ELECTRON
CONFIGURATION
Electron Configuration

The way electrons are arranged around
the nucleus.
Atomic Spectra and Bohr
Bohr said classical view is wrong.
Need a new theory — now called
e- can only exist in certain discrete orbits
e- is restricted to
energy
state (quanta = bundles of energy)
Quantum or Wave Mechanics
E. Schrodinger
1887-1961
Schrodinger applied idea of ebehaving as a wave to the problem
of electrons in atoms.
He developed the
Solution gives set of math
expressions called
Each describes an allowed energy
state of an e-
Heisenberg Uncertainty Principle
W. Heisenberg
1901-1976
Problem of defining nature of
electrons in atoms solved by
W. Heisenberg.
Cannot simultaneously define
the
(= m•v) of an electron.
We define e- energy exactly
but accept limitation that we
do not know exact position.
Quantum Mechanical Model




1920’s
Werner Heisenberg (
)
Louis de Broglie (
)
Erwin Schrodinger (mathematical equations using
probability,
numbers)
Principal Quantum Number, n

Indicates main
n = 1, 2, 3, 4…
 positive
integers
 As n increases, the electron’s
nucleus and the electron’s energy increases.

Each main energy level has sub-levels
 There

are 4 sublevels
from the

The principle quantum number, n, determines
the number of sublevels within the principle
energy level.
Electron Configuration



Electrons always go into the
lowest possible energy
level (nearest the nucleus).
The atomic number is the
number of protons, and hence
the number of electrons.
The electrons are arranged in
different levels, by filling up
an inner level before filling an
outer one. The energy
sublevels are filled in a
specific order as shown by the
arrow diagram given below:
Orbital Quantum Number, ℓ
(Angular Momentum Quantum Number)


This quantum number indicates the
or type
of orbital that corresponds to a particular sublevel.
ℓ = n-1
ℓsublevel
0
1
2
3
s
p
d
f
Orbital
The space where there is a
that it is occupied by a pair of electrons.
 Orbitals are solutions of Schrodinger’s equations.

Orbitals in Sublevels
Sublevel
# Orbitals
s
1
p
3
d
5
f
7
# electrons
Magnetic Quantum Number, m
(subset of the ℓ quantum number)
 Also
indicates the numbers and orientations of orbitals around
the nucleus.
 The value of m takes whole-number values, depending on the
value of ℓ.
 The




number of orbitals includes
1 s orbital
3 p orbitals
5 d orbitals
7 f orbitals
Spin Quantum Number
(orientation of an electron’s magnetic field)

The
quantum number is represented by:
+1/2 or -1/2 (

or
)
A single orbital can hold a maximum of
must have
spins.

Remember: Opposites attract and Like
electrons, which
Arrangement of
Electrons in Atoms
Electrons in atoms are arranged as
QUANTUM NUMBERS
The shape, size, and energy of each orbital is a function of 3
quantum numbers which describe the location of an electron
within an atom or ion
n (principal) --->
l (orbital) --->
ml (magnetic) --->
The fourth quantum number is not derived from the wave
function
s (spin) --->
of the electron
(clockwise or counterclockwise: ½ or – ½)
Three rules are used to build the
electron configuration:
 Aufbau
principle
 Pauli Exclusion Principle
 Hund’s Rule
Aufbau Principle

Electrons occupy orbitals of
energy first.
 So,
the order in which the orbitals are filled matches the
order of energies.

Aufbau
Diagram
-Pauli Exclusion Principle
(Wolfgang Pauli, Austria, 1900-1958)
-Electron Spin Quantum Number


An orbital can hold only
must have
spin.
electrons and they
Electron Spin Quantum Number (ms):
+1/2, -1/2
Pauli Exclusion Principle
•
•
•
•
Two electrons can have the same value of n by being in
the same main energy level.
These two electrons can also have the same value of l by
being in orbitals that have the same shape.
These two electrons may also have the same value of m
by being in the same orbital.
But these two electrons
quantum number.
•
have the same spin
If one electron has the value of 1/2, then the other electron
must have the value of –1/2.
Hund’s Rule
In a set of orbitals, the electrons will fill the orbitals in a
way that would give the
number of
parallel spins (maximum number of unpaired
electrons).
Analogy: Students could fill each seat of a school bus,
one person at a time, before doubling up.
Aufbau
Diagram for
Hydrogen
Aufbau
Diagram for
Helium
Aufbau
Diagram for
Lithium
Aufbau
Diagram for
Beryllium
Aufbau
Diagram for
Boron
Aufbau
Diagram for
Carbon
Aufbau
Diagram for
Nitrogen
Notations of Electron Configurations

Lets take a look at an example and write it in both
standard and shorthand notation.



A Li atom has 3 electrons; the third one is added to the
next lowers energy orbital, 2s and is unpaired.
Standard
 We would write it as Li:
Shorthand
 We would write it as Li:
Aufbau
Diagram for
Fluorine
Standard Notation
of Fluorine
Number of electrons
in the sub level
2
1s
2
2s
5
2p
Sublevels
Shorthand Notation




Use the
noble gas that is located in the
periodic table right before the element.
Write the symbol of the noble gas in
.
Write the
configuration after the
brackets.
Ex: Fluorine:
Blocks in the Periodic Table