chemistry - njctlcontent.njctl.org/courses/science/chemistry/atomic-origins/atomic... · call this...
TRANSCRIPT
3
Table of Contents: Creation of Matter
• The Big Bang
• Formation of the Elements
• Electrons & Protons
• The Nucleus
Click on the topic to go to that section
• Isotopes
• Radioactive Decay• HalfLife
4
The Big Bang
Return to Tableof Contents
5
Chemistry
The observable Universe is made up of amazing stuff. We more formally call this stuff matter.
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?
6
Chemical ElementsScientists 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.
http://www.periodictable.com/
7
The Beginning...Where did the elements that makes up the Universe come from?
8
The Beginning...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.
9
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.
10
Big Bang Theory
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!
11
1 Scientists believe the Big Bang happened:
A 14 million years ago
B 14 trillion years ago
C 14 billion years ago
D within the last 3000 years
Answer C
12
Energy and Matter14 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.
Charged Matter
Oppositely Charged Antimatter
Photons Photons
13
2
Answer protons, neutrons, electrons
+1.6x1019 C, neutral, 1.6x1019 C1.67x1027 kg, 1.67x1027 kg, 9.1x1031 kg
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
+
What are these positive, negative and neutral particles called?What is the magnitude of their charge?What are their masses?
Energy and Matter
14
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."
http://www.universetoday.com/79777/cosmicbackgroundradiation/
15
3 Following the Big Bang, the universe:
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.
Answer
16
Formation of the Elements3 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.
+ +
17
Formation of the ElementsAbout 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.
+
Hydrogen1
+
Hydrogen2
Deuterium
+
Hydrogen3
Tritium
Helium4
++
Lithium7
+ 3
4
Beryllium9
+ 4
5
18
During the formation of the universe only atoms of the lightest elements hydrogen, helium, lithium and beryllium were formed.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.
Stellar Furnaces
The high pressure and temperature within Stars caused protons and neutrons to fuse together.
In smaller stars like our Sun, the temperatures are 15.5 million C at the core, hot enough to make Helium from Hydrogen only.
19
Larger Elements
hydrogens fuse to make helium
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 Red Giant
Blue Supergiant
Blue Giant
Sun
In the core of hotter, larger giant stars:
20
Formation of Heavier ElementsAtoms 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.
21
Periodic Table of Nucleosynthesis
22
"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.
23
Atomic Structure:Electrons & Protons
Return to Tableof Contents
24
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.
+
POWEROFF
+
POWERON
.
Actual Cathode Ray Tube
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.
25
.
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.
Waves vs. Particles
26
4
+
POWERON
+
Scientists found that they could deflect this beam by subjecting it to an additional electrical field.
Why would the beam deflect toward the positive plate?Does that indicate the rays are light rays or particles?
Answer
27
5
+
POWERON
x x x x x x x x xx x x x x x x x xx x x x x x x x xx x x x x x x x xx x x x x x x x xx x x x x x x x x
Scientists found that they could also deflect this beam by subjecting it to a magnetic field. Why would the beam deflect upward in the magnetic field above? Does that indicate the rays are light rays or particles?
Answer
28
6 Scientists determined that a very weak electrical field could deflect the beam a great deal. If the particles are really easy to deflect they either have a very small _______ or a very large _________ or both.
+
POWERON
+
Answer
29
.
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
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.
Charge to Mass Ratio
30
.
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.
Negatively Charged Particles Electrons
31
7 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
Answer
D b and c
32
8 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 Answer
33
Millikan Oil Drop Experiment.
A scientist named Millikan squirted oil drops into a box and then passed high energy xrays at the box hoping to knock electrons off the air molecules and onto the oil drops.
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.
Xrays
Oil drops+
Click here to see an animation of the experiment
34
.
Here are some sample data points from Millikan's experiment.
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?
= 1.6x1019 Coulombs
He correctly interpreted this to be the charge of an electron.
move for answer
Millikan Oil Drop Experiment: Sample Data
35
9 If the charge of an electron is 1.6 x 1019 C and the charge to mass ratio is 1.76 x1011 C/kg, what is the mass of an electron?
A 1.6 x 1019 kg
B 2.82 x 108 kg
C 9.1 x 1031 kg
D 1.1 x 1030 kg
Answer
36
10 Which of these could be the charge of a drop in the Millikan oil drop experiment?
A 0.80 x 1019 CB 2.0 x 1019 CC 8.0 x 1019 C
D 4.0 x 1019 C Answer
C
37
11 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
D atomic theory of matter
Answer
38
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
+
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.
