Location of Electrons
Quantum Mechanics
Describes the arrangement of electrons
in atoms in terms of:
Main or principal energy levels (n)
Energy subshells
Orbitals (space occupied within the
atom)
Location of Electrons
1.
2.
3.
4.
Principal Quantum Number
Angular Momentum Quantum Number
Magnetic Quantum Number
Electron Spin Quantum Number
Important Information
 Orbital- A 3 dimensional space around a
nucleus in which electrons are most likely
to be found
 Shape represents electron density (not a path
the electron follows)
 Each orbital can hold up to 2 electrons.
Principal Quantum Number (n)
Contain electrons that are
Close in energy; similar distance
from nucleus
Have values of n = 1+ (1,2,3…∞)
Maximum number of electrons = 2n2
n =1
2(1)2 = 2
n =2
2(2)2 =8
n=3
Energy Levels (Shells)
• A group of electrons in an atom all having the
same principal quantum number (n)
n = 1, 2, 3, …
• The first shell (n = 1) is lowest in energy,
2nd level next and so on 1<2<3<4
n=1
n=2
n=3
Angular Momentum Quantum
Number (l)
Energy sublevels within energy level
Describes shape of the volume of
space that the electron occupies
For a given value of n, l = 0, 1, 2, 3, …
n-1
l=0
l=1
l=2
l=3
s orbital
p orbital
d orbital
f orbital
Sublevel energy: s<p<d<f
l = 0 (s orbitals)
l = 1 (p orbitals)
l = 2 (d orbitals)
Electrons Allowed
 There is a limit to the number of electrons each
sublevel can hold:
s sublevel
2 electrons
p sublevel
6 electrons
d sublevel
10 electrons
f sublevel
14 electrons
Electron Configuration
 List of subshells containing electrons
 Written in order of increasing energy
 Superscripts give the number of electrons
Example: Electron configuration of neon
number of electrons
1s2
2s2
main shell (n)
2p6
subshell (l)
Order of Filling
 Total energy of a subshell =
energy of the main shell + the subshell
Additional Note
Aufbau principle – electrons are filled starting
with the lowest energy orbital
Writing Electron Configurations
H
1s1
He
1s2
Li
1s2
2s1
C
1s2
2s2
2p2
S
1s2
2s2
2p6
3s2
3p4
Blocks in the Periodic Table
Timberlake LecturePLUS 2000
Learning Check S1
Choose the correct electron configuration for
potassium. Give an explanation for your
selection.
A.
1s22s22p63s1
B.
1s22s22p63s23p6
C.
1s22s22p63s23p64s1
D.
1s22p83s1
E.
1s22s22p63s23p7
Solution E1
For phosphorus, indicate if each
configuration is (1) correct or (2) incorrect.
Explain why or why not.
A.
1s22s22p63s1
B.
1s22s22p63s23p6
C.
1s22s22p63s23p64s1
D.
1s22p83s1
E.
1s22s22p63s23p7
Learning Check S2
Using the periodic table, write the complete
electronic configuration for each:
A. Cl
B. Sr
C. I
Solution S2
Using the periodic table, write the complete
electronic configuration for each:
A. Cl
1s2 2s2 2p6 3s2 3p5
B. Sr
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2
C. I
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5
Shorthand Notation
• Use the last noble gas that is located in the
periodic table right before the element.
• Write the symbol of the noble gas in brackets.
• Write the remaining configuration after the
brackets.
• Ex: Fluorine: [He] 2s2 2p5
Learning Check S3
A.
The final two notations for Co are
1) 3p64s2
2) 4s24d7
3) 4s23d7
B. The final three notations for Sn are
1) 5s25p24d10
2) 5s24d105p2
3) 5s25d105p2
Solution S3
A.
The final two notations for Co are
3) 4s2 3d7
B. The final three notations for Sn are
2) 5s2 4d10 5p2
Magnetic Quantum Number (ml)
• Describes orientation of electrons
• For a given value of l
ml = -l, …., 0, …. +l
• If l = 1 (p orbital), ml = -1, 0, or 1
• If l = 2 (d orbital), ml = -2, -1, 0, 1, or 2
ml = -1
ml = -2
ml = 0
ml = -1
ml = 0
ml = 1
ml = 1
ml = 2
Learning Check S4
A. Number of electrons in a s orbital
1) 1e
2) 1e or 2e 3) 3e
B. Number of orbitals in a p subshell
1) 1
2) 2
3) 3
C. Number of orbitals in 4d subshell
1) 1
2) 3
3) 5
D. Number of electrons (maximum) in a 3d
subshell
1) 2e
2) 5e
3) 10e
Solution S4
A. Number of electrons in a p orbital
2) 1e or 2e
B. Number of orbitals in a p subshell
3) 3
C. Number of orbitals in 4d subshell
3) 5
D. Number of electrons in a 3d subshell
3) 10e
Additional Note
Pauli exclusion principle - no two electrons in
an atom can have the same four quantum
numbers.
Each seat is uniquely identified (E, R12, S8)
Each seat can hold only one individual at a
time
Spin Quantum Number (ms)
• Describes direction of spin
• ms= + ½ or – ½
• Remember an orbital can hold only two electrons
and they must have opposite spins
Orbital diagram
H
1s1
Additional Note
Hund’s Rule- the electrons will fill the orbitals in
a way that would give the maximum number of
parallel spins (maximum number of unpaired
electrons)
Analogy: Students could fill each seat of a
school bus, one person at a time, before
doubling up.
Learning Check 5
A. What does Aufbau Principle state?
B. What does Pauli Exclusion Principle state?
C. What does Hund’s Rule State?
Solution 5
A. What does Aufbau Principle state?
Electrons fill starting with the LOWEST
energy level
B. What does Pauli Exclusion Principle state?
No two electrons can have the same four
quantum numbers
C. What does Hund’s Rule State?
Electrons fill orbitals so max number of
parallel spins (max number of unpaired
electrons)