Quantum Mechanics
Electron Density
Representation of the electron density
distribution surrounding the nucleus in the
hydrogen atom; shows a high probability of
finding the electron closer to the nucleus
• Gives the probability that
an electron will be found
in a particular region of
an atom
• Regions of high electron
density represent a high
probability of locating the
electron
Atomic Orbital
• Way to distinguish Bohr’s model from the current
quantum mechanical model
• Probability of locating the electron in 3D space
around the nucleus
• Has a characteristic energy
Quantum numbers
•
•
•
•
Principle quantum number (n)
Angular momentum quantum number
Magnetic quantum number
Electron spin quantum number
Principal Quantum
Number n
n = 1, 2, 3, 4, ….
distance of e- from the nucleus
n=1
n=2
n=3
5
Energy levels are like rungs of a
ladder. You cannot be in
between a rung
Energy levels in an atom’s
electron are unequally spaced.
The higher energy levels are
closer together.
Angular Momentum
Quantum Number
Shape of the “volume” of space that the e- occupies
s orbital
p orbital
d orbital
f orbital
Magnetic
Quantum Number
Describes the orientation of the orbitals in space
s orbital
Sphere
p orbital
3 orientations
dumbbell
d orbital
5 orientations
Double dumbbell
f orbital
7 orientations
Electron Spin
Quantum Number
Energy of Orbitals
Shielding Effect
• Why is the 2s orbital
lower in energy than the
2p?
• “shielding” reduces the
electrostatic attraction
• Energy difference also
depends on orbital shape
Electron Configuration
number of electrons
in the orbital
1s1
principal quantum
number n
Shape
Orbital diagram
H
1s1
Aufbau Principle
Orbitals in the Periodic Table
Pauli Exclusion Principle
• No two electrons can have the same 4 quantum
numbers
• Only two electrons may occupy the same atomic
orbital, and these electrons must have opposite
spins
• Electrons that have opposite spins are said to
be paired Orbital name # of orientations Total # of electrons
s
p
d
f
Hund’s Rule
The most stable
arrangement of
electrons in an
orbital is the one
with the greatest
number of parallel
spins
Practice
Fill in the orbital diagram, and write the electron
configuration for the following atoms
Exceptions
Element
Copper
Should be
1s22s22p63s23p63d44s2
Chromium 1s22s22p63s23p63d94s2
Actually is
1s22s22p63s23p63d54s1
1s22s22p63s23p63d104s1
Noble Gas Configuration
What is the electron configuration for Ne?
Ne:
What is the electron configuration for Mg?
Mg:
What do both electron configurations have in
common?
To figure out which noble gas to use find the
noble gas that is closest to the element without
going over in atomic number
Which noble gas is closest without going over?
Rb
Cl
Ra
Practice
Write the noble gas electron configuration for the
following atoms:
Na:
Mn:
Co:
Sn:
Valence Electrons
• Electrons in the outermost s and p orbitals
(highest n shell)
• These electrons participate in chemical
reactions