Unit 3 Notes – Atomic Structure
Section 4.1 – Early Ideas About Matter (pages 102 – 105)
Main Idea: The ancient Greeks tried to explain matter, but the scientific study of the atom began with
John Dalton in the early 1800s.
1. Greek Philosophers – Many believed that matter was composed of things such as _________,
_________, _________, and _________. It was also commonly believed that matter could be
endlessly divided into ________________________ pieces.
a) Democritus (460-370 B.C.) – was the first person to propose the idea that matter was not
infinitely divisible. His ideas:
 Matter is composed of atoms which move through empty space.
 Atoms are solid, homogeneous, indestructible, and indivisible.
 Different kinds of atoms have different _________ and _________.
 Size, shape, and movement of atoms determine the properties of matter.
However, his ideas were met with criticism from other philosophers because he could not
explain…
Question: Why was it hard to Democritus to defend his ideas?
b) Aristotle (384-322 B.C.) – He rejected the idea of atoms. His ideas:
 __________ __________ cannot exist
 Matter is composed of earth, fire, air, and water.
2. Law of Conservation of Mass – states that mass is conserved in any process, such as a chemical
reaction. Proposed by _____________________________.
3. Law of Definite Proportions – (Joseph Proust – 1794)
The Law of Definite Proportions states that a compound is always composed of the same
elements in the same proportion by mass, no matter how large or small the sample is.
“The proportions of masses of chemicals in reactions are always the same.”
Example:
8g O
+
1 g H yields
16g O
+
2 g H yields _____ g H2O
___ g O +
9 g H2O
__ g H yields 36 g H2O
4. John Dalton (1766-1844) – Dalton revived and revised Democritus’s ideas based on the results
of his scientific research. He studied ____________ ____________, making careful
measurements and observations along the way. He was able to determine the mass ratios of
the elements involved in these reactions. His research led to Dalton’s atomic theory and the law
of multiple proportions.
Dalton’s Atomic Theory (1803) – Consists of 5 Postulates





All matter consists of indivisible and indestructible particles called
atoms.
All atoms of a given element are ______________ in mass and
properties but are different from atoms of different elements.
Atoms cannot be created or destroyed during chemical reactions.
Atoms are ________________ during chemical reactions.
Atoms of different elements may combine with each other in a fixed,
simple, ___________ number ratios to form compounds
Question: Which of the statements from above are incorrect according to our current
understanding of matter?
Dalton’s model of atoms was called the _______________ ________________ model.
5. Law of Multiple Proportions (Dalton-1804)
Dalton observed in the laboratory that some elements can combine in multiple proportions.
Originally he believed that he found a flaw in Proust’s work. However, he realized that
elements can combine in more than one ration depending on the environmental conditions.
The Law of Multiple Proportions that when two or more different compounds are formed by
a combination of the same elements, different masses of one element combine with the
same relative mass of the other element in a ratio of small whole numbers.
Example – Carbon and oxygen link up to form two different compounds. CO and CO 2
Mass of carbon in
one mole of each
compound.
Mass of oxygen in
one mole of each
compound.
Mass Ratio (mass of
O / Mass of C)
CO
CO2
Example 2 – Law of Multiple Proportions
Nitrogen can combine with oxygen in three different proportions
Nitrogen
1.0 g
1.0 g
1.0 g
Oxygen
1.75 g
.875 g
.438 g
Chemical Formula
NO2
Section 4.2 – Defining the Atom (pages 106 – 114)
Main Idea: An atom is made of a nucleus containing protons and neutrons; electrons move around the
nucleus.
1. The Atom – The smallest particle of an element that retains the _______________ of that
element.
a) Atoms are extremely small. To try to help you visualize how small atoms really are, look at
the following examples:



There are 2.9 x 1022 atoms of copper in a penny.
The diameter of a single copper atom is 1.28 x 10-10 m
If you increased the size of an atom to the size of an orange, you would have to
increase the size of an orange to the size of _______________ to keep the same
proportions.
b) Scanning tunneling microscope (STM)




