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The Quantum Mechanical
Picture of the Atom
 The allowed energy states of atoms
and molecules can be described by sets
of numbers called quantum numbers
n

m
ms
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Quantum Numbers



n - the principal quantum number
 n = 1, 2, 3, 4, ...... “shells”
 - the angular momentum quantum number

 = 0, 1, 2, 3, … , n – 1 “subshells”
 = s, p, d, f, ......
m - the magnetic quantum number


m = –, – + 1, … ,  – 1, 
ms - the spin quantum number

ms = –½, +½
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Quantum Numbers and Orbitals

n +






Define the energy of the electron
Defines the shape of the orbital
Orbital
 The volume around the nucleus where
the electron appears 90-95% of the time
The Pauli principle
 No two electrons in an atom may have
identical sets of four quantum numbers
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s orbital ( = 0)

There is only one s orbital per n level
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p orbital ( = 1)

There are 3 p orbitals per n level

They are named px , py , and pz
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d orbital ( = 2)

There are 5 d orbitals per n level
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f orbital ( = 3)

There are 7 f orbitals
per n level
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Order of Orbital Energy
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Building Atoms
1) Build the nucleus by adding the required
# of protons and neutrons according to
the atomic # and mass # of the atom
2) Fill energy levels (orbitals) with the
required # of electrons starting from
the lowest available energy level and
following Pauli and Hund rules (this is
called Aufbau principle)
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Building Atoms - Period 1
n

m
ms
H
He
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Building Atoms - Period 2
n

m
ms
Li
Be
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Building Atoms - Period 2
n

m
ms
B
C
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Hund’s Rule
Electrons occupy all the orbitals of a given
subshell (with the same ) singly before
pairing begins. These unpaired electrons
have parallel spins (the same sign of ms).
n

m
ms
C
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Building Atoms - Period 2
N
O
F
Ne
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Building Atoms - Period 3
Na
P
Mg
S
Al
Cl
Si
Ar
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Building Atoms - Period 4
K
Ca
Sc – 4p or 3d ?
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