EMR and the atom: Part Deux
Electron Configurations
http://imagers.gsfc.nasa.gov/ems/waves3.html
What you’ve seen so far….
Model of an atom
Which is really not true- why?

Because orbitals- the
“electron cloud” are

3-D, not flat

e-s are spread out as much as
possible
are not round in most cases
 e-s are moving very rapidly


(We can’t see this in a still
image)
Orbitals

The electrons are spread out in orbitals that
have varying



Shapes
Energy (distance from nucleus)
The orbitals are described in regards to their
quantum numbers


Descriptions that are descriptive and hierarchical
There are 4 numbers that describe an orbital

Written as follows: (#, #, #, ±#)
Principal quantum number (n)
The first number (1, #, #,±#)
 Describe the



Values for n are integers


distance from the nucleus to the orbital
The energy of the orbital
The smallest possible value is 1
As the distance from the nucleus (and
therefore energy) increases, the number
increases
Quantum numbers
There periodic table and n


The 7 periods on the periodic table
correspond to n values
Each period has a unique n value



For the 1st period, n=1
For the 2nd period, n=2
And so on….
Angular Momentum (l)
(this is a script l, as in llama)



Is the shape of the orbital
It is the second number in the description
(#,1,#, ±#)
Range from 0 to n-1 (although we never
deal with anything above l=3)




s =0
p =1
d =2
f=3
The s orbital
http://www.sfu.ca/~nbranda/28xweb/images/s_orbital.gif
p orbitals
d orbitals
d orbitals
f orbitals
General tutorials for electron
configuration stuff

some slides in this PowerPoint are from this
site already

http://www.wwnorton.com/college/che
mistry/gilbert/tutorials/ch3.htm

See key equations and concepts (select
from menu on the left), as well as the
looking through the overview where to the
tutorials are listed (links for just those are on
the left, too)
Magnetic number (ml)


Denote the orbital sublevel that is filled
It is the third number in the description
(#,#,1, ±#)




s orbitals have one sublevel; a sphere has one
orientation in space
p orbitals have three sublevels; 3 orientations in
space
d orbitals have five sublevels; 5 orientations in
space
f orbitals have seven sublevels; 7 orientations in
space
“Flavors” of ml

s sublevels
have one
orbital; a
sphere has
one orientation
in space
“Flavors” of ml

p sublevels have three orbitals; 3
orientations in space
“Flavors” of ml

d sublevels
have five
orbitals; 5
orientations in
space
“Flavors” of ml

f sublevels
have seven
orbitals; 7
orientations
in space
Magnetic number (ml)




Denote the orbital sublevel that is filled
It is the third number in the description
(#,#,1, ±#)
Values of –l to l, (integers only)
For




s
p
d
f
ml = 0 only since l= 0
ml = -1,0,1 since l= 1
ml = -2,-1,0,1,2 since l= 2
ml = -3,-2,-1,0,1,2,3 since l= 3
Spin


It is the last number in the description
(#,#,#,±½)
Spin is +½ or -½

Up or down
Summary, excluding spin
How we use this….

There is a specific order to how the efill the orbitals; it is not random

Although there are exceptions to the
rules (last thing we do)
The principles of econfiguration

The Aufbau (next) Principle:


The Pauli Exclusion Principle:


That e- fill the lowest energy sublevel before
going to the next sublevel
That e-s are paired according to opposite spins
Hund’s Rule:

e-s spread out in equal energy sublevels before
placing electrons

The first level to fill is the 1s level


It is the lowest energy sublevel
It holds two electrons


They are oppositely paired (up and down- ↑↓)
Each sublevel (each __) holds 2 electrons
Next…



The second sublevel is the 2s sublevel
It also holds 2 electrons (because s
holds 2, not because of the number),
also oppositely paired ↑↓
1s2, 2s2,then comes 2p6

So, as it states
above

1s fills, 2s fills ,then
comes 2p


It holds up to six
electrons
Because p orbitals
hold 6 electrons
Next…

From 2p,







3s fills with 2e-, then onto
3p, with 6e- then
4s with 2e- followed by
3d with 10e- (because d
holds 10e-)
Then 4p with 6e-
Notice, you follow the
arrows
Remember, the number
of electrons comes from
the letter (the orbital’s
momentum,ml)



The sublevels of the
orbitals are first filled,
then you continue onto
the next level (Aufbau)
Also be sure to place
one electron in each
sublevel prior to filling
the level (↑ ↑ ↑ and
not ↑↓ ↑ _) (Hund)
e-s must be paired with
e-s of opposite spin
(↑↓, not ↑↑ or ↓↓)
(Pauli)
Putting it all together…


Carbon (neutral, so 6 electrons)
What this would look like:
↑↓ ↑↓ ↑ ↑ _
1s 2s 2p
(notice there are 6 arrows for 6 electrons)


This can also be written as 1s2 2s2 2p2
Notice the superscripts add up to 6
There are some exceptions…

This is because some energy levels are very close
together



electrons are able to move between close orbitals in order
to minimize repulsion
Example: the 4s and 3d orbitals are very close in
energy
So exceptions for some period 4 d block elements
occur



Cr is not 1s2 2s2 2p6 3s2 3p6 4s2 3d4
Cr is 1s2 2s2 2p6 3s2 3p6 4s1 3d5
Because it takes less energy to split the electrons between
the 5 sublevels than it does to put them together in the 4s
and 3d