An expensive tool first developed in 1981 that allows scientists to see and manipulate
materials at the atomic level.
Has resolution of .1 nm
Operates at various temperatures but often will operate at close to 0 K.
Using an STM, scientists are now able to move individual atoms around to form
shapes, patterns, and even simple machines (field of nanotechnology)
2. – Basic Information about Subatomic Particles
Particle
Symbol
Location
p+
n0
e-
1 atomic mass unit = ____________
Relative Electric
Charge
Relative Mass
Actual Mass
CATHODE RAY TUBE (CRT) EXPERIMENT
Lead Scientist
Year
Location of Experiment
Experimental Design & Observed Results
No Magnetic
Field Present
With Magnetic
Field Present
Question - In the second image, where is the positive end of the magnet located?
Notes about the experiment:
Using a voltage source, Thomson shot a beam of electricity across the cathode ray tube. He was able to
__________________________________________________________ using a magnet. Therefore he concluded
that negatively-charged particles must exist and he called these particles ________________________. He
changed the model of the atom from Dalton’s _______________ _________________ to a new model that he
called the _______________ _________________ model. He determined that the mass of an electron was
almost 2000x less than the mass of a hydrogen atom (which was the lightest known particle at the time)
PLUM PUDDING MODEL (1904)
This model was proposed by JJ Thomson in 1904 to try to explain questions that scientists
had due to the discovery of the electron such as how can all matter be _______________ if they
contain negatively-charged particles.
Recommended Videos
https://www.youtube.com/watch?v=IdTxGJjA4Jw
Discovery of the Electron – In Search of Giants
https://www.youtube.com/watch?v=CsjLYLW_3G0
Cathode Ray Tubes
GOLD FOIL EXPERIMENT
Lead Scientist
Year
Location of Experiment
EXPERIMENTAL DESIGN
In his experiment, Rutherford “fired” positively-charged alpha particles (made up of 2 protons and 2 neutrons) at a thin sheet
of gold foil. The detecting screen was made of zinc sulfide coated material that produced a flash when struck by an alpha
particle. Rutherford witnessed that _______ alpha particles went straight through the thin sheet of gold foil as expected.
However, a very small number (1 out of approximately 8000) of alpha particles were deflected at various angles, even
straight back at the alpha-particle emitter. How could this be possible? What was causing a small number to ricochet off of
the very thin sheet of gold foil?
Expected Results vs. Observed Results
Expected Results
If atoms were like plum pudding…..
All of the positively-charged alpha particles
would pass straight through the gold atoms
as shown to the right
Actual Observations
A small number of alpha particles ricocheted
off of the thin sheet of gold foil.
CONCLUSIONS
The gold foil experiment lead to two new atomic developments
1.
2.
RUTHERFORD’S NUCLEAR MODEL (1911)
Recommended Videos
https://www.youtube.com/watch?v=XBqHkraf8iE
GOLD FOIL EXPERIMENT – BACKSTAGE SCIENCE
https://www.youtube.com/watch?v=wzALbzTdnc8
DISCOVER OF THE NUCLEUS – IN SEARCH OF
GIANTS
ADDITIONAL EVENTS

In 1918, Ernest Rutherford discovered the ___________________ which led us to the
proton model of the atom.

In 1932, Rutherford’s coworker, English Physicist_____________ ___________
discovered that the nucleus contained a second subatomic particle called the
_________________.

