Electron
Configurations
“Any one who is not shocked
by Quantum theory does
not understand it.”
Niels Bohr
Electron Configurations

The quantum mechanical model of the atom
predicts energy levels for electrons; it is
concerned with probability, or likelihood, of
finding electrons in a certain position.
Electron Configurations

Regions where electrons are likely to be
found are called orbitals. EACH ORBITAL
CAN HOLD UP TO 2 ELECTRONS!
Electron Configurations

In quantum theory, each electron is assigned
a set of quantum numbers
 analogy: like the mailing address of an
electron
Principal Quantum Number (n)


Describes the energy level that the electron occupies
n=1, 2, 3, 4
n=1
n=2
n=3
n=4

The larger the value of n, the farther away from the
nucleus and the higher the energy of the electron.
**Electrons always start filling in the lowest
possible energy level available (AUFBAU
PRINCIPLE)
Sublevels (l)

The number of sublevels in each energy level is
EQUAL to the quantum number, n, for that energy
level.

Sublevels are labeled with a number that is the
principal quantum #, and a letter: s, p, d, f (ex: 2 p
is the p sublevel in the 2nd energy level)
***REMEMBER THIS… let’s start filling in the chart
Sublevels (l)
Principal Energy Level
Sublevels
Orbitals
n=1
1s
2s
2p
one (1s)
one (2s)
three (2p)
n=3
3s
3p
3d
one (3s)
three (3p)
five (3d)
n=4
4s
4p
4d
4f
one (4s)
three (4p)
five (4d)
seven (4f)
n=2
Sublevels (l)
Sublevel
# of orbitals
Max # of electrons
s
1
2
p
3
6
d
5
10
f
7
14
ONLY 2 electrons per 1
orbital!!!!!
Electron Configurations
NOTICE!!!!!! 3d is on the 4th
energy level (d block is n-1)
Complete electron configurations

helium
1s2

boron
1s22s22p1

neon
1s22s22p6

aluminum
1s22s22p63s23p1

Uranium
(follow your
periodic table)
1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p67s25f4
Abbreviated electron diagrams





helium
boron
(AKA: noble gas config.)
1s2
[He]2s22p1

N3-
= [Ne]
aluminum [Ne]3s23p1
cobalt
[Ar]4s23d7
uranium
[Rn]7s25f4
[He]2s22p6


Se2-
Mg2+
= same # of
electrons as a Kr
atom: [Ar]
4s23d104p6
= same # of
electrons as a
Ne atom: [He]
2s22p6
Spin quantum number (ms)

Labels the orientation of the electron

Electrons in an orbital spin in
opposite directions; these
directions are designated as
+½ and -½
Pauli Exclusion Principle

States that no 2 electrons have an
identical set of four quantum #’s to
ensure that no more than 2 electrons
can be found within a particular
orbital.
Hund’s Rule

Orbitals of equal energy are each occupied by
one electron before any pairing occurs.



Repulsion between electrons in a single orbital is
minimized
(ex: you want your own bedroom before you would
have to share a bedroom with a sibling)
All electrons in singly occupied orbitals must have
the same spin (such as in the p sublevel).
When 2 electrons occupy the same orbital they must
have opposite spins
(want to be a little different. You wouldn’t want to
wear the exact same clothes as your brother or
sister, right?!)
Orbital Diagrams


Each orbital is represented by a box.
Each electron is represented by an arrow.
Orbital Diagrams

hydrogen
1s

helium
1s

carbon
1s
2s
2p