in an atom tend to occupy the lowest energy levels available to them.
Atomic Shells
•
How many electrons can we put in an energy level defined by n ?
– n=1
l =0
ml=0
ms= ±½
– n=2
l =0
l =1
ml=0
ms= ±½
ml=-1
ml=0
ml=1
ms= ±½
ms= ±½
ms= ±½
l =0
l =1
ml=0
ms= ±½
ms= ±½
ms= ±½
ms= ±½
l =2
ml=-1
ml=0
ml=1
ml=-2
ml=-1
ml=0
ml=1
ml=2
ms= ±½
ms= ±½
ms= ±½
ms= ±½
ms= ±½
– n=3

2

8

18
Designation
•
•
n – label of the electron shell
l – electron sub-shell
l 0 1 2 34 5
6
s p d f g h g
Fine structure is changing slightly the
ordering of the shells that are filled.
Large l-value shells appear more
shielded (do not feel the full intensity
of the attractive force) than small lvalue shells  higher energy levels.
The periodic table of elements
• Dimitri Mendeleev 1834–1907
– Creates the periodic table of elements,
by ranging the 60 known elements (at
the time) according to their chemical
properties
Not well accepted, until elements are discovered
with the chemical properties he predicted.
- Gallium (1875)
- Scandium (1879)
- Germanium (1886)
Groups / Periods
Groups:
 Vertical columns.
 Same number of electrons in an ℓ orbit.
 Can form similar chemical bonds.
Periods:
 Horizontal rows.
 Correspond to filling of the subshells.
•
Some properties of elements are compared by the ionization energies of
elements and atomic radii.
Elements in the
periodic table (I)
• Chemical reactivity of certain elements reveal
the basic factors controlling binding atoms to
form molecules
• Noble gases / Inert gases are chemically inert:
He (Z=2), Ne (Z=10), Ar(Z=18), etc…
–
–
–
–
He: (1s)2
Ne: (1s)2 (2s)2 (2p)6
Ar: (1s)2 (2s)2 (2p)6 (3s)2 (3p)6
Etc…
 Their electronic configurations correspond to
completely filled major shells.
Rn
Xe
Kr
Ar
Ne
He
Elements in the periodic table (II)
Alkalis:
• Single s electron outside an inner core
• Easily form positive ions with a charge +1e
• Lowest ionization energies, largest atomic radii
• Electrical conductivity is relatively good
Halogens:
• Need one more electron to fill outermost
subshell
• Form strong ionic bonds with the alkalis
• More stable configurations occur as the p
subshell is filled
H: (1s)1
Li: (1s)2(2s)1
Na: (1s)2(2s)2(2p)6(3s)1
Etc…
F: (1s)2(2s)2(2p)5
Cl: (1s)2(2s)2(2p)6(3s)2(3p)5
Etc…
Elements in the periodic table (III)
Alkaline Earths:
• Two s electrons in outer subshell
• Large atomic radii (similar to Alkaline metals)
• High electrical conductivity
Lanthanides (rare earths):
• Have the outside 6s2 subshell completed
• As occurs in the 3d subshell, the electrons in the 4f subshell have
unpaired electrons that align themselves
• The large orbital angular momentum contributes to the large
ferromagnetic effects
Actinides:
• Inner subshells are being filled while the 7s2 subshell is complete
• Difficult to obtain chemical data because they are all radioactive
• Have longer half-lives
Elements in the periodic table (IV)
Transition Metals:
• Three rows of elements in which the 3d, 4d, and 5d are being filled
• Properties primarily determined by the s electrons, rather than by
the d subshell being filled
• Have d-shell electrons with unpaired spins
• As the d subshell is filled, the magnetic moments, and the tendency
for neighboring atoms to align spins are reduced