Chemistry Unit Five
The Electron
What is an electron?
Sometimes electrons behave like waves.
(ex. Color of light emitted when electrons
of a sample are heated)
Sometimes electrons behave like particles.
(ex. Photoelectric effect: Increased
intensity of light ejects more electrons
from surface.)
Quantum Theory and the Atom
Ground State – The lowest allowable
energy state of an atom. Electrons
are in lowest energy levels possible.
Excited State – When atoms are
heated, electrons can jump to higher
energy levels.
Each energy level represents a
“quantum” of energy.
Electron Configurations
Show the arrangement of electrons in
an atom.
Describe where the electrons are
found and what energies they
possess.
Usually represent the ground state of
the electrons.
The Rules
1. The Aufbau Principle
Would you rather drive 10 min. to a
restaurant or 1 hour to the exact
same restaurant?
Same with electrons
Aufbau Principle - Electrons enter
orbitals of lowest energy level first.
Order of Filling
The Diagonal Rule
2 6
10
14
1s2
2s2 2p6
3s2
4s2
5s2
6s2
3p6
4p6
5p6
6p6
7s2 7p6
3d10
4d10 4f14
5d10 5f14
6d10
2. Pauli Exclusion Principle
Pauli Exclusion Principle – An orbital can
hold a maximum of 2 electrons. To occupy
the same orbital, the 2 electrons must spin
in opposite directions.
3. Hund’s Rule
Hund’s Rule - one electron enters each
orbital until each orbital contain one
electron with parallel spins before a second
electron is added.
Electrons like to stay hund paired for as
long as possible
Orbitals of equal energy are each occupied
by one electron first before any orbital is
occupied by another electron
Unpaired electrons in the same orbital have
the SAME spin
Determining Electron Configurations
Quantum Numbers describe the amount of
energy in that level. The lower the number, the
less energy it has. (n = 1, 2, 3, 4, etc.)
Sublevels are divisions of the principle energy
levels. The main sublevels are called s, p, d and f.
Each sublevel has a different shape caused by
the different energy levels.
Each sublevel has a certain number of orbitals,
which is a space that can take two electrons.
Number of Electrons per Sublevel
Sublevel
(suite)
Number of
Orbitals
Maximum
# of e-
s
1
2
p
3
6
d
5
10
f
7
14
s and p Orbital Electrons
d Orbital Electrons
f Orbital Electrons
Let’s Try an Example
An atom of carbon has 6 electrons
arranged in the following order.
1s22s22p2
Carbon’s electrons require two energy
levels. (Second quantum number.)
It has 4 electrons in its highest level.
Superscripts must add up to the atomic
number of the element. (#p+ = #e-)
Carbon has 6 p+ and 6 e-. (2+2+2)
How about another one???
An atom of sodium has 11 electrons
arranged in the following order.
1s22s22p63s1
Sodium’s electrons require three energy
levels. (Third quantum number.)
It has 1 electron in its highest level.
Superscripts must add up to the atomic
number of the element. (#p+ = #e-)
Sodium has 11 p+ and 11 e-. (2+2+6+1)
How About a Tougher One???
An atom of gold has 79 electrons arranged in
the following order.
1s22s22p63s23p64s23d104p65s24d105p66s24f145d9
Gold’s electrons require six energy levels.
(Sixth quantum number.)
It has 2 electrons iits highest level.
Superscripts must add up to the atomic number
of the element. (#p+ = #e-)
Gold has 79 p+ and 79 e-.
(2+2+6+2+6+2+10+6+2+10+6+2+14+9)
Orbital Notation
Very similar to electron
configuration.
Shows spins of individual electrons.
Example: Nitrogen has 7 electrons.
1s
2s
2p
Orbital Notation
Carbon
1s
2s
2p
Sodium
1s
2s
2p
3s
Electron Configuration
Short Form
Notice that large portions of each electron
configuration is repetitive.
To write an electron configuration in short
form:
1. Find the element on the periodic table and
“back up” to the previous noble gas.
2. Write the symbol for the noble gas in
brackets, then continue with the remainder
of the electron configuration.
Noble Gas Configuration
Lets looks at platinum
How many electrons does it have?
If we used orbital notation we would
have to draw 78 arrows
If we used electron configuration
notation we’d have to write
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10
5p6 6s2 4f14 5d8
THIS TAKES FOREVER!
Noble Gas Configuration – Short
Cut
Calcium
Atomic Number 20
Electron Configuration Long Form
1s22s22p63s23p64s2
Electron Configuration Short Form
[Ar]4s2
Thank you, may I have
another???
Tin
Atomic Number 50
Electron Configuration Long Form
1s22s22p63s23p64s23d104p65s24d105p2
Electron Configuration Short Form
[Kr]5s24d105p2
So, what if it is a noble gas?
Neon
Atomic Number 10
Electron Configuration Long Form
1s22s22p6
Electron Configuration Short Form
[He] 2s22p6
Lewis Dot Structures
One more type of atomic model…
(In addition to Bohr models and long and
short electron configurations)
Consists of the element’s symbol and the
atom’s valence electrons.
Symbol = kernel (represents the protons,
neutrons and full electron shells).
Dots = valence electrons.
Lewis Dot Structures Con’t
B
B = Kernel
(The protons,
neutrons and full
electron shells.)
Valence shell
electrons
You can use the Electron
Configuration to get the Lewis Dot
Structure…
Ca
1s22s22p63s23p64s2
Locate the highest quantum number. (4)
Add the s and p orbital electrons, and place
them around the element symbol. (2)
Look how easy this is with the short form.
[Ar]4s2
Ca
One Final Example
Tin
1s22s22p63s23p64s23d104p65s24d105p2
Or
[Kr]5s24d105p2
Locate the highest quantum number (5)
Add the s and p orbital electrons (4)
Sn
How to place electrons on a
Lewis Dot
First two dots represent the s orbital
electrons and are placed at the top of
the element’s symbol.
Then the p orbital electrons are
placed in this order: right, bottom,
left, right, bottom, left.
So, it goes like this…
8
5
1 2
Ne
7 4
3
6