Electron Configuration
Chapter 5
Electrons have 3 levels of
organization
Energy levels are the
first/biggest level of
organization
Sub levels are
second – energy
levels contain
sublevels
Definition: an orbital is
the space occupied by
2 (a pair of) electrons
Orbitals are third –
sublevels contain
orbitals
Electron Sub-Levels
s
holds 2 e-
1 orbital
On all
energy
levels
p
d
f
holds 6 e- holds 10 e- holds 14 e-
3 orbitals
5 orbitals
7orbitals
On energy On energy On energy
levels 2
levels 3 -6 levels 4 -5
and up
Sublevels and orbitals are too complicated to draw,
so when we plot the location of electrons, we use a
special notation.
Each orbital is represented by a box.
Electrons are represented by an up and a down
arrow.

(means 2 electrons)
S sub level
S sub level has one orbital,
so we draw one box.
P sub level
P sub level has 3 orbitals,
so we draw 3 boxes.
D sub level
D sub level has 5 orbitals, so we draw 5 boxes.
F sub level
F sub level has 7 orbitals, so we draw 7 boxes.
Orbital and sublevel information is like a
map, telling you where an electron can be
found in an atom.
There are rules that govern why an electron
will be in one sub-level rather than another.
Next are the directions for filling in electrons
in sublevels and orbitals.
Aufbau Principle – each electron must occupy
the lowest energy orbital available in a sub level
and energy level. Not all orbitals have the
same energy. See the chart on page 135.
s  p  d  f
energy increases
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p
This chart shows the order that electrons fill sublevels.
Although the D and F sublevels are on lower energy levels, they
have high energy and do not fill until after the S and P for higher
energy levels.
Pauli Exclusion Principle – 2 electrons may
occupy one orbital but they must have
opposite spin direction.
This is why we draw electrons as arrows
facing opposite directions.

Hund’s Rule – Before 2 electrons will occupy the
same orbital of a sub level, there must be at least
one electron in every orbital of that sub level.
 
WRONG -This is not stable.

 

RIGHT -This is stable.
Hund’s Rule paraphrased – do not double up electrons in
an orbital (a box) until you have put one in each orbital
for a sub level.
Example: Nickel (Ni) has 28 electrons. Draw the Electron
Configuration Notation.
1s
2s
2p
3s
3p
4s
3d
Use Aufbau Principle to see what order to fill sublevels and
orbitals .
Example: Nickel (Ni) has 28 electrons. Draw the Electron
Configuration Notation.

1s


2s

3s

4s


2p



3p






3d
Use Pauli Exclusion Principle to place arrows facing the
opposite direction in each orbital
Example: Nickel (Ni) has 28 electrons. Draw the Electron
Configuration Notation.

1s


2s

3s

4s


2p



3p









3d
Follow Hund’s rule in the 3d sublevel so each orbital gets
one electron before one orbital has 2 electrons.
Lewis Dot Structures
Aka Dot Diagrams
Electron Dot Diagrams
Valence electrons are
the electrons on the
outermost energy level.
6 valence electrons on this atom.
Image from Wikipedia


1s
2s



2p
Valence electrons
The final s and p valence electrons are
especially important because they determine
what kinds of reactions an atom will
undergo.
If you write the Shorthand Configuration for an element,
the electrons on the highest energy level are the valence
electrons.
Example: Tin, Sn:
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p2
Lewis Dot Structures
• A Lewis Dot Structure is a way of showing only
the Valence electrons for an element.
• This is very useful when showing how atoms
react with one another.
• A Lewis Dot Diagram consists of two parts:
– the element symbol
– dots that represent the S and P block electrons
Rules for Writing Lewis Dot
Diagrams:
• Dots may only go on the sides of the
element symbol
• Only use s and p valence electrons
• If there are d or f valence electrons, they
are left out of the dot diagram.
• You must put 1 electron dot on each side
of the element symbol before you put 2 on
one side (like Hund’s rule)
Example: Tin, the valence electrons are
5s2 4d10 5p2.
For the dot diagram, we will use only
5s2 and 5p2.
Therefore we will place 4 dots around
the element symbol.
Sn
= places where electrons can go
Example: Bromine
Shorthand Configuration is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5.
Use only the 4s2 and the 4p5.
A total of 7 dots.
Br
Practice problems
• Sodium
• Neon
• Aluminum
Octet Rule - Atoms are chemically stable
when they have 0 or 8 electrons in their S &
P sublevels (0 or 8 dots in a dot diagram).
Ne
Noble gases are chemically stable
because they have full octets.
Na
Alkali metals are unstable and very
reactive because they are so close to
having a stable octet.
Br
Halogens are unstable and very reactive
because they are so close to having a
stable octet.
To become chemically stable, atoms can
gain or lose electrons to obtain a stable
octet.
+2
Ca
Calcium has 2 electron dots. It will lose them so it will
have 0 in the dot diagram.
To determine the charge on the atom, you add up the
number of positive and negative charges.
+20 protons (atomic number)
-18 electrons (20 electrons to start; lose 2; equals 18)
+2 charge
Ionization Rules
• If an atom has 1, 2, or 3 electrons in a dot
diagram, it will lose those electrons.
• If an atom has 5, 6, or 7 electrons in a dot
diagram, it will gain more to make 8.
• If an atom has 4 electrons in a dot
diagram, it can either gain or lose 4.
Alternatively, there are two rules
you can memorize:
• If atom loses electrons, it becomes
positive.
• If atom gains electrons, it becomes
negative.
Example: If it gains 1 electron, the atom becomes -1.
If it loses 2 electrons, the atom becomes +2.
Example Chlorine:
Shorthand Electron Configuration: 1s2 2s2 2p6 3s2 3p5
:
Lewis Dot Diagram:
-1
Stable Octet 
.
:Cl:
Now decide whether Chlorine will gain or lose electrons.
To calculate the charge:
+17 protons
-18 electrons (17 electrons to start and gain 1 to get 18)
-1 charge
Alternatively, remember the rule, if it gains one electron, the
atom becomes -1 charge.
The correct answer is Cl-1, it is an anion.
Assignment
• Complete the Dot Diagram Periodic Table
Assignment.
• Answer all questions on the back.
• Be sure to label your Dot Diagram Periodic
Table when the instructions say to.
• Turn in the answers and the periodic table
when you finish