39
.
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.
The number of protons an atom has is different for each element on the periodic table.
Discovery of the Proton
40
12 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 Answer
41
13 Which of the following is NOT true regarding protons and electrons?
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
Answer
42
14 The mass of an electron was found to be 9.1 x 1031 kg. What is the mass of a proton?
A 1.67x1027 kg
B 4.95x1034 kg
C 9.1x1031 kg
D 1.6x1019 kg
Answer
1 proton = 1840 x mass of electron
43
The Nucleus
Return to Tableof Contents
44
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 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.
Models of the Atom: Plum Pudding
45
RadioactivityOf 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.
46
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.
Radioactivity
Radioactive Decay: Nucleus breaking into smaller nuclei and releasing energy.
47
RadioactivityThree types of radiation were discovered by Ernest Rutherford:
αrays alpha particles (positively charged particles with a mass roughly 4x that of the proton)
βrays beta particles (electrons)
γrays gamma rays (form of light with very high energy)
48
15 Of the three types of radioactivity characterized by Rutherford, which are particles?
A αrays, βrays, and γraysB γraysC αrays and γraysD αrays and βrays
Answer
49
16 Betaparticles are attracted to a ________ charged plate, indicating they are __________ charged.
A positively, negatively
B negatively, positively
C neutrally, negatively
D neutrally, positively Answer
50
17 Alpha particles are __________ charged. A negativelyB positivelyC neutrallyD unknown
Answer
B
51
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
52
Discovery of the NucleusIn 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.
53
Discovery of the Nucleus
What actually happened was very surprising.
Most of the particles flew right through the foil with no deflection at all.
54
18 While most particles went straight through some bounced back...totally unexpected?
What does this indicate about the location of protons in an atom?
Answer
55
The Nuclear Atom Model
The only way to account for the large angles was to assume that all the positive charge was contained within a tiny volume.
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.
gold foil
alpha particle
gold atom
nucleus
56
In Rutherford's words...
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 alphaparticles 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 15inch shell at a piece of tissue paper and it came back and hit you.
Rutherford
57
19 The gold foil experiment performed in Rutherford's lab __________. A confirmed the plumpudding model of the atom
B led to the discovery of the atomic nucleus
C was the basis for Thomson's model of the atom
D utilized the deflection of beta particles by gold foil Answer
58
20 In the Rutherford nuclearatom model:
A the heavy subatomic particles reside in the nucleus
B the principal subatomic particles all have essentially the same mass
C the light subatomic particles reside in the nucleus
D mass is spread essentially uniformly throughout the atom
Answer
A
59
.
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)
Discovery of the Neutron
60
.
Example Helium (He)
Helium = 2 protons, 2 electrons
Expected mass = 2 x (mass of proton)
Actual mass = 4 x (mass of proton)
Discovery of the Neutron
Where is the extra mass coming from?
Rutherford guessed it came from another particle called a neutron and verified its existence.
61
.
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).
Particle Charge Mass
proton +1.6 x 1019 C 1.6726 x1027 kg = 1.0073 u
neutron no charge 1.6749 x1027 kg = 1.0087 u
electron 1.6 x 1019 C 9.1 x1031 kg = 0.00055 u
1 u = 1.66053892x1027 kg
62
.
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
63
The Nuclear Atom
Rutherford postulated a very small, dense nucleus containing protons and neutrons with the electrons around the outside of the atom.
Most of the volume of the atom is empty space.
104 Ao
15Ao
Nucleus containing protons and neutrons
Volume occupied byby electrons
10 A = 1 nmo
scale:
A = 1010 mo
Click here to see Atom animation
64
21 What is the mass of an element that has 10 protons and 11 neutrons (in u)?
Answer
65
22 How many neutrons are present in an oxygen atom with a mass of 18 u?
Answer
66
23 How many protons are present in atom with a mass of 13 u if it has 7 neutrons?
Answer
67
24 What is the mass of an element with 18 protons, 18 electrons, and 22 neutrons?
Answer
68
NomenclatureThe 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.
H1
1.0079Hydrogen
U92
238.029Uranium
Atomic NumberAtomic Symbol and Name
Atomic Mass
69
NomenclatureTogether, protons and neutrons are referred to as nucleons.
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.
70
.
There are two common ways to indicate the mass of a particular atom.
Atomic Symbols and Atomic Masses
Method 1 Method 2 (Nuclear Symbol)
Where X is the chemical symbol, Z is the atomic number, and A is the mass number.
Example:
X A XA
Z
Ag 107 Ag107
47
71
25 How many neutrons are present in a neutral atom of Sr80?
A 32
B 38
C 80
D 42 Answer
72
26 Find the mass number.
Na2311
Sodium Atom
Answer
73
27 How many protons does this element have?
Na2311
Sodium Atom
Answer
74
28 How many electrons does this element have?
Na2311
Sodium Atom
Answer
75
29 How many neutrons does this element have?
Na2311
Sodium Atom
Answer
76
30 How many neutrons does this element have?
Br8035
Bromine Atom
Answer
77
31 Which of the following has 45 neutrons?
A 80KrB 80BrC 78Se
D 103Rh
Answer
78
Formation of the Elements
Return to Tableof Contents
79
AtomsRecall, after the Big Bang, hydrogen, the lightest type of atom, was the first to form.
Hydrogen contains one proton and one electron.
+
Hydrogen1
What is hydrogen's nuclear symbol?
Answer
80
AtomsProtons and neutrons continued to collide and were held together by the Nuclear Strong Force, creating more massive versions of Hydrogen called Deuterium and Tritium.
+
Hydrogen1
+ +
Hydrogen2
Deuterium
Hydrogen3
Tritium
81
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 massenergy equivalence.
Eb = Δmc2
+ +
Energy
Helium4
++
82
Binding Energy and Mass Defect*To calculate the binding energy we start by converting Atomic mass units to kilograms.
Then use the energymass equivalence to solve for binding energy.
The binding energy is measured in Joules.
∆m = 0.069513u x1.6605 x 1027 kg
1u= 1.1543 x 1028 kg
E = ∆mc2
E = ∆mc2 = 1.1543 x 1028kg)(3 x 108 m/s)2 = 1.0388 x 1011 J
83
Binding Energy and Mass DefectFor example, if we want to calculate the mass defect andbinding energy of a Boron isotope B.
There are 5 protons, 5 electrons and 5 neutrons. The mass of Hydrogen is equivalent to the mass of a proton .
To calculate the mass defect:
5
n: 5 x 1.008665uH: 5 x 1.007825uB: 10.012937u
10
510
11
∆m = (5 x 1.008665u) + (5 x 1.007825u) (10.012937u)
∆m = 5 x mass(neutron) + 5 x mass(proton) mass(Boron)
*
84
32 Binding Energy is
A the energy required to separate the nucleus into its constituent parts.
B the energy required to split an atom into its constituent parts.
C the energy that holds the electrons in orbit about the nucleus.
D the energy that pushes the protons apart.
Answer
A
*
85
33 What is the mass defect of ?
Answer
12
6C: 12.000000u
12
6C
1
0n: 1.008665u1 H: 1.007825u1
*
86
34 What is the binding energy (in Joules) of ?
Answer
12
6C
12
6C: 12.000000u1
0n: 1.008665u1 H: 1.007825u1
*
87
35 What is the mass defect of U?
Answer
238
92U: 238.05078826u1
0n: 1.008665u1 H: 1.007825u1
238
92*
88
36 What is the binding energy (in Joules) of U?
Answer
238
92U: 238.05078826u1
0n: 1.008665u1 H: 1.007825u1
238
92*
89
Nuclear FusionMaking Helium occurs in 3 steps in the core of the star.
Step 1: Two hydrogen atoms fuse...
Producing a deuterium atom, a positron, and a neutrino.
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.
H + H H + e++ v1 1
1 1
2 1
90
Nuclear Fusion
Producing a Helium3 atom and a gamma ray.
H + H He + γ1 1
2 1
3 2
Making Helium occurs in 3 steps in the core of the star.
Step 2: A hydrogen and a deuterium atom fuse...
91
Producing a Helium4 atom and two hydrogen atoms.
H + H 1 1
1 1
4 2 He + 3
2 3 2 He + He
Nuclear FusionMaking Helium occurs in 3 steps in the core of the star.
Step 3: Two helium3 atoms fuse...
Note: Steps 1 & 2 must occur twice to produce the required helium3 atoms.
92
Nuclear Fusion
The net effect is to transform four protons into a helium nucleus plus two positrons, two neutrinos and two gamma rays.
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.
