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Atomic Therory
Introduction to Electron
Configurations
Atomic Theory
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Each energy level eventually leads
to an atomic orbital: devised by
Schrödinger
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Atomic Orbital: Region of space
where there is a high probability of
finding and electron
Lets discuss energy levels…
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Energy level (shells) is described by the
principal quantum number
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Abbreviated by the letter (n)
Whole numbers: 1-7
 Each energy level has sublevels (subshells)
represented by letters: s, p, d, f
 Different shaped orbitals within each
sublevel
Energy level to sublevel…
Energy level (shell) one has 1 different
sublevel (subshell) (s)
 Energy level two has 2 different
sublevel (s, p)
 Energy level three has 3 different
sublevel (s, p, d)
 Energy level 4 has 4 different sublevels
(s, p, d, f)
 How about energy level 5?
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Sublevel to orbital
s sublevel: there is only 1 orbital
called s
 p sublevel: there are 3 orbitals called
px, py & pz
 d sublevel: there are 5 orbitals called
dxy, dxz, dyz, dx2y2 & dz2
 f sublevel: there are 7 orbitals
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Examples:
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Energy level 1:
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1 sublevel (1s)
one orbital of s
Total of 1 orbital
Energy level 2:
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2 sublevels (2s and 2p)
Orbitals: 2s has the one orbital of s and 2p has
the three orbitals of px py pz
Total of 4 orbitals
Examples
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Energy level 3:
 3 sublevels (3s, 3p, 3d)
 Orbitals: 3s has one orbital, 3p
has three orbitals, and 3d has
five orbitals.
 Total of 9 orbitals
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How about the 4th energy level?
What Do
Orbitals
Look Like?
p orbitals
s orbitals
d orbitals
So what about electrons?
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Maximum number of electrons that can
occupy a principle energy level is given
by the formula: 2n2 (n=principle
quantum number)
OR
Each orbital can hold 2 electrons
Example:
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Energy level 4
n=4
2n2=2(4)2=32 electrons
Energy level 4 has 4 sublevels (s, p, d, f)
s:1 orbital, p:3 orbitals, d:5 orbitals, f:7
oritals. Total=16 orbitals
16 orbitals x 2 electrons=32 electrons!!
5.2 Electron Arrangement
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Electrons actually fill the orbitals we
just discussed.
How?
The electrons arrange themselves
around the nucleus from the lowest to
highest amount of energy
Call this: electron configurations
Delete before presenting
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Use pic from before of circle orbits and
talk about how the inner orbitals are
less energy and the outer are higher
energy
Pull is less on outer so those are the
most reactive…valence electrons…key
players in bonding
Electron Configurations
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Three primary rules determine
the electron configuration of an
element:
 1) Aufbau Principle
 2) Pauli Exclusion Principle
 3) Hund’s Rule
Rule #1: Aufbau Principle
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Describes the filling order of energy
levels and sublevels
Electrons fill the orbitals of LOWEST
energy first.
Begin filling with the 1s orbital and
move higher (see next slide)
Aufbau Principle-filling order
1s,
2s,
3s,
4s,
5s,
6s,
7s,
2p,
3p,
3p,
5p,
6p,
7p,
3d
4d,4f
5d, 5f
6d, 6f
7d, 7f
Learn
this order
Rule # 2: Pauli Exclusion
Principle
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Rule: One orbital may have ONLY 2
electrons
 Electrons must have opposite
spins
 One electron points up (↑)
 One electron points down (↓)
 Ex: ↑↓
Rule # 3: Hund’s Rule
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Rule: Must fill all of the same energy
orbitals (degenerate) with one electron
before a 2nd electron is placed in the
same orbital.
Ex: If we have 3 electrons and 3
orbitals at the same energy level:
↑ ↑ ↑ NOT
↑↓ ↑ __
Unpaired electrons have PARALLEL
spins!
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Stess that it’s a lot of abstract thinking,
but it will come together when we do
electron configs coming up.