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Chemistry
Atomic Origins
2015-08-14
www.njctl.org
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Table of Contents: Creation of Matter
Click on the topic to go to that section
· The Big Bang
The Big Bang
· Electrons & Protons
· The Nucleus
· Formation of the Elements
· Isotopes
· Radioactive Decay
· Half-Life
Return to Table
of Contents
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Chemistry
The observable Universe is made up
of amazing stuff. We more formally
call this stuff matter.
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Chemical Elements
Scientists have discovered all of the matter in our Universe is
made up of 116 different types of chemical elements.
About 90 of these elements occur naturally.
Humans have always been curious
about the nature of matter:
where did matter come from?
what is it made out of?
why does it behave the way it
does?
http://www.periodictable.com/
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The Beginning...
The Beginning...
Where did the elements that makes up the Universe come from?
You were correct if you said the prevailing theory is that the
Universe began with the "Big Bang," which is an event thought
to have occurred about 14 billion years ago.
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Big Bang Theory
Big Bang Theory
It is believed our Universe
began at a single point. This
one spot was thousands of
times smaller than the head of a
pin. It was also hotter and more
dense than any object we know
of today.
This heat still remains as
Cosmic Background Radiation.
This Universe began expanding suddenly and rapidly from this
single point. Consequently, every piece of matter, all the "stuff" in
the universe came from this small, dense spot!
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Doppler Effect
1 In 1929, Edwin Hubble observed the "redshift" of the universe
and concluded
galaxies
Students type that
their answers
here are moving away from each other.
What phenomenon causes the universe to appear red?
The Doppler effect occurs when a source of the wave is moving
with respect to an observer. A source moving toward an observer
has a higher frequency and shorter wavelength; the opposite is
true when a source is moving away from an observer. This works
with sound waves and the waves of an electromagnetic spectrum.
"Schema Redshift" by Rogilbert - Own work. Licensed under Public Domain via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Schema_Redshift.png#/media/File:Schema_Redshift.png
Click here for a video on
the doppler effect
http://njc.tl/pu
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2 Red light has the longest visible spectrum (wavelength), so
when Hubble
looked
at the
Students type
their answers
herenight sky the galaxies appeared
red because they were moving away from us!
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3 Scientists believe the Big Bang happened:
A 14 million years ago
How would they appear if they were moving toward us?
B 14 trillion years ago
C 14 billion years ago
D within the last 3000 years
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Energy and Matter
14 billion years ago, in the flash of the Big Bang high energy
photons (light particles) collided with each other, forming
oppositely charged particles. Typically, when this happened
the oppositely charged matter and antimatter annihilated
each other instantly, converting back into high energy
photons.
Photons
Photons
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4
Energy and Matter
Students type their answers here
In the first seconds of the Universe, for reasons scientists
cannot explain, it is estimated that one particle of matter for
approximately every one billion particles of antimatter were
not annihilated. (You could win a Nobel Prize if you figure out
why!)
In this environment three major particles formed:
positively charged particles
+
neutrally charged particles
negatively charged particles -
Charged
Matter
Oppositely Charged
Antimatter
What are these positive, negative and neutral particles called?
What is the magnitude of their charge?
What are their masses?
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Cosmic Background Radiation
"As the universe expanded, both the plasma and the radiation filling
it grew cooler. When the universe cooled and stable atoms could
form, they eventually could no longer absorb the thermal radiation
and the universe became transparent instead of being an opaque
fog. The photons that from that time have been propagating ever
since, growing fainter and less energetic."
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5 Which of these pieces of evidence support the
Big Bang theory?
A red-shift
B cosmic background radiation
C both a and b
http://www.universetoday.com/79777/cosmic-background-radiation/
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Formation of the Elements
6 Following the Big Bang, the universe:
3 minutes after the Big Bang, the Universe began to cool
down from (1x 1032 °C to 1 x 109 °C) and protons and
neutrons began to combine.
A expanded and then rapidly stopped expanding.
B expanded and has not stopped expanding since.
C
rapidly expanded and then shrunk back to its
original size.
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Formation of the Elements
Stellar Furnaces
About 300,000 years later, the universe had cooled enough
for positively charged protons to attract the negatively
charged electrons, and the first atoms were formed.
-
-
Hydrogen-1
+
+
-
-
Hydrogen-3
Deuterium
Tritium
-
-
+4
4
-
-
Lithium-7
In smaller stars like our Sun, the
temperatures are 15.5 million C at the
core, hot enough to make Helium from
Hydrogen only.
-
5
-
Helium-4
As the cloud of cosmic dust and gases from the Big Bang cooled,
stars formed, and these then grouped together to form galaxies
and stars.
The high pressure and temperature
within Stars caused protons and
neutrons to fuse together.
+
Hydrogen-2
+3
During the formation of the universe only atoms of the lightest
elements - hydrogen, helium, lithium and beryllium were formed.
-
+
+
-
Beryllium-9
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Larger Elements
Formation of Heavier Elements
In the core of hotter, larger giant stars:
hydrogens fuse to make helium
Sun
heliums fuse to make atoms
with 4 protons - beryllium
helium and beryllium fuse to
make atoms with 6 protons carbon
carbon and helium fuse to make
atoms with 8 protons - oxygen,
etc., and in this manner
elements with up to 12 protons
formed.
+
+
Red Supergiant
Blue
Supergiant
Red Giant
Blue
Giant
Atoms of elements
aluminum to iron formed in
Super Giant stars.
26
.
-
+
26
30
The most massive elements
from iron to uranium were
created in star explosions
called supernovae.
.
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Periodic Table of Nucleosynthesis
"We Are Made from Star Stuff"
Atoms, the building blocks of matter, formed in the intense
heat and pressure of the early universe, stellar furnaces and
supernovae.
Everything around us was once part of a star.
In this course we will explore the nature of matter and apply
principles of physics to understand atomic structure, chemical
properties and predict chemical behavior.
Click here to watch a video on the
formation of the Elements.
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.
Discovery of the Electron
In the late 1800's scientists were passing electricity through glass
tubes containing a very small amount of gas like oxygen. When the
power was turned on, the tube emitted light and glowed.
Atomic Structure:
Electrons & Protons
POWER
OFF
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POWER
ON
The positive electrode is called the
anode and the negative called the
cathode. They called the rays
"cathode rays" because they
appeared to be coming from the
negative end of the tube.
Return to Table
of Contents
.
+
-
+
Actual Cathode Ray Tube
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Waves vs. Particles
There was much speculation about what these "cathode rays" were.
When an object was placed in the path of the rays, the rays cast
shadows of the objects placed in their path.
Light waves casts
a shadow - so it
could be light.
Or, it could be a
stream of tiny
particles.
7
Cathode
Rays in an Electric Field
Students type their answers here
Scientists found that they could deflect this beam by subjecting it to
an additional electrical field.
-
+
+
POWER
ON
Why would the beam deflect
toward the positive plate?
Does that indicate the rays are
light rays or particles?
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Cathode Rays in a Magnetic Field
8
9
Students type their answers here
Scientists found that they could also deflect this beam by subjecting
it to a magnetic field.
+
Charge to Mass Ratio
Students type their answers here
Scientists determined that a very weak electrical field could deflect the
beam a great deal.
-
-
-
+
+
POWER
ON
POWER
ON
If the particles are really easy to deflect they either have a
very small _______ or a very large _________ or both.
Why would the beam deflect upward in the magnetic field above?
Does that indicate the rays are light rays or particles?
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.
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.
Negatively Charged Particles - Electrons
Charge to Mass Ratio
J.J. Thomson and team were able to determine this charge to
mass ratio to be:
1.76 x1011 Coulombs of charge/ kg of mass or C/kg
Physicists proposed these negatively charged particles be called
electrons. These particles have the same charge to mass ratio as the
negative particles generated by static electricity, heated materials, and
illuminated materials.
Keep in mind, at this point they knew neither the charge nor the mass,
just that the ratio was large indicating either a large charge or a
small mass.
What was very interesting was that these negatively charged particles
were found in all gases they experimented on and they all had the
same charge to mass ratio.
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10
What characteristic about the cathode rays led them to
believe they were negatively charged?
A They were small
B Their behavior in an electric field
C Their behavior in a magnetic field
D b and c
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11
Which of the following indicated the cathode rays had a
large charge to mass ratio?
A They were small
B They were easily deflected
C They were deflected towards a positive electrode
D They were deflected towards a negative electrode
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.
Millikan Oil Drop Experiment
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.
Millikan Oil Drop Experiment: Sample Data
Here are some sample data points from Millikan's experiment.
A scientist named Millikan squirted oil drops into a box and then
passed high energy x-rays at the box hoping to knock electrons off
the air molecules and onto the oil drops.
Oil drops
+
X-rays
-
Drop
Charge (Coulombs)
1
4.8 E -19
2
3.2 E -19
3
6.4 E -19
4
9.6 E -19
Interestingly, he found that the charges on each drop were a
multiple of a number. Can you find what number they are all a
multiple of?
Click here to see an animation of the experiment
By measuring the energy necessary to stop the drops from
descending, he was able to determine the charge per drop. The more
energy needed to prevent the drop from falling, the smaller the
charge of the drop.
-19
= 1.6x10for
Coulombs
move
answer
He correctly interpreted this to be the charge of an electron.
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13 Which of these could be the charge of a drop in
the Millikan oil drop experiment?
12 If the charge of an electron is 1.6 x 10-19 C and the
charge to mass ratio is 1.76 x1011 C/kg, what is the
mass of an electron?
A 0.80 x 10-19 C
B 2.0 x 10-19 C
A 1.6 x 10-19 kg
C
8.0 x 10-19 C
B 2.82 x 10-8 kg
D
4.0 x 10-19 C
C 9.1 x 10-31 kg
D 1.1 x 1030 kg
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.
14
The magnitude of the charge on an electron
was determined in the __________.
A cathode ray tube, by J. J. Thomson
B Millikan oil drop experiment
C Dalton atomic theory
Discovery of the Proton
After the discovery of the electron, scientists believed that there
must also be a positively charged particle in the atom. To look for
these, they used an anode ray tube.
-
Power
D atomic theory of matter
Positive
anode rays
+
By placing holes in the cathode so particles could move through
it, they found that particles were indeed moving from the anode
to the cathode. Since they move towards a negative plate, they
must be positive.
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.
Discovery of the Proton
The anode rays were referred to as protons, which were found to be
significantly heavier than electrons.
1 proton = 1840 x mass of electron
Since the heaviest anode rays in oxygen were found to be 8 x heavier
than those in hydrogen, it was assumed that oxygen had 8 protons
compared to hydrogen's 1.
15
Which of the following is TRUE regarding
protons?
A They were originally called cathode rays
B They move faster than cathode rays
C They have a larger mass than electrons
D They moved from the cathode to the anode
The number of protons an atom has is different for each element on the
periodic table.
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16
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Which of the following is NOT true regarding protons
and electrons?
17 The mass of an electron was found to be 9.1 x 10-31 kg.
What is the mass of a proton?
A 1.67x10-27 kg
A Both were found in all atoms
B Their charges are equal in magnitude
C Protons are significantly heavier than electrons
D All elements have the same number of protons
and electrons
B 4.95x10-34 kg
C 9.1x10-31 kg
D 1.6x10-19 kg
1 proton = 1840 x mass of electron
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Models of the Atom: Plum Pudding
Once it was determined that atoms
are made up of negatively and
positively charged particles, J.J.
Thompson and team proposed that
the structure of an atom resembled
"plum pudding."
The Nucleus
The model featured a positive
sphere of matter with negative
electrons embedded in it. It was
based around the idea that positive
and negative charges attract and
like charges repel.
Return to Table
of Contents
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Radioactivity
Of course, models must be tested and the search was on to find
evidence to support the "plum pudding" model.
Ernest Rutherford used radioactivity used to test this theory.
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Radioactivity
Radioactivity is the spontaneous emission of radiation (energy) by
an atom. Rutherford studied emissions from the unstable element
uranium.
Larger elements like uranium contain an atomic nucleus that can be
either stable and does not change, or radioactive, meaning that it
transforms, or decays, into another element after a certain amount
of time. Decay can be as short as a fraction of a second and as
long as a few million years.
Radioactive Decay: Nucleus breaking
into smaller nuclei and releasing energy.
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Radioactivity
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18
Three types of radiation were discovered by Ernest Rutherford:
Of the three types of radioactivity characterized
by Rutherford, which are particles?
A
B
C
D
α-rays - alpha particles (positively charged particles with a mass
roughly 4x that of the proton)
β-rays - beta particles (electrons)
α-rays, β-rays, and γ-rays
γ-rays
α-rays and γ-rays
α-rays and β-rays
γ-rays - gamma rays (form of light with very high energy)
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19
Beta-particles are attracted to a ________ charged
plate, indicating they are __________ charged.
A positively, negatively
B negatively, positively
C neutrally, negatively
D neutrally, positively
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20
Alpha particles are __________ charged.
A
B
C
D
negatively
positively
neutrally
unknown
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Rutherford's Gold Foil Experiment
Physicists Geiger
and Marsden
under the direction
of Ernest
Rutherford shot a
beam of alpha
particles at a thin
sheet of gold foil
and observed the
scatter pattern of
the particles.
Click here to see an animation of the experiment
Discovery of the Nucleus
In the Plum Pudding Model of the atom, positive and negative
charges are dispersed evenly throughout the atom. If this
model were correct, the high energy alpha particles would be
slightly deflected by weak electric fields as they passed
through the foil.
Rutherford and team expected all alpha particles to pass through
the atoms in the gold foil and be deflected by only a few degrees.
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Discovery of the Nucleus
What actually
happened was
very surprising.
21
Discovery of the Nucleus
Students type their answers here
While most particles went
straight through
some bounced back...totally
unexpected?
What does this indicate about
the location of protons in an
atom?
Most of the
particles flew
right through
the foil with no
deflection at all.
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The Nuclear Atom Model
gold foil
The only way to account for
the large angles was to
assume that all the positive
charge was contained
within a tiny volume.
Then I remember two or three days later Geiger coming to me
in great excitement and saying "We have been able to get some
of the alpha-particles coming backward …"
It was quite the most incredible event that ever happened to me
in my life. It was almost as incredible as if you fired a 15-inch
shell at a piece of tissue paper and it came back and hit you.
A small very dense
nucleus must lie within a
mostly empty atom.
Now we know that the
radius of the nucleus is
1/10,000 that of the atom.
In Rutherford's words...
nucleus
alpha particle
- Rutherford
gold atom
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22
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23
The gold foil experiment performed in
Rutherford's lab __________.
In the Rutherford nuclear-atom model:
A confirmed the plum-pudding model of the atom
A the heavy subatomic particles reside in the nucleus
B led to the discovery of the atomic nucleus
B the principal subatomic particles all have
essentially the same mass
C was the basis for Thomson's model of the atom
C the light subatomic particles reside in the
nucleus
D mass is spread essentially uniformly throughout the
atom
D utilized the deflection of beta particles by gold
foil
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.
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.
Discovery of the Neutron
Discovery of the Neutron
Since electrons were so much smaller than protons, Rutherford
believed the mass of an atom would be simply related to the
number of protons present. However, they found that atoms were
heavier than predicted!!
Example - Helium (He)
Helium = 2 protons, 2 electrons
Expected mass = 2 x (mass of proton)
Actual mass = 4 x (mass of proton)
Example - Helium (He)
Helium = 2 protons, 2 electrons
Where is the extra mass coming from?
Expected mass = 2 x (mass of proton)
Rutherford guessed it came from another particle called a neutron
and verified its existence.
Actual mass = 4 x (mass of proton)
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.
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.
Subatomic Particles
Neutrons have a mass that is essentially the same as
a proton and no charge.
The mass of a proton or neutron is described as an
atomic mass unit (u).
1 u = 1.66053892x10-27 kg
Particle
Charge
Mass
proton
+1.6 x 10-19 C 1.6726 x10-27 kg = 1.0073 u
neutron
no charge
1.6749 x10-27 kg = 1.0087 u
electron
-1.6 x 10-19 C
9.1 x10-31 kg = 0.00055 u
Neutrons, Protons, and Atomic Masses
Since electrons have a much smaller mass than a proton or
neutron, the mass of an atom (in amu) is generally
considered to be equal to the sum of the protons and neutrons in
an atom.
(# of protons) + (# of neutrons) = atomic mass (A) in amu
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The Nuclear Atom
24 What is the mass of an element that has 10 protons and
11 neutrons (in u)?
Volume occupied by
by electrons
Nucleus containing
protons and neutrons
Rutherford postulated a
very small, dense
nucleus containing
protons and neutrons
with the electrons around
the outside of the atom.
o
Most of the volume of
the atom is empty space.
10 A
-4
o
1-5A
o
scale: 10 A = 1 nm
Click here to see Atom animation
o
A = 10-10 m
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25 How many neutrons are present in an oxygen
atom with a mass of 18 u?
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26 How many protons are present in atom with a
mass of 13 u if it has 7 neutrons?
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27 What is the mass of an element with 18 protons, 18
electrons, and 22 neutrons?
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Nomenclature
The number of protons in a nucleus is called the atomic number,
and it is designated by the letter Z.
This number is given for each element on the periodic table, often
directly above the chemical symbol.
1
H
Hydrogen
1.0079
Atomic Number
Atomic Symbol and Name
Atomic Mass
92
U
Uranium
238.029
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.
Nomenclature
Atomic Symbols and Atomic Masses
Together, protons and neutrons are referred to as nucleons.
There are two common ways to indicate the mass of a particular
atom.
Method 2 (Nuclear Symbol)
Method 1
The number of nucleons in a nucleus is called the mass number,
and it is designated by the letter A.
The neutron number, N, is given by N = A - Z.
A
Z
X-A
X
Where X is the chemical symbol, Z is the atomic number, and A is the
mass number.
Example:
107
Ag - 107
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28
How many neutrons are present in a neutral atom of
Sr-80?
47
Ag
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29 Find the mass number.
A 32
B 38
C 80
23
11
D 42
Na
Sodium Atom
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30
How many protons does this element have?
23
11
Na
Sodium Atom
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31
How many electrons does this element have?
23
11
Na
Sodium Atom
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32
How many neutrons does this element have?
23
11
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33 How many neutrons does this element have?
Na
80
35
Sodium Atom
Br
Bromine Atom
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34 Which of the following has 45 neutrons?
