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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
The basic electron configuration is listed on the
periodic table
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Look, it’s Calcium!
How many electrons are in each of
calcium’s PEL’s?
How many of calcium’s PEL’s are
full?
How many valence electrons does
calcium have?
Bohr model of calcium
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PEL’s and The Periodic 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.
The number of PEL’s increases by one as you move
to higher (lower, really) periods of the table.
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 sublevel’s.
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.
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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
3p5
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Filling can get crazy
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
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Damn, that’s confusing
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”:
Follow the arrows!
You can always make
your own!!!
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Use the Aufbau
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?
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d9
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There has to be a lazier way!
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
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3. Orbital (Box) Diagrams
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
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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.
Hund’s Rule
Electrons in orbitals of the same sub-level will
always occupy empty orbitals before they will pair
up.
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Fun with Box Diagrams
Lithium’s Box Diagram:
Nitrogen’s Box Diagram:
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Magnesium’s Box Diagram:
Chlorine’s Box Diagram:
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Why should we care about Boxes?
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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Shells, Sublevels, and Orbitals
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.
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4. Lewis Dot Diagrams
Named after Gilbert Newton
Lewis.
Only show an atom’s valence
electrons, surrounding its
chemical symbol.
Since the valence electrons
are involved in bonding,
Gilbert Newton Lewis
that’s usually the only part of
(1875 – 1946)
the electron configuration
we care about.
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Lewis Diagrams
Li
B
Be
C
N
F
O
Ne
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More about Valence Electrons
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.
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Any
Questions?
What now?
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