3-2 Discovering Atomic Structure*
The “Holy Grail” Of Chemistry:
To Understand the Structure of the Atom
*Modified from a presentation by Mr. Matt Davis.
Sept 2006
OBJECTIVES
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Define the term “atom.”
Describe the history of Atomic Theory.
List the postulates of Dalton’s Atomic Theory.
Show how atomic structure and
electricity are related.
Explain what studies of cathode rays
and radioactivity revealed about atoms.
Discuss Rutherford’s alpha-scattering
experiment and how it showed the
existence of the nucleus.
Name/describe the three subatomic particles in an
atom.
Determine the number of protons, neutrons and
electrons in an atom or ion.
Define “isotope” and atomic mass.
Understanding the Structure of the Atom
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Michael Faraday suggested…
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Benjamin Franklin proposed…
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…there are two kinds of charge (positive & negative);
…like charges repel, and opposite charges attract.
Cathode Ray Tube (“CRT”) was developed and…
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…atomic structure is somehow related to electricity.
…it showed an ‘odd glow’ in electrified gases.
(Demonstration of CRT.)
J.J. Thompson proved…
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…CRT ‘glow’ came from the negative end (cathode).
The glow was actually particles. (It moved a pinwheel.)
He is credited with discovering the electron.
He measured the ratio of an electron’s charge to its mass.
Understanding the Structure of the Atom
(cont’d)
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Robert Millikan measured…
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Henri Becquerel discovered…
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…the charge of a single electron (Oil Drop Experiment)
as 1.60 X 10-19 coulomb.
So the calculated mass of electron is 9.11 X 10-28 g.
…radioactivity is emitted from uranium ore.
Marie Curie & Pierre Curie discovered…
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..other radioactive elements, radium & polonium.
J. J. Thomson proposed the “Plum Pudding” model
of the atom.
(Think of it as a “chocolate chip cookie” model instead.)
Atoms have negative charges
evenly distributed throughout
the atom’s positive interior.
Understanding the Structure of the Atom
(cont’d)
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Ernest Rutherford discovered…
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…alpha & beta radiation.
Both are charged. (Alpha particles have 2+
charge; beta particles have 1- charge.)
(Gamma radiation was discovered later.)
Rutherford also did a very important
experiment: the Gold Foil Experiment.
Rutherford’s Gold Foil Experiment
Try the following websites at home! (I’ll demo them here
just to show how they work.)
http://www.waoen.screaming.net/revision/nuclear/rsanim.htm
http://www.micro.magnet.fsu.edu
(Use the search tool to find the “Rutherford Experiment.”)
Gold Foil Experiment Observations:
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99% of alpha particles went straight through
the gold foil.
But, ½% deflected, and ½% reflected!
Unexpected & remarkable results!
Rutherford said, “It was about as credible
as if you had fired a 15-inch [artillery]
shell at a piece of paper and it came back
and hit you!”
Rutherford’s Conclusions
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The ‘Plum Pudding’ model is wrong.
An atom basically consists of a lot of
empty space!
All of an atom’s positive charge is
concentrated in a very small core at the
atom’s center, which Rutherford called
the nucleus.
The negatively charged electrons move
around the nucleus.
Note: If the entire atom is the size of a football
stadium, the nucleus would be about the size of
a marble sitting on the 50-yd line.
OBJECTIVES
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Define the term “atom.”
Describe the history of Atomic Theory.
List the postulates of Dalton’s Atomic Theory.
Show how atomic structure and electricity are related.
Explain what studies of cathode rays and radioactivity
revealed about atoms.
Discuss Rutherford’s alpha-scattering experiment and
how it showed the existence of the nucleus.
Name/describe the three subatomic
particles in an atom.
Determine the number of protons,
neutrons and electrons in an atom or
ion.
Define “isotope” and atomic mass.
Understanding the Structure of the Atom
(cont’d)
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James Chadwick discovered…
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…another subatomic particle: the neutron.
Neutrons have no charge, but their mass is nearly
equal to that of a proton.
Neutrons reside in the nucleus, along with
protons.
Neutrons act as a ‘glue’ that holds the nucleus
together.
 Recall that “like charges repel,” so atoms
with lots of protons would be very unstable
without lots of neutrons.
 Strong Nuclear Force – the name for the
attraction that holds a nucleus together, thus
preventing it from flying apart.
Understanding the Structure of the Atom
(cont’d)
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Henry Moseley found that…
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…atoms of each element contain a unique
positive charge in their nucleus.
An atom’s identity comes from the number of
protons in its nucleus.
The number of protons in an atom is called its
Atomic Number (Z).
3-3 Modern Atomic Theory
“Parts of the Atom”
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Atoms are composed of protons, neutrons and
electrons. (Plus many other smaller particles that
do not impact the chemistry.)
The nucleus contains protons (+) and neutrons
(neutral).
Electrons (-) move in space around the nucleus (in
the “planetary” model).
The number of electrons always equals the number
of protons when an atom is neutral.
Atomic Number (Z): the number of protons in an
atom.
Mass Number (A): the total of protons and
neutrons.
Atomic mass units (amu) are used to express mass.
Models of the Atom (So far!)
Ancient Greek Model: Tiny particles (atomos).
Thomson Model: Ball of positive charge with
embedded electrons.
Rutherford Model: An atom’s mass concentrated
in a small, positively charged region (nucleus)
with electrons around it.
Other Models to follow?
Subatomic Particles
Particle
Location
Charge
(C)
Mass (g)
p
Nucleus
+1.602
X10^-19
1.673
X10^-24
n
Nucleus
0
1.675
X10^-24
e
Outside
nucleus
-1.602
X10^-19
9.109
X10^-28
Text Fig. 3-19, page 104.
Atom Notation
A
Z
X
X = Element Symbol
A = Mass Number
Z = Atomic Number (Often omitted. Why?)
Ions
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Atoms are electrically neutral. (Why? Demonstration.)
If an atom loses or gains electrons it becomes charged,
forming an ion.
ION: an atom or group of atoms that has a
positive or negative charge because it lost or
gained electrons.
Lithium: Z = 3, A = 7
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Oxygen: Z = 8, A = 16
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Losing one electron, Li forms Li1+ and the e1Gaining two electrons, O plus 2 e1- forms O2-
Ions are written with chemical symbols by placing its
charge on the upper right.
Practice this!
Isotopes
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Contrary to Dalton’s idea, all atoms of a given
element are NOT identical!
They have the same number of protons, but
may not have the same number of neutrons.
Isotopes: atoms of the same element that
have different masses. (This is caused by
the different numbers of neutrons.)
Examples
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H-1 (hydrogen or protium, H), H-2 (deuterium, D),
H-3 (tritium, T).
C-12, C-13, C-14.
U-235, U-238.
Let’s practice p, n, e counting. In your notes,
set up the grid on the next slide.
p, n, e Counting
Element
H
He
Li
B
F
At #
Mass
#
p
n
e
Counting the Mass of Atoms
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Recall that total mass of an atom is (p + n +
e), but electrons are only 1/2000th the mass of
a proton, and may generally be neglected.
So an atom’s mass is basically just the mass of
its protons & neutrons.
Very tiny masses; not practical to use.
Chemists compare the relative masses of atoms
vs. a carbon-12 atom standard, which has 6p,
6n & 6e.
C-12 defined as exactly 12 atomic mass units
(amu), so 1 amu = 1/12th the mass of carbon
12.
All atoms are then compared with this. (We
will do a lab to see how this works.)
Let’s complete the Table we saw before.
Subatomic Particles
Particle
Location
Charge
(C)
Mass (g) Mass
(amu)
p
Nucleus
+1.602
X10^-19
1.673
X10^-24
1.0073 =
n
Nucleus
0
1.675
X10^-24
1.0087 =
e
Outside
nucleus
-1.602
X10^-19
9.109
X10^-28
0.0006 =
Text Fig. 3-19, page 104.
1
1
0
Counting the Mass of Atoms (cont’d)
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We learned:
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Mass of a single atom depends on the
number of protons and neutron only.
(Why?)
Therefore, mass of an element should just
be whole numbers, right?
Then how can the mass of chlorine be
35.453 amu?
Explanation: relative abundance of
natural isotopes!
Relative Abundance of Isotopes
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Chlorine has two isotopes, Cl-35 (35 amu) & Cl-37 (37
amu).
Fractional abundance of Cl-35 is ~75%, and of Cl-37 is
~25%.
Use a weighted average to get the atomic mass of the
element. This reflects the mass and relative
abundance of isotopes.
Example: Three Cl-35 atoms for every Cl-37 atom.
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Total proton mass = 17 + 17 + 17 +17 = 68 amu.
Total neutron mass = 18 + 18 + 18 +20 = 74 amu.
Weighted average mass of Cl = (68 + 74)/4 = 35.5 amu.
Actual value = 35.453 amu if more precise data used.
We will always round atomic mass data to the nearest 0.1
amu.
What are the most common isotopes of He; Ne; Kr?
VOCABULARY (Chapter 3)
3-1 
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3-2 
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3-3
Atom
Law of Constant
Composition
Atomic Theory of
Matter
Cathode Ray
Cathode Ray Tube
Electron
Radioactivity
Nucleus
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Proton
Neutron
Atomic Mass Unit
(amu)
Atomic Number
Ion
Isotope
Mass Number
Atomic Mass
Strong Nuclear
Force
Did we meet the OBJECTIVES?
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Define the term “atom.”
Describe the history of Atomic Theory.
List the postulates of Dalton’s Atomic Theory.
Show how atomic structure and electricity are related.
Explain what studies of cathode rays and radioactivity
revealed about atoms.
Discuss Rutherford’s alpha-scattering experiment and
how it showed the existence of the nucleus.
Name/describe the three subatomic particles in
an atom.
Determine the number of protons, neutrons and
electrons in an atom or ion.
Define “isotope” and atomic mass.
NOTE: We will skip Section 3-4 (Changes in the
Nucleus). You are NOT responsible for it now.