A
80
B
80
C
78
D
103
Kr
Br
Formation of the Elements
Se
Rh
Return to Table
of Contents
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Atoms
Recall, after the Big Bang, hydrogen, the lightest type of
atom, was the first to form.
Hydrogen contains one proton and one electron.
-
+
Hydrogen-1
What is hydrogen's nuclear symbol?
Atoms
Protons and neutrons continued to collide and were held
together by the Nuclear Strong Force, creating more massive
versions of Hydrogen called Deuterium and Tritium.
+
Hydrogen-1
-
-
-
+
+
Hydrogen-2
Hydrogen-3
Deuterium
Tritium
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Nuclear Fusion Reactions
When protons and neutrons bind in a nuclear reaction, they lose a
bit of mass, which is released as energy.
The amount of energy released is called the "binding energy" and
its magnitude can be found using mass-energy equivalence.
*
Binding Energy and Mass Defect
For example, if we want to calculate the mass defect and
10
binding energy of a Boron isotope 5 B.
There are 5 protons, 5 electrons and 5 neutrons. The mass of
1
Hydrogen is equivalent to the mass of a proton1 H .
1
0
1
1
10
5
Eb = Δmc2
Energy
+
+
+
+
-
n: 5 x 1.008665u
H: 5 x 1.007825u
B: 10.012937u
To calculate the mass defect:
#m = 5 x mass(neutron) + 5 x mass(proton) - mass(Boron)
#m = (5 x 1.008665u) + (5 x 1.007825u) - (10.012937u)
-
#m = 0.06951
Helium-4
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*
Binding Energy and Mass Defect
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* 35
To calculate the binding energy we start by converting
Atomic mass units to kilograms.
#m = 0.069513u x
1.6605 x 10-27 kg
1u
Binding Energy is
A the energy required to separate the nucleus into its
constituent parts.
= 1.1543 x 10-28 kg
B the energy required to split an atom into its
constituent parts.
Then use the energy-mass equivalence to solve for binding energy.
E = #mc2
The binding energy is measured in Joules.
C the energy that holds the electrons in orbit about
the nucleus.
E = #mc2 = 1.1543 x 10-28kg)(3 x 108 m/s)2 = 1.0388 x 10-11 J
D the energy that pushes the protons apart.
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* 36
12
What is the mass defect of 6 C ?
12
6
1
0
1
1
C: 12.000000u
n: 1.008665u
H: 1.007825u
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*37
12
What is the binding energy (in Joules) of 6 C?
12
6
1
0
1
1
C: 12.000000u
n: 1.008665u
H: 1.007825u
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* 38
What is the mass defect of
238
92
1
0
1
1
238
92
U?
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* 39
What is the binding energy (in Joules) of
238
92
U?
238
92
1
0
1
1
U: 238.05078826u
n: 1.008665u
H: 1.007825u
U: 238.05078826u
n: 1.008665u
H: 1.007825u
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Nuclear Fusion
Nuclear Fusion
Making Helium occurs in 3 steps in the core of the star.
Making Helium occurs in 3 steps in the core of the star.
Step 1: Two hydrogen atoms fuse...
Step 2: A hydrogen and a deuterium atom fuse...
1
1
1
1
2
1
H+ H
H+e+v
+
Producing a deuterium atom, a positron, and a neutrino.
1
1
2
H + 1H
3
2
He + γ
Producing a Helium-3 atom and a gamma ray.
Positrons (e+) are the opposite of electrons with the same mass
and charge - only positive. Positron emission causes a proton to
become a neutron.
A neutrino has no charge and does not affect the reaction.
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Nuclear Fusion
Making Helium occurs in 3 steps in the core of the star.
Step 3: Two helium-3 atoms fuse...
3
2
3
He + 2He
4
2
1
He + 1H + 11H
Producing a Helium-4 atom and two hydrogen atoms.
Note: Steps 1 & 2 must occur twice to produce the required
helium-3 atoms.
Nuclear Fusion
The net effect is to transform four protons into a helium nucleus
plus two positrons, two neutrinos and two gamma rays.
1
41H
4
2
He + 2e+ + 2v + 2γ
A conservation law applies to these reactions. The Law of the
Conservation of Nucleon Number states that the total number of
nucleons (A) remains constant for all nuclear reactions.
A proton can change into a neutron (positron emission) or a neutron
can change into a proton (electron emission) - but the total number
of nucleons stays constant.
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40 Which of the following is true regarding a positron
emission?
41 In the following fusion reaction, how many nucleons are
in the unknown nucleus?
12
A increases the number of protons
6
1
C + 1H
X+γ
B increases the number of electrons
C increases the number of neutrons
D does not affect the nucleus of the atom
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42 Identify the unknown element in the nuclear reaction.
A Boron
B Carbon
12
6
1
C + 1H
43 In the following fusion reaction, how many nucleons are
in the unknown nucleus?
2
1
X+γ
3
1
H + 1H
X + 0n
C Nitrogen
D Oxygen
Slide 101 / 145
Slide 102 / 145
Nuclear Fission
44 Identify the unknown element in the nuclear reaction.
A Hydrogen-1
B Hydrogen-2
C Helium-3
D Helium-4
2
1
3
H + 1H
1
X + 0n
While nuclear fusion reactions release energy while generating
more massive elements, nuclear fission reactions also release
energy.
The target nucleus fissions into two nuclei of smaller masses and a
number of neutrons.
For example, the general equation for the fission of Uranium-235 is:
235
92
1
U + 0n
236
92
U*
X + Y + neutrons + Q
Note: Q represents energy released.
Slide 103 / 145
Slide 104 / 145
45 Identify the missing element in the following fission
reaction.
Nuclear Fission
Here are two examples of possible fission reactions:
235
92
1
236
92
1
236
92
U + 0n
235
92
U + 0n
U*
U*
92
1
94
1
141
56
Ba + 36 Kr + 30n + Q
140
54
Xe + 38 Sr + 20n + Q
235
92
1
U + 0n
236
92
U*
141
56
Ba +
__ + 310n + Q
A Kr
B Sr
Note that in either case the total number of nucleons is conserved.
C U
D Pu
Slide 105 / 145
46 Identify the missing element in the following fission
reaction.
235
1
137
___ + 52 Te + 2 10 n
92 U +0 n
Slide 106 / 145
47 Identify the missing element in the following fission
reaction.
235
92
1
133
U + 0n
1
___ + 55 Cs + 3 0n
A Kr
A Rb
B Zr
B Np
C Pd
C Cf
D Bk
D Cm
Slide 107 / 145
Nuclear Fission
The energy release in a fission
reaction is quite large. The smaller
nuclei are stable with fewer neutrons,
so multiple neutrons emerge from
each fission.
The neutrons can be used to induce
fission in surrounding nuclei,
causing a chain reaction.
Enrico Fermi built the first self
sustaining nuclear reaction in Chicago
in 1942. Here's a nice simulation:
http://njc.tl/s7
Slide 108 / 145
Nuclear Reactions
*
First fill in the missing component:
2
1
14
H + 7N
3
2
He + ___?
Next, find the mass defect:
m = 2.014102u+14.003074u-3.016029u-13.003355u = -0.002207u
Find the reaction energy:
E = #mc2 = -0.002207u x
E = -3.294 x 10-13 J
1.6605 x 10-27 kg
= 2.9979 x 108 m/s2
1u
Slide 109 / 145
* 48
Slide 110 / 145
* 49
Compute the Q value of the reaction.
2
1
3
H + 1H
2
1
H: 2.014u
3
1
H: 3.016u
1
0
4
235
92
n + 2 He?
1
U + 0n
94
38
1
140
Sr + 54 Xe + 2 0n
235
92
U: 235.044u
4
2
He: 4.003u
Slide 111 / 145
*
Compute the Q value of the reaction.
94
38
Sr: 93.9154u
140
54
Xe: 132.9059
Slide 112 / 145
*
Nuclear Fission
Nuclear Fission
The reactor is controlled by regulating how many neutrons are
free to strike other Uranium atoms. Cadmium and Boron control
rods are excellent neutron absorbers and are carefully adjusted
to absorb the right amount of neutrons to allow a self sustained,
controlled reaction.
Critical Mass is the mass of the fissionable material that is
required for nuclear fission to occur.
Nuclear reactors are designed with layers upon layers of safety
features and there is no possible way for a reactor to ever cause
a nuclear explosion.
This is a schematic of a nuclear power plant. The fission process
occurs in the Reactor Vessel (red), which heats water in a primary
loop, which boils water in the secondary loop. Then, you just have
a regular steam/turbine generator which generates electricity.
Slide 113 / 145
Nuclear weapons are designed to explode in a massively
uncontrolled chain reaction and are very, very different from a
nuclear reactor.
Slide 114 / 145
Isotopes
As you have seen, atoms of the same element can have different
numbers of neutrons. For example, some Carbon atoms have 6
neutrons, some carbon atoms have 8 neutrons.
Atoms of the same element that have differing numbers of neutrons are
called isotopes.
Isotopes
C-12
C-14
6 protons 6
6 neutrons 8
6 electrons 6
Return to Table
of Contents
Note: Isotopes of an element will always have the same number of
protons but differing masses due to the differing numbers of
neutrons.
Slide 115 / 145
Slide 116 / 145
Isotopes
Write the complete symbol for each of these isotopes.
50 Which pair of atoms constitutes a pair of
isotopes of the same element?
14
6
A
Neon 20
10 protons
10 neutrons
10 electrons
Ne
Neon 21
10 protons
11 neutrons
10 electrons
Neon 22
10 protons
12 neutrons
10 electrons
Ne
Ne
B
14
6
C
D
X
X
X
12
6
X
17
9
X
17
8
X
19
10
X
19
9
X
Slide 117 / 145
51 Which of the following is TRUE of isotopes of an
element?
14
7
Slide 118 / 145
52 An atom that is an isotope of potassium (K) must...
A They have the same number of protons
A Have 20 protons
B The have the same number of neutrons
B Have 19 neutrons
C They have the same mass
C Have 19 protons
D They have the same atomic number
D A mass of 39
E A and D
Slide 119 / 145
*
53
Which species is an isotope of 39Cl?
A
40
B
34 2-
C
36
D
39
Ar+
Slide 120 / 145
Isotopes and Atomic Masses
Not all isotopes are found in the same abundances in nature.
S
Cl Ar
Neon 20
10 protons
10 neutrons
10 electrons
90.48%
Neon 21
10 protons
11 neutrons
10 electrons
Neon 22
10 protons
12 neutrons
10 electrons
0.27%
9.25%
So in a 10,000 atom sample of neon, you would on average find...
9048
27
(atoms of each isotope of neon)
925
Slide 121 / 145
Slide 122 / 145
Calculating Atomic Masses
Atomic Masses and Mass Number
To determine the atomic mass of an element, one must know the
masses of the isotopes and how commonly they are found in
nature. Then a weighted average is calculated as shown below.
The atomic mass indicates the average atomic mass of allthe
isotopes of a given element. This is the number reported on the
periodic table.
The mass number indicates the exact relative mass of aparticular
isotope of that element. These numbers are NOT reported on the
periodic table.
Example: As we have seen, a sample of neon will consist of three
stable isotopes - Ne-20, Ne-21, and Ne-22. If the relative abundance
of these are 90.48%, 0.27%, and 9.25% respectively, what is the
atomic mass of neon?
10
Ne
Atomic mass
(an average - no single
neon atom has this mass)
How to calculate average atomic mass:
1. Multiply each isotope by its % abundance expressed as a decimal
2. Add the products together
20.18
20(.9048) + 21(0.0027) + 22(0.0925) = 20.18 amu
Slide 123 / 145
Example: Calculate Atomic Mass
Slide 124 / 145
*
54
Calculate the atomic mass of oxygen if it's
abundance in nature is:
99.76% oxygen-16,
0.04% oxygen-17, and
0.20% oxygen-18.
Carbon consists of two isotopes that are
stable (C-12 and C-13). Assuming that
98.89% of all carbon in a sample are
C-12 atoms, what is the atomic mass of
carbon?
First, 100-98.89 = 1.10% C-14
move
for answer
then...
12(.9889) + 13(.011) = 12.01 amu
(liquid oxygen)
Slide 125 / 145
55
Calculate the atomic mass of copper.
Copper has 2 isotopes. 69.1% has a mass of
62.9 amu, the rest has a mass of 64.93 amu.
Slide 126 / 145
56
Sulfur has two stable isotopes: S-32 and S-34. Using
the average atomic mass on the periodic table, which
of the following best approximates the natural
relative abundances of these isotopes of sulfur?
A
B
C
D
50% S-32
25% S-32
75% S-32
95% S-32
and 50% S-34
and 75% S-34
and 25% S-34
and 5% S-34
Slide 127 / 145
Slide 128 / 145
Application of Isotopes
Elephants are hunted for the ivory
in their tusks. Game wardens use
isotopes to track where elephants
are going so they can help protect
them.
If an elephant eats plants from a
wet climate, the ratio of N-15 to
N-14 in the hair will be lower
than is typically found in nature.
If they graze plants grown in a
dry climate, they will have a
higher ratio of N-15 to N-14 than
normal.
Radioactive Decay
Where would you look for an elephant
that had a hair sample with a ratio of
0.0045 N-15/N-14 where the normal
ratio is 0.0034 N-15/N-14?
Return to Table
of Contents
Slide 129 / 145
Slide 130 / 145
Radioactivity
Nuclear Stability Curve
There are around 260 stable
nuclear isotopes. The curve on
the right plots N (neutron number)
vs. Z (proton number). The most
stable nuclei are shown in red, with
the least stable shown in blue.
More neutrons are required in stable
higher mass nuclei - the short range
nuclear force's ability to counteract
the repulsive Coulomb force is
reduced as the nucleus grows
larger.
http://njc.tl/rv
Non stable nuclei become stable nuclei by emitting radiation.
This is called radioactivity and was first observed and studied by
Henri Becquerel, Marie Curie and Pierre Curie.
Recall there are three types:
Alpha particles - helium nuclei.