Why do you think the neutron was the last subatomic particle to be discovered?
CURRENT MODEL OF THE ATOM: THE ELECTRON CLOUD MODEL
Electrons do not ______________ the nucleus like planets orbit the sun, but instead they move
randomly in areas outside of the nucleus called the electron cloud. The shaded region represents the
area where electrons will be found at least _______________ of the time. We will learn a lot more
about electrons and where they are located within the atom in unit 4!
Section 4.3 – How Atoms Differ (pages 115 – 121)
If there are 118 known elements on the periodic table, then what makes atoms different from each
other?
1) Atomic number - the number of ______________ contained in the nucleus of an atom.
a. Each element contains a unique number of ______________ in its nucleus.
b. The number of _____ in an atom identifies it as an atom of a particular element.
Gold atoms must contain _____________.
Copper atoms must contain ____________.
Oxygen atoms must contain ____________.
c. Because all atoms are neutral, then there must be an equal number of
__________________ and ________________.
d. Therefore, the atomic number will also tell you the number of ______________ for an
atom.
e. Atoms that gain or lose electrons are no longer _____________ and therefore are
referred to as ____________ not atoms.
f. ________ - atoms that have a charge.
2. Mass Number – the number of ______________ and _________________ in an atom.
2. Dalton was incorrect about atoms being indivisible and in stating that all atoms of the same
element are ______________________.
3. Atoms of the same element can have different numbers of _________________ in the
nucleus.
4. _______________ - atoms with the same number of protons, but different numbers of
________________.
A) To distinguish between different isotopes of the same element, scientists use the element’s
name followed by the specific atom’s mass number. This is called the isotope name.
For example, copper has two isotopes. The isotope with 29 protons and 34 neutrons is
called copper-63. The isotope with 29 protons and 36 neutrons is called copper-65.
B) Isotope Symbols: Scientists will also often use the chemical symbol of an element with up to 3
numbers located around it to tell us more about the specific isotope.
C) Write the isotope name and the isotope symbol of a carbon atom that contains 6 p+ and 8 n0.
D) Natural abundance of isotopes – Most elements have more than one isotope.
E) The most ____________ isotope of an element is the most abundant form in nature.
F) Usually, no matter where a sample of an element is obtained, the relative abundance of each
isotope is ___________.
G) Potassium has three naturally occurring isotopes; potassium-39, potassium-40, and potassium41.
Potassium-39
Potassium -40
Potassium -41
p+
19
19
19
n0
20
21
22
e-
19
19
19
Question: Can you determine the most abundant isotope of potassium by looking at the
periodic table?
3. Mass of Atoms
Subatomic Particle
p+
n0
e-
Relative Mass (amu)
1
1
1/1840
Actual Mass (g)
1.6727 x 10-24
1.6750 x 10-24
9.110 x 10-28

Atomic mass unit (amu) – Is defined as 1/12 of the mass of a carbon-12 atom.

Carbon-12 has six protons and six neutrons, therefore scientists assign each proton and
neutron a relative mass of one amu. (note that the values are slightly different). We refer
to these values as the relative atomic mass.

When finding the mass of an atom, we ignore _______________ because there mass is so
small in comparison.
4. Average Atomic Mass or Atomic Mass– the weighted average mass of all of the isotopes of an
element on Earth.


If you look at the periodic table and look at the atomic mass, it is usually not a whole
number. This is because it is the average mass of all of the isotopes of an element that
factors in their abundance in nature.

How is atomic mass calculated? Let’s take a look at chlorine, Cl.
Isotope
Percent Abundance
chlorine-35
76%
chlorine-37
24%
Calculate the atomic mass for lehmanium, Le.
Isotope
Percent Abundance
Le-343
12%
Le-347
24%
Le-349
64%
Section 4.4 – (pages 122 – 125)
Main Idea: Unstable atoms emit radiation to gain stability
1) Radioactivity – the spontaneous process in which substances emit ________________.
2) Radiation – the rays and particles that are emitted by radioactive materials.

There are 3 types of radiation – alpha particles, beta particles, and gamma rays
3) Radioactive decay – the spontaneous process in which unstable nuclei lose energy. Unstable
atoms undergo radioactive decay until they form stable atoms, often of a different element.
Effect of Magnetic Field
Penetrating Power
Radiation Type
Description
Penetrating Power
alpha particle
beta particle
gamma ray
Alpha decay – occurs when an alpha particle is ejected from an unstable nuclei. Results in a
more stable atom that has 2 fewer protons and neutrons. Note: The mass number stays the
same!
Examples:
Beta decay – occurs when a beta particle is ejected from an unstable nucleus. Results in a more
stable atom that has one additional proton, but one fewer neutron.
Examples:
Mass Spectrometer: