1. Noble Gas Notation Electron Configuration of Ions & Aufbau Exceptions
Lecture 5D

2. Noble Gas Notation
Inside an atom the most important electrons are found in the highest energy level.
These electrons are called valence electrons (usually in s and p orbitals). They determine the chemical reactivity of an element.
All other electrons in the atom are below the valence shell and are called core electrons. The core electrons are not directly involved in chemical reactions of the element.

3. Noble Gas Notation For example: Co: 1s 2 2s 2p 6 3s 4s 3d 7 3p Ar: 1s
Core electrons
Co: 1s 2 2s 2p 6 3s 4s 3d 7 3p
Ar: 1s 2 2s 2p 6 3s 3p
Note: The core electrons for Cobalt are exactly the same electrons as the full e- configuration for Argon!

4. Noble Gas Notation
Now we can shorten the e- config. and orbital diagrams for Cobalt:
Co: 1s 2 2s 2p 6 3s 4s 3d 7 3p
Co:
[Ar] 4s 2 3d 7
[Ar] 4s 3d
Co:

5. Noble Gas Notation Let’s practice this! [??] Sr:
How do we know which Noble gas to use as the “core” electrons?
[??]
Sr:
Use our P.T. of the Electrons!

6. Noble Gas Notation [Kr] Sr: 5s2 Place your finger on Sr.
Then move up one period and over to the Noble gas.
Place your finger on Sr.
It’s Krypton! Kr
[Kr]
Then continue on after Kr, writing in all electrons until you get to Sr.
Sr:
5s2

7. Ion Electron Configuration
cations – cations – cations - cations 3
valence electrons
Al: 1s 2 2s 2p 6 3s 3p 1
Al+3: 1s 2 2s 2p 6 Ne
Note: Metals will lose their valence electrons in order to “look like” a Noble gas!
Al+3 ion has the same electron configuration as
Neon!

8. Ion Electron Configuration
anions – anions – anions - anions 6
valence electrons S: 1s 2 2s 2p 6 3s 3p 4 8
valence electrons
S-2: 1s 2 2s 2p 6 3s 3p Ar
Note: Nonmetals will gain valence electrons in order to “look like” a Noble gas!
S-2 ion has the same electron configuration as
Argon!

9. Aufbau Exceptions The Filled “d” Orbital
We would expect the electron configuration of Copper to be…
Cu:
[Ar] 4s 2 3d 9
Cu:
[Ar] 4s 3d
But nature likes symmetry and really wants to completely fill orbitals with electrons. And copper only needs one more – just one more - electron to have a filled “d” orbital...

10. Aufbau Exceptions The Filled “d” Orbital
So it “steals” an electron away from the closest s-orbital to completely fill the d-orbital.
Cu:
[Ar]
4s2
3d9
Cu:
[Ar]
4s1
3d10

11. Aufbau Exceptions The Filled “d” Orbital
What other element would you expect to behave the same way?
Silver
Ag:
[Kr]
5s2
4d9
Ag:
[Kr]
5s1
4d10

12. Aufbau Exceptions The Half-Filled “d” Orbital
We would expect the electron configuration of Chromium to be…
Cr:
[Ar] 4s 2 3d 4
Cr:
[Ar] 4s 3d
But nature likes symmetry and really wants at least half-fill orbitals with electrons. Chromium only needs one more – just one more - electron to have a half-filled “d” orbital...

13. Aufbau Exceptions The Half-Filled “d” Orbital
So it “steals” an electron away from the closest s-orbital to build a half-filled d-orbital.
Cr:
[Ar]
4s2
3d4
Cr:
[Ar]
4s1
3d5

14. The Principal Quantum Number (n)
Energy levels are in designated quantum #’s (n).
n = 1, 2, 3, 4, 5, 6 &
A Quantum number is equal to the period
Higher the quantum number, the greater average distance from the nucleus
Nucleus
n = 3
n = 2
n = 1

15. Example: What is the quantum number (n) of: Potassium Helium Lead

16. The Angular Momentum Quantum Number (l)
Tells us the shape of the orbitals.
The value of l is dependent on the Principal Quantum Number.
For a given value of n, l has possible values of 0 to (n-1). l 1 2 3 4 5
Name of Orbital s p d f g h

17. Example: What would l be for these orbitals? 2s 3d 4p 3f2 1 3

18. The Magnetic Quantum Number (ml)
Describes the orientation of the orbital in space.
For a given value of l, there are (2l +1) integral values of ml as follows:
-l, (-l + 1)…0…(+l – 1), +l l 1 2 3 ml
-1, 0, 1
-2, -1, 0, 1, 2
-3, -2, -1, 0, 1, 2, 3