4 H He + 2e+ + 2v + 2γ1 1
4 2
93
37 Which of the following is true regarding a positron emission?
A increases the number of protons
B increases the number of electrons
C increases the number of neutrons
D does not affect the nucleus of the atom
Answer
94
38 In the following fusion reaction, how many nucleons are in the unknown nucleus?
Answer
C + H X + γ12 6
1 1
95
39 Identify the unknown element in the nuclear reaction.
A Boron
B Carbon
C Nitrogen
D Oxygen
Answer
C + H X + γ12 6
1 1
96
40 In the following fusion reaction, how many nucleons are in the unknown nucleus?
Answer
4
H + H X + n2 1
3 1
1 0
97
41 Identify the unknown element in the nuclear reaction.
A Hydrogen1
B Hydrogen2
C Helium3
D Helium4
Answer
H + H X + n2 1
3 1
1 0
98
Nuclear Fission 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 Uranium235 is:
Note: Q represents energy released.
U + n235 92
1 0 U* X + Y + neutrons + Q
236 92
99
Nuclear Fission
Here are two examples of possible fission reactions:
Note that in either case the total number of nucleons is conserved.
U + n U* Ba + Kr + 3 n + Q235 92
1 0
1 0
236 92
141 56
92 36
U + n U* Xe + Sr + 2 n + Q235 92
1 0
1 0
236 92
140 54
94 38
100
42 Identify the missing element in the following fission reaction.
A Kr
B Sr
C U
D Pu
Answer
U + n U* Ba + __ + 3 n + Q235 92
1 0
1 0
236 92
141 56
101
43 Identify the missing element in the following fission reaction.
A Kr
B Zr
C Pd
D Bk
Answer
U + n ___ + Te + 2 n 235 92
1 0
1 0
137 52
102
44 Identify the missing element in the following fission reaction.
A Rb
B Np
C Cf
D Cm
Answer
U + n ___ + Cs + 3 n 235 92
1 0
1 0
133 55
103
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://phet.colorado.edu/en/simulation/nuclearfission
104
Nuclear Reactions
First fill in the missing component:
Next, find the mass defect:
Find the reaction energy:
H + N He + ___?2 1
14 7
32
m = 2.014102u+14.003074u3.016029u13.003355u = 0.002207u
E = ∆mc2 = 0.002207u x1.6605 x 1027 kg
1u= 2.9979 x 108 m/s2
E = 3.294 x 1013 J
*
105
45 Compute the Q value of the reaction.
H + H n + He?2 1
3 1
42
1 0
H: 2.014u
H: 3.016u
He: 4.003u
2 13 1
42
*
Answer
106
46 Compute the Q value of the reaction.
U + n Sr + Xe + 2 n 235 92
1 0
9438
1 0
14054
235 92 U: 235.044u
Sr: 93.9154u3894
Xe: 132.905914054
*
Answer
107
Nuclear Fission
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.
*
108
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.
Nuclear weapons are designed to explode in a massively uncontrolled chain reaction and are very, very different from a nuclear reactor.
*
109
Isotopes
Return to Tableof Contents
110
IsotopesAs 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.
C12 C14
protonsneutronselectrons
666
686
Note: Isotopes of an element will always have the same number of protons but differing masses due to the differing numbers of neutrons.
111
Write the complete symbol for each of these isotopes.
Neon 2010 protons10 neutrons10 electrons
Neon 2110 protons11 neutrons10 electrons
Neon 2210 protons12 neutrons10 electrons
Ne Ne Ne
Isotopes
112
47 Which pair of atoms constitutes a pair of isotopes of the same element?
X147
14 6 XA
B
C
D
X612X14
6
X817X17
9
X919X19
10
Answer
113
48 Which of the following is TRUE of isotopes of an element?
A They have the same number of protons
B The have the same number of neutrons
C They have the same mass
D They have the same atomic number
E A and D Answer
114
49 An atom that is an isotope of potassium (K) must...
A Have 20 protons
B Have 19 neutrons
C Have 19 protons
D A mass of 39 Answer
115
50 Which species is an isotope of 39Cl?
A 40Ar+
B 34S2
C 36Cl
D 39Ar
Answer
*
116
Isotopes and Atomic MassesNot all isotopes are found in the same abundances in nature.
Neon 2010 protons10 neutrons10 electrons
Neon 2110 protons11 neutrons10 electrons
Neon 2210 protons12 neutrons10 electrons
90.48% 0.27% 9.25%
So in a 10,000 atom sample of neon, you would on average find...
9048 27 925(atoms of each isotope of neon)
117
Atomic Masses and Mass Number
The atomic mass indicates the average atomic mass of all the isotopes of a given element. This is the number reported on the periodic table.
The mass number indicates the exact relative mass of a particular isotope of that element. These numbers are NOT reported on the periodic table.
10
Atomic mass (an average no single neon atom has this mass)
20.18
Ne
118
Calculating Atomic MassesTo 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.
Example: As we have seen, a sample of neon will consist of three stable isotopes Ne20, Ne21, and Ne22. If the relative abundance of these are 90.48%, 0.27%, and 9.25% respectively, what is the atomic mass of neon?
How to calculate average atomic mass:
1. Multiply each isotope by its % abundance expressed as a decimal2. Add the products together
20(.9048) + 21(0.0027) + 22(0.0925) = 20.18 amu
119
Example: Calculate Atomic Mass
Carbon consists of two isotopes that are stable (C12 and C13). Assuming that 98.89% of all carbon in a sample are C12 atoms, what is the atomic mass of carbon?
First, 10098.89 = 1.10% C14then...
12(.9889) + 13(.011) = 12.01 amumove for answer
120
51 Calculate the atomic mass of oxygen if it's abundance in nature is:
99.76% oxygen16, 0.04% oxygen17, and 0.20% oxygen18.
(liquid oxygen)
Answer
*
121
52 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.
Answer
122
53 Sulfur has two stable isotopes: S32 and S34. 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 50% S32 and 50% S34B 25% S32 and 75% S34C 75% S32 and 25% S34D 95% S32 and 5% S34
Answer
123
If an elephant eats plants from a wet climate, the ratio of N15 to N14 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 N15 to N14 than normal.
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.
Where would you look for an elephant that had a hair sample with a ratio of 0.0045 N15/N14 where the normal ratio is 0.0034 N15/
N14?
Answer
124
Radioactive Decay
Return to Tableof Contents
125
Nuclear Stability Curve
There are around 260 stable nuclear isotopes. The curve on the right plots N (neutron number)vs. Z (proton number). The moststable nuclei are shown in red, withthe least stable shown in blue.
More neutrons are required in stablehigher mass nuclei the short rangenuclear force's ability to counteractthe repulsive Coulomb force isreduced as the nucleus grows larger.
126
RadioactivityNon 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 antineutrino. 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.
127
Radioactivity Stopping Power
Alpha particles are stoppedby a sheet of paper.
Beta particles are stoppedby a thin sheet of aluminum.
Gamma rays are the most penetrating and are stopped by several meters of lead.
128
Decay NomenclatureAlpha 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:
Beta Decay is when a neutron converts into a proton and emits an electron and an antineutrino (to conserve momentum) OR a proton converts into a neutron and emits a positron and a neutrino.
Gamma Radiation is the emission of a photon when an excited nucleus decays to a lower energy level.
X + Y + He A Z 2
4 A 4Z 2
X + Y + e + vA Z
4Z + 1
X + Z + e+ + vA Z
4Z 1
X* X + γAZ
A Z
129
Alpha DecayAn 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).
Another example is Radium 218.
Po Pb + He2 4208
82
21284
218 88Ra Rn + He
214 86
42
?
?
130
Beta Decay
Here are two examples of Beta Decay.
Electron & Antineutrino
Positron & Neutrino
Be B + e11 4
11 5
01?
Na B + e22 11
22 10
01?
131
54 Which type of radiation is the hardest to shield a person from?
A Alpha particles.
B Beta particles.
C Gamma rays.
D Xrays.
Answer
132
55 Which type of radiation is stopped by the shirt you wear?
A Alpha particles.
B Beta particles.
C Gamma rays.
D Xrays.
Answer
133
56 What is the missing component?
B C + ? 12 5
12 6
Answer
134
57 What is the missing component?
Po He + ? 190
84 4 2
Answer
135
58 What is the missing component?
Answer
U Th + ? 238 92
23490
136
Nuclear Halflife
Return to Tableof Contents
137
Nuclear HalflifeA macroscopic sample of any radioactive substance consists 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 halflife.
The isotopes of a specific element have very different halflives; ranging from μseconds to never decaying at all.
138
Nuclear Halflife
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 halflife), 100 g are left.
After 4 years (two halflives), 50 g are left.
After 6 years (three halflives), 25 g are left.
139
Nuclear Halflife Another way of solving this problem is to recognize that a time interval of 6 years will include 3 halflife periods of 2 years.
n = number of halflives = 3x = original sample sizey = sample size after 3 halflives
The 2 in the denominator represents the sample size being cut in half after each halflife.
y = x = 200g = 25g 2n 23
140
59 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?
Answer
141
60 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?
Answer
First, find out how many halflives need to occurto reduce the mass of the isotope to 62.50 g.
Three halflives occurred.
Total time = number of halflives x halflife period
Total time = 3 x 3 hours = 9 hours.