Modern Atomic Theory
How do electrons create the chemical
trends on the
Periodic Table
Wave-Mechanical Model
Assumptions
• We never know exactly how electrons move
in an atom
• At any point in time there are areas around
the nucleus where the electron is more likely
to be (higher electron probability)
• We can make a probability map of where the
electron is more likely to be
– The more intense the color on the map, the more
likely the electron will be there
Wave-Mechanical Model
Assumptions (cont)
• We call the probability maps orbitals
– An orbital is the three dimensional space
around the nucleus where electron
probability is high
Wave-Mechanical Model
Assumptions (cont)
• The “edge” of the orbital is fuzzy, like
the outer edge of the earth’s
atmosphere
• Chemists and physicists arbitrarily
define the orbital as containing 90%
electron probability
Three Rules for electron
orbital filling
• Aufbau Principle
• Pauli Exclusion Principle
• Hund’s Rule
– When electrons occupy orbitals of equal energy,
one electron enters each orbital until all of the
orbitals contain one electron with parallel spins
– Then the second electron enters the orbitals with
opposite spins
Principal Components of
the Wave-Mechanical Model
1. Atoms have a series of energy levels
called Principal Energy Levels
A. Designated by whole numbers
B. Symbolized by n
-
n=1, n=2, n=3, n=4
Principal Components cont
2. The energy level increases as the
value of n increases
3. Each Energy Level contains one or
more types of orbitals called sublevels
4. The number of sub-levels (or types of
orbitals) equals n (number of the
energy level)
Principal Components cont
5. The number of the energy level, and
the letter of the sub-level (orbital
shape) are used to designate the
energy level, sub-level, and orbital
shape (I.e. 1s, 2p)
6. An orbital can be empty or it can
contain up to two electrons as long as
they have opposite spins
Principal Components cont
7. The shape of the orbital indicates 90%
electron probability
-NOT the details of the electron’s movement
Energy Levels
• Electrons fill the orbitals in order of
energy, from lowest energy to highest
• The order of energy is:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p,
6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p
What this means is….
• 3d has a higher energy than 4s
• Even though 3d is closer to the nucleus as far
as the number of the energy level
• Electrons will fill 4s before 3d
• 4s is still the outer orbital compared to 3d
• It is not Mrs. Erfft’s fault
• Mrs. Erfft is not confused
This creates a pattern on
the Periodic Table
• Explains the trends seen in much of the
chemical behavior of the elements
Electron Configuration
Patterns
The whole Periodic Table
Short Form Electron
Configuration
The electron configuration for Sr is
1s22s22p63s23p64s23d104p65s2
The electron configuration for Kr
is1s22s22p63s23p64s23d104p6
Note that everything is the same through
the fourth energy level (4p6 is complete)
Short Form Cont
We can shorten the electron configuration
for Sr by writing the electron
configuration for Kr as [Kr]
1s22s22p63s23p64s23d104p6=[Kr]
Then we can write the configuration for Sr
as [Kr]5s2
To write the short form:
1. Take the electron configuration for the noble
gas on the period above the element you
are focusing on
2. Shorten the electron configuration of the
noble gas by putting the symbol of the noble
gas into brackets
3. Write the rest of the electron configuration
to the right of the bracketed noble gas
symbol in the correct order
Valence Electrons
Valence Electrons are the electrons in the
outermost energy level of an atom
These are the electrons in the s and p
orbitals specifically
All of the other electrons are called the
core electrons
Valence Electrons
• Are the most important electrons to
chemists
• Being the outermost electrons, they are
the electrons that govern the chemical
behavior of the element/atom
• Valence electrons are the electrons that
form bonds and are lost or gained to
form ions
Valence Electrons of
Groups
• Note that the atoms in the same vertical
groups on the periodic table have the same
numbers of electrons in the same orbitals
• Just higher energy levels as you go down a
column
• Therefore elements in the same groups show
the same valence electron configuration
Valence electron
configurations
For Chlorine
Electron Configuration:
[Ne]3s23p5
Valence Configuration: 3s23p5