1. The Spin Quantum Number2

2. Summary of Bohr’s Model (1913)
Electrons are in different orbits at fixed distances from nucleus.
Electrons that leave one orbit must move to another orbit.
Electrons only change orbits if specific amounts (quanta) of extra energy from the outside world are involved.
Electrons that receive enough extra energy from the outside world can leave the atom they are in.
Electrons that return to orbits they used to reside in give up the extra energy they acquired when they moved in the first place.
Electronic energy given up when electrons move back into an original orbit often shows up as a specific color light.

3. Three Quantum Number Bohr Atom Model1s 2s 2p y x z 1s 2s 2p y x
Three Quantum Number Bohr Atom Model
Two quick review questions.
(1)
What are the 5 lowest energy terms (orbits) for a nitrogen? 2 1s , 2 2s , 1 2p x , 2p y 1 , 2p z 1
(2)
What are the names of the lowest energy terms (orbits) for a carbon atom.
(Remember that carbon’s atomic number is 6) 1s 2 , 2s 2 , 2p x 1 , 2p y 1
The carbon atom has 6 protons and 6 electrons.

4. Four Quantum Number Bohr Atom Model1s 2s 2p x 2p y
The three quantum number atom model was very successful explaining various light energies observed in the spectra of atoms. 2p z
However, in some cases, there were still spectra lines that were clearly different colors but very close together.
A fourth and final quantum number was added to the Bohr model to account for these light waves that differed by only a small amount of energy.
The spin quantum number

5. Four Quantum Number Bohr Atom Model1s 2s 2p y x z
Four Quantum Number Bohr Atom Model n
the principal quantum number l
the angular momentum (azimuthal) quantum number m
the magnetic quantum number s
the electron spin quantum number
Since each electron has a negative charge, two electrons do not like to be near each other.
Originally, it was imagined that an electron spins on its axis just as the earth spins on its axis.
If two electrons had to be close to each other, one would spin one way (clockwise) and the other would spin the other way (counter clockwise.)

6. Four Quantum Number Bohr Atom Model1s 2s 2p y x z
Four Quantum Number Bohr Atom Model
the principal quantum number
the angular momentum (azimuthal) quantum number n l
the magnetic quantum number m s
the electron spin quantum number
the electron spin quantum number
Each spin direction, clockwise and counter clockwise, was assigned a value and identified as the s quantum number value.
The spin quantum number value for an electron spinning clockwise
is equal to
Most of the time, people say 1/2 instead of 0.5.

7. Four Quantum Number Bohr Atom Model1s 2s 2p y x z
Four Quantum Number Bohr Atom Model
the principal quantum number
the angular momentum (azimuthal) quantum number n l
the magnetic quantum number m s
the electron spin quantum number
EXAMPLE: Using the four quantum number model, uniquely identify the lowest energy term (orbit) for a hydrogen atom.
1, 0, 0, +1/2
The spin quantum number value for an electron spinning clockwise
is equal to

8. Four Quantum Number Bohr Atom Model1s 2s 2p y x z
Four Quantum Number Bohr Atom Model
the principal quantum number
the angular momentum (azimuthal) quantum number n l
the magnetic quantum number m s
the electron spin quantum number
The spin quantum number value for an electron spinning clockwise
is equal to + 1/2.
The spin quantum number value for an electron spinning counter clockwise
is equal to - 1/2.