Beta particles - a neutron is converted into a proton and emits
an electron and an anti-neutrino. When a proton is converted
into a neutron, it emits a positron (postively charged electron)
and a neutrino. The beta particles are these electrons and
positrons emitted from the nucleus.
Gamma rays - high energy (high frequencey) electromagnetic
radiation released when an excited nucleus moves to a lower
energy level and releases the excess energy in the form of a
photon.
http://njc.tl/rv
Slide 131 / 145
Slide 132 / 145
Radioactivity Stopping Power
Alpha particles are stopped
by a sheet of paper.
Beta particles are stopped
by a thin sheet of aluminum.
Decay Nomenclature
Alpha Decay is when a nucleus emits a Helium nucleus (2
protons, 2 neutrons, 0 electrons, with a charge of +2e). It is
represented as shown below:
A
Z
A
Z
X+
Z+1
Y+e +v
X+
4
Z-1
Z + e+ + v
Gamma Radiation is the emission of a photon when an excited
nucleus decays to a lower energy level.
A
Z
http://njc.tl/rv
Y + 2 He
Beta Decay is when a neutron converts into a proton and emits an
electron and an anti-neutrino (to conserve momentum) OR a
proton converts into a neutron and emits a positron and a
A
4
neutrino.
Z
Gamma rays are the most
penetrating and are stopped by
several meters of lead.
4
A-4
Z-2
X+
http://njc.tl/rv
X*
A
Z
X+γ
Slide 133 / 145
Slide 134 / 145
Alpha Decay
Beta Decay
An example of a nucleus that undergoes alpha decay is the
following isotope of polonium. We can find out what it decays into
by balancing out the atomic (Z) and mass numbers (A).
212
84
Po
Ra
Electron & Anti-neutrino
4
208?
82
11
Pb + 2He
4
Be
?
214
86
22
4
Rn + 2He
http://njc.tl/rv
Na
11
0
B +-1 e
22
?
10
0
B +-1 e
http://njc.tl/rv
Slide 135 / 145
57 Which type of radiation is the hardest to shield a person
from?
Slide 136 / 145
58 Which type of radiation is stopped by the shirt you wear?
A Alpha particles.
A Alpha particles.
B Beta particles.
B Beta particles.
C Gamma rays.
C Gamma rays.
D X-rays.
D X-rays.
http://njc.tl/rw
http://njc.tl/rx
Slide 137 / 145
59 What is the missing component?
Students type their answers here
12
5
http://njc.tl/ry
11
?
5
Positron & Neutrino
Another example is Radium 218.
218
88
Here are two examples of Beta Decay.
B
12
6
Slide 138 / 145
60 What is the missing component?
Students type their answers here
C+?
190
84
http://njc.tl/rz
Po
4
2
He + ?
Slide 139 / 145
Slide 140 / 145
61 What is the missing component?
Students type their answers here
238
92
U
234
90
Th + ?
Nuclear Half-life
Return to Table
of Contents
http://njc.tl/s0
http://njc.tl/s1
Slide 141 / 145
Slide 142 / 145
Nuclear Half-life
A macroscopic sample of any radioactive substanceconsists of a
great number of nuclei. These nuclei do not decay at one time.
The decay is random and the decay of one nucleus has nothing
to do with the decay of any other nuclei.
The number of decays during a specific time period is proportional
to the number of nuclei as well as the time period.
Mathematically, it is defined as an exponential decay. After each
specific time period, half of the nuclei decay. This specific time
period is called the isotope's half-life.
Nuclear Half-life
The half life of an isotope is defined as the amount of time it
takes for half of the original amount of the isotope to decay.
For example, find how much of a starting sample of 200 g of an
isotope, whose half life is 2 years, is left after 6 years:
After 2 years (one half-life), 100 g are left.
After 4 years (two half-lives), 50 g are left.
After 6 years (three half-lives), 25 g are left.
The isotopes of a specific element have very different half-lives;
ranging from μseconds to never decaying at all.
http://njc.tl/s1
Slide 143 / 145
Nuclear Half-life
Another way of solving this problem is to recognize that a time
interval of 6 years will include 3 half-life periods of 2 years.
Slide 144 / 145
62 The half life of an isotope is 5.0 seconds. What is the
mass of the isotope after 30.0 seconds from a starting
sample of 8.0 g?
n = number of half-lives = 3
x = original sample size
y = sample size after 3 half-lives
The 2 in the denominator represents the sample size being cut
in half after each half-life.
y = x = 200g = 25g
23
2n
http://njc.tl/s1
http://njc.tl/s2
Slide 145 / 145
63 The half life of an isotope is 3 hours. How long (in hours)
will it take for a sample of 500.0 g to be reduced to 62.50
g?
http://njc.tl/s3