CH 4
ATOMIC STRUCTURE
1
WHAT IS IT AGAIN?
 An
atom is a unit of matter, the smallest
unit of an element that retains the
chemical properties of that element.
 An element is a substance that cannot be
broken down into simpler substances and
retain the same qualities.
 Everything around us is composed of
atoms. Your desk, the air, even you are
made up of atoms!
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HOW DO WE KNOW ANY OF THIS?
 Scientists
came up with ideas and
theories and performed experiments to
test their ideas.
 No
one scientist suddenly figured it all
out-it took the work of many separate
scientists working over a period of time to
gather the knowledge that we have today!
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DALTON’S THEORY (1803)
1)ALL ELEMENTS ARE COMPOSED OF ATOMS.
2)ALL ATOMS OF THE SAME ELEMENT HAVE THE
SAME MASS, AND ATOMS OF DIFFERENT ELEMENTS
HAVE DIFFERENT MASSES.
3)COMPOUNDS CONTAIN ATOMS OF MORE THAN ONE
ELEMENT.
4)IN A PARTICULAR COMPOUND, ATOMS OF
DIFFERENT ELEMENTS ALWAYS COMBINE IN THE
SAME WAY.
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DALTON’S THEORY (1803)
A THEORY MUST EXPLAIN THE DATA
FROM MANY EXPERIMENTS. BECAUSE
DALTON’S ATOMIC THEORY MET THAT
GOAL, THE THEORY BECAME WIDELY
ACCEPTED.
OVER TIME, SCIENTISTS FOUND THAT
NOT ALL OF DALTON’S IDEAS ABOUT
ATOMS WERE COMPLETELY CORRECT.
THEY REVISED THE THEORY TO TAKE
INTO ACCOUNT NEW DISCOVERIES.
THOMSON’S MODEL OF THE ATOM
(1897)
When some materials are rubbed
together, they gain the ability to
attract or repel other materials. Such
materials are said to have either a
positive or a negative electric charge.
• Objects
with like charges repel, or
push apart.
• Objects with opposite charges
attract, or pull together.
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THOMSON’S MODEL OF THE ATOM (1897)
Thomson revised Dalton’s model to
account for newly discovered subatomic
particles.
• The atom has neither a positive nor a
negative charge, but there must
always be some positive charge in the
atom.
• The atom is filled with a positively
charged mass of matter that has
negative charges evenly scattered
throughout it.
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THOMSON’S MODEL OF THE ATOM (1897)
 Thomson’s
model is called the “plum
pudding” model.
 The negatively charged particles
(electrons) are spread evenly through
a mass of positively charged matter.
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RUTHERFORD’S ATOMIC THEORY
(1911)
Ernest Rutherford designed an
experiment to find out what happens to
alpha particles (fast-moving, positively
charged particles.) when they pass
through a thin sheet of gold.
Alpha
particles
He expected the particles to go
straight through the gold foil…
Thin Sheet of Gold
WHAT ACTUALLY HAPPENED IN
RUTHERFORD’S EXPERIMENT…
Alpha
particles
Nucleus
Rutherford proposed a new model.
•The positive charge of an atom is not evenly spread
throughout the atom.
•Positive charge is concentrated in a very small, central
area.
•The nucleus of the atom is a dense, positively charged10
mass located in the center of the atom.
BOHR’S MODEL OF THE ATOM (1913)
Bohr’s atomic model had a
nucleus surrounded by a large
volume of space. But Bohr’s
model focused on the electrons
and their arrangement.
In Bohr’s model, electrons move
with constant speed in fixed
orbits around the nucleus, like
planets around a sun. Each
electron in an atom has a specific
amount of energy.
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WHAT ARE ATOMS MADE OF?
 Atoms
are made up of three basic building
blocks:
Protons
2) Neutrons
3) Electrons
1)
We know that these building blocks are made up
of even smaller components called Elementary
Particles, but we’ll worry about these later!
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PROTONS, ELECTRONS
AND NEUTRONS CAN
BE DISTINGUISHED BY
MASS, CHARGE, AND
LOCATION IN AN
ATOM.
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Particle
Charge
Location
Relative
Mass
Proton
+
nucleus
1
Neutron
none
nucleus
1
Electron
-
Electron
cloud
1/1836 or
0.0005
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Protons
and Neutrons join
together to form the
Nucleus-the dense,
positively charged central
part of the atom
Electrons
are attracted to
the protons in the nucleus
by what is called the
electromagnetic force.
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USING THE PERIODIC TABLE …
Atomic Number –
Nitrogen
Number of protons
7
N
14.007
Mass Number –
Number of protons plus
neutrons
So … how would you figure out 16
how many neutrons an atom has?
 Using

Nitrogen as our example…
Mass Number – Atomic Number= Number of Neutrons
 14.007
– 7 = 7.007 Neutrons
Atomic Number –
Nitrogen
N
14.007
Number of protons
7
Mass Number –
Number of protons plus
neutrons
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WHAT'S THE DEAL WITH THE .007? WHY
ISN'T IT A WHOLE NUMBER?

Atoms of the same element that have different numbers of
neutrons are called Isotopes.

Their Atomic number is the same but their Mass number
changes

Mass numbers are calculated by figuring out how many
atoms of each type are out there in the universe.

When you average out all of the masses, you get a number
that isn’t quite a whole number

Using Nitrogen as our example…
-Most naturally occurring and stable Nitrogen atoms are N
14, but a few are N 15. Which is why our mass # is 14.007
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WHAT ABOUT ELECTRONS?
If the atom is neutral (does not have a + or – charge),
then
 Electrons = Protons

How many …
Sodium
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Na
22.99
1. Protons?
2. Neutrons?
3. Electrons?
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http://www.miamisci.org/af/sln/phantom/spectroscope.html
NUMBER OF ELECTRONS CONT.


If the atom has a charge (ion)…

Cations are atoms with a + charge

Anions are atoms with a – charge

Start with the number of electrons a neutral atom would
have (same as # of protons)

Then add electrons if it is a – charged anion

Or subtract electrons if it is a + charged cation
Example: Nitrogen with a +1 charge has _?_ electrons.
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BOHR’S MODEL OF THE ATOM (1913)
Energy Levels
When an atom gains or
loses energy, the energy of
an electron can change.
Electrons gain or
lose energy
when they move
between fixed
energy levels
The possible energies that
electrons in an atom can
Nucleus
have are called energy
levels.
• An electron cannot exist
between energy levels.
•
Electron
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Remember this
guy…?
-IN 1913 NIELS BOHR CAME UP
WITH THE IDEA THAT
ELECTRONS MOVED IN
SPHERICAL ORBITS AROUND
THE NUCLEUS.
-THESE ELECTRONS MOVE IN
ENERGY LEVELS THAT RADIATE
OUT FROM THE NUCLEUS.
-AN ELECTRON CAN MOVE FROM
ONE ENERGY LEVEL TO
ANOTHER WHEN THE ATOM
GAINS OR LOSES ENERGY. 22
WE NOW KNOW THAT BOHR’S "SOLAR SYSTEM
LIKE" PICTURE OF AN ATOM, WITH AN
ELECTRON NEATLY MOVING AROUND A
NUCLEUS IN A CIRCLE ISN'T REALLY CORRECT.
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TURNS OUT, THERE'S NO REASON TO ASSUME THAT
ELECTRON ORBITS ARE CIRCULAR. IN FACT IT'S
VERY RARE FOR AN ATOM'S ELECTRON TO BE IN A
CIRCULAR ORBIT.
THE ELECTRON MOVES AT DIFFERENT SPEEDS.
FAST NEAR THE NUCLEUS AND SLOW WHEN IT'S
FAR FROM THE NUCLEUS.
THE ELECTRON IS NOT ALWAYS THE SAME
DISTANCE FROM THE NUCLEUS. SOMETIMES IT'S
CLOSE, OTHER TIMES, IT'S FAR AWAY.
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ELECTRON CLOUD MODEL…( BASED ON WORK
BY
AUSTRIAN PHYSICIST SHRODINGER IN 1924)
 An
electron cloud is a visual model of the
most likely locations for electrons in an
atom. The cloud is denser at those locations
where the probability of finding an electron
is high.
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ENERGY LEVELS CONTINUED…
-We now know that the electrons
don’t go around the nucleus in a neat
circle, but we can still imagine
distinct circular orbitals or energy
levels within this electron cloud.

-It is the electromagnetic force, or
the attraction between the opposite
charges (the positively charged proton
and the negatively charged electron)
that keep the electrons from flying off

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Each of the orbitals can hold a fixed number of electrons




The lower levels closer to the nucleus must be full before
filling the upper levels.
Chemical properties are based on the number of electrons
in the outermost shell
Elements with full outer shells are usually
nonreactive
Elements try to give up, take, or share electrons with the
goal of having a full outer shell-these are more reactive
elements
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