9. Four Quantum Number Bohr Atom Model
the principal quantum number
the angular momentum (azimuthal) quantum number n l
the magnetic quantum number m s
the electron spin quantum number
If two electrons have to reside close together, they will spin in opposite directions.
s = +1/2
s = +1/2
-1/2 = s

10. Four Quantum Number Bohr Atom Model
the principal quantum number
the angular momentum (azimuthal) quantum number n l
the magnetic quantum number m s
the electron spin quantum number
Using the four quantum number model, describe the lowest energy term (orbit) for a hydrogen atom.
1, 0, 0, +1/2
Using the four quantum number model, describe the two lowest energy term (orbits) for a helium atom.
1, 0, 0, +1/2
1, 0, 0, -1/2

11. Four Quantum Number Bohr Atom Model
Try two more examples.
(1)
Name the three lowest energy terms (orbits) for a lithium atom.
1, 0, 0, +1/2
1, 0, 0, -1/2
Remember that lithium is atomic number 3.
2, 0, 0, +1/2
1, 0, 0, +1/2
1, 0, 0, -1/2
2, 0, 0, +1/2
(2)
Name the four lowest energy terms (orbits) for a beryllium atom
Remember that beryllium is atomic number 4.
2, 0, 0, -1/2
EVERY electron has its own individual set of 4 quantum number values

12. Four Quantum Number Bohr Atom Model
The Exclusion Principle
No two electrons in the same atom can have the same set of 4 quantum number values.
Wolfgang Ernst Pauli
Wolfgang Ernst Pauli
Nobel prize in 1945

13. Four Quantum Number Bohr Atom Model
the principal quantum number
the angular momentum (azimuthal) quantum number n l
the magnetic quantum number m s
the electron spin quantum number
EVERY electron has its own individual set of quantum number values.
Short hand way to use 4 quantum numbers to describe electron energy levels.
The three lowest energy terms (orbits) for a lithium atom.
1, 0, 0, +1/2
1, 0, 0, -1/2
2, 0, 0, +1/2
The short hand notation for lithium atom energy levels. 1s 2s

14. Four Quantum Number Bohr Atom Model
The short hand notation for lowest lithium atom energy levels 2s 1s
Try three more examples.
(1)
What are the lowest energy terms (orbits) for a nitrogen atom (atomic number 7)? 1s 2s 2p 2p 2p x y z
(2)
What are the lowest energy terms (orbits) for a fluorine atom (atomic number 9). 1s 2s 2p 2p 2p x y z
(3)
What are the lowest energy terms (orbits) for a neon atom (atomic number 10). 1s 2s 2p 2p 2p x y z

15. Four Quantum Number Bohr Atom Model
Atoms from atomic number 1 (hydrogen) through atomic number 18 (argon) fill their energy levels (orbits) using a repetitive pattern.
2 electrons 1s
A maximum of 9 energy states
2 electrons
6 electrons 2s [ 2p x 2p y 2p z ]
A maximum of 2 electrons in each energy state
2 electrons
6 electrons 3s [ 3p x 3p y 3p z ]

16. Four Quantum Number Bohr Atom Model1 He 2 Li 3 Be 4 B 5 C 6 N 7 O 8 F 9 Ne 10 Na 11 Mg 12 Al 13 Si 14 P 15 S 16 Cl 17 Ar 18 K 19
Various shorthand ways to describe the electron energy levels for beryllium, fluorine and some of the other first 18 atoms in the periodic table are shown below. 2p x 1s 2s 3s 4s Ne Be Ar Y 2 6 Z
Are there other common shorthand notations for Fluorine? 2s 1s 2p x y z 1s 2 , 2s 2p 5
And/or

17. Summary of Four Quantum Number Bohr Atom Model
Electrons within an atom can be completely identified as unique electrons with the aid of 4 quantum numbers.
These 4 quantum numbers are called
the principal quantum number
the angular momentum (azimuthal) quantum number n l
the magnetic quantum number m
the electron spin quantum number s
No two electrons in the same atom can have the same value for all four of these quantum numbers.

18. Summary of Four Quantum Number Bohr Atom Model
Electrons returning to their “ground” state can emit light with a unique frequency (energy).
Atoms are filled with electrons from the orbit closest to the nucleus to the orbit furthest from the nucleus.
The diagonal fill rule predicts the way electrons fill orbits.
The Bohr model does not explain why the 4s orbit is closer to the nucleus than the 3d orbit.