10/23/2014
Unit 4: Electrons in Atoms
Lesson 4.3: Electron Configuration
Why Should You Care About Electrons?
An atom’s electrons tell us about how the atom
will behave physically and chemically.
Kernel- All of an atom’s electrons except for
the ones in the outermost energy level
Valence- the electrons in the outermost energy
level.
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1. Basic Electron Configuration
In a basic configuration, we know how many
electrons are in each of the atom’s principal
energy levels (PEL’s – aka “shells”).
Each PEL has a a specific number of electrons it
can fit:
PEL 1 – maximum of 2 electrons
PEL 2 – maximum of 8 electrons
PEL 3 – maximum of 18 electrons
PEL 4 – maximum of 32 electrons
PEL 5 - ?
The basic electron configuration is listed on the
periodic table
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Look, it’s Chlorine!
How many electrons are in
each of chlorine’s PEL’s?
How many of chlorine’s PEL’s
are full?
How many valence electrons
does chlorine have?
Bohr model of chlorine
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PEL’s and The Periodic Table
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The number of PEL’s increases by one as
you move to higher (lower, really) periods
of the table.
As you can probably see, elements in the
same row (aka “period”) of the periodic
table all have the same number of electron
shells.
Q: What do all of the members of a
column (aka “group”) of the
periodic table have in common?
A: They all have the same number
of valence electrons!
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2. Expanded Electron Configuration
Each PEL has sublevels.
The expanded electron configuration tells you the
PEL’s and sublevels of an atom that are filled, and how
many electrons are in each sublevel.
Sub-levels are designated as s,p,d, and f.
We use superscript numbers to denote how many
electrons are in a particular sublevel.
Writing Expanded Configuration
Lithium has the basic configuration of 2-1
Its expanded configuration is 1s2 2s1
Nitrogen has the basic configuration of 2-5
Its expanded configuration is 1s2 2s2 2p3
Magnesium has the basic configuration of 2-8-2
It’s expanded configuration is 1s2 2s2 2p6 3s2
Chlorine has the basic configuration of 2-8-7
It’s expanded configuration is 1s2 2s2 2p6 3s2
5
3p
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Filling can get crazy
Ouch, that’s confusing
The basic rule for filling is that electrons will
always go in the lowest energy sublevel possible.
The only problem with this, is that some sublevels
are at a higher energy than other sublevels of
higher shells:
– The d sublevel of any shell that has one is at
a higher energy than the s sublevel of the
next shell.
– The f sublevel of any shell that has one is at
a higher energy than the s and p sublevels of
the next shell
I know, and I’m sorry.
The German’s who figured this stuff out came up
with a filling order diagram to help us keep it all
straight.
It’s called the “Aufbau” rule:
Electrons fill the lowest
Energy level available
to them
Follow the arrows!
You can always make
your own!!!
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Use the Aufbau rule
There has to be a lazier way!
What is the electron configuration of
Potassium?
1s2 2s2 2p6 3s2 3p6 4s1
What is the electron configuration of Iron?
1s2 2s2 2p6 3s2 3p6 4s2 3d6
What is the electron configuration of Bromine?
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5
What is the electron configuration of Gold?
Of course there is.
When the electron configuration get’s too
funky, we can say that our atom has the
configuration of the noble gas (group 18) atom
in the previous period by putting that atom’s
symbol in square brackets, and then list the
remaining expanded notation following that
gas.
e.g.
Gold: [Xe] 6s2 4f14 5d9
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14
5d9
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3. Orbital (Box) Diagrams
Hund’s Rule
In a box diagram we show how many electrons are in
each orbital of the sublevel and what the spin of the
electron is.
Each sublevel has a different number of orbitals:
s has 1 orbital
p has 3 orbitals
d has 5 orbitals
f has 7 orbitals
Each orbital can fit a maximum of two electrons.
These electrons have opposing spins, which we call “up
and “down”.
Box notation shows all of this
Electrons in orbitals of the same sub-level will
always occupy empty orbitals before they will
pair up.
The Pauli Exclusion Principle
An orbital can hold 0, 1, or 2 electrons and if
there are two electrons in an orbital, they must
have opposite spins.
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Fun with Box Diagrams
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Magnesium’s Box Diagram:
Lithium’s Box Diagram:
Chlorine’s Box Diagram:
Nitrogen’s Box Diagram:
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Why should we care about Boxes?
Orbitals (Box) notation is useful for helping us
see the unpaired electrons in an atom.
Unpaired electrons are involved in chemical
bonds.
Unpaired electrons are always found only in a
ground-state atom’s valence.
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Oh, I also Lied about Gold
Earlier, I told you that Gold’s electron
configuration was:
[Xe] 6s2 4f14 5d9
It’s actually:
Gold: [Xe] 6s1 4f14 5d10
Can you use box notation to figure out why?
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Shells, Sublevels, and Orbitals
4. Lewis Dot Diagrams
These are three concepts that are related, but
can be confusing. Think about it like this:
“If electrons were people, they would live in towns
called shells (PEL’s), on streets called sublevels
and in houses called orbitals. They prefer to live
alone, but will live together if they have to. Two
electrons can live in each orbital house, as long
as they spin in opposite directions.”
Different town’s have different numbers of streets
and different streets have different numbers of
houses.
Named after Gilbert Newton
Lewis.
Only show an atom’s
valence electrons,
surrounding its chemical
symbol.
Since the valence electrons
are involved in bonding,
that’s usually the only part of Gilbert Newton Lewis
(1875 – 1946)
the electron configuration we
care about.
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Lewis Diagrams
Li
Be
N
O
More about Valence Electrons
B
F
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The number of valence electrons is very important for
most of the rest of chemistry.
Atom’s do what they do because of their valence
electrons.
Every atom is most stable when it has a full valence
shell.
The maximum number of valence electrons an atom
can have is 8 (except Hydrogen and Helium).
Once an atom has 8 valence electrons, it has a
“stable octet” configuration.
Chemical Bonding is the way that atoms achieve a
stable octet.
C
Ne
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Any
Questions?
What now?
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