The Big Bang Electrons & Protons The Nucleus Formation of the - - PDF document

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Chemistry

Atomic Origins

2015-10-27 www.njctl.org

Slide 3 / 141 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 · Half-Life

Slide 4 / 141

The Big Bang

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• f Contents

Slide 5 / 141 Chemistry

The observable Universe is made up

• f 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?

Slide 6 / 141 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.

http://www.periodictable.com/

Slide 7 / 141 The Beginning...

Where did the elements that makes up the Universe come from?

Slide 8 / 141 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.

Slide 9 / 141 Big Bang Theory

It is believed our Universe began at a single point. This

• ne spot was thousands of

times smaller than the head of a

• pin. It was also hotter and more

dense than any object we know

• f today.

This heat still remains as Cosmic Background Radiation.

Slide 10 / 141 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!

Slide 11 / 141

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

Slide 12 / 141 Energy and Matter

14 billion years ago, in the flash of the Big Bang high energy photons (light particles) collided with each other, forming

• ppositely 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

Slide 13 / 141

2

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

+

• What are these positive, negative and neutral particles called?

What is the magnitude of their charge? What are their masses?

Energy and Matter Slide 14 / 141 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/cosmic-background-radiation/

Slide 15 / 141

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

• riginal size.

Slide 16 / 141 Formation of the Elements

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.

+ + Slide 17 / 141 Formation of the Elements

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-2

Deuterium

+

• Hydrogen-3

Tritium

• Helium-4

+ +

• Lithium-7

+

• 3

4

• Beryllium-9

+

• 4

5

• Slide 18 / 141

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.

Slide 19 / 141 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:

Slide 20 / 141 Formation of Heavier Elements

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.

Slide 21 / 141 Periodic Table of Nucleosynthesis Slide 22 / 141 "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.

Slide 23 / 141

Atomic Structure: Electrons & Protons

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• f Contents

Slide 24 / 141 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.

+

• POWER

OFF +

• POWER

ON

.

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.

Slide 25 / 141

.

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 Slide 26 / 141

4

Students type their answers here

+

• POWER

ON +

• 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?

Slide 27 / 141

5

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+

• POWER

ON 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?

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6

Students type their answers here

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. +

• POWER

ON +

• Slide 29 / 141

.

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 Slide 30 / 141

.

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

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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 D b and c

Slide 32 / 141

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

Slide 33 / 141 Millikan Oil Drop 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. 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.

X-rays Oil drops

+

• Click here to see an animation of the experiment

Slide 34 / 141

.

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? = 1.6x10-19 Coulombs He correctly interpreted this to be the charge of an electron.

move for answer Millikan Oil Drop Experiment: Sample Data Slide 35 / 141

9 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 1.6 x 10-19 kg B 2.82 x 10-8 kg C 9.1 x 10-31 kg D 1.1 x 1030 kg

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10 Which of these could be the charge of a drop in the Millikan oil drop experiment? A 0.80 x 10-19 C B 2.0 x 10-19 C C 8.0 x 10-19 C D 4.0 x 10-19 C

Slide 37 / 141

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

Slide 38 / 141 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.

Slide 39 / 141

.

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 Slide 40 / 141

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

Slide 41 / 141

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

Slide 42 / 141

14 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 B 4.95x10-34 kg C 9.1x10-31 kg D 1.6x10-19 kg

1 proton = 1840 x mass of electron

Slide 43 / 141

The Nucleus

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• f Contents

Slide 44 / 141

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 Slide 45 / 141 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.

Slide 46 / 141

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

• f 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.

Slide 47 / 141 Radioactivity

Three types of radiation were discovered by Ernest Rutherford: α-rays - alpha particles (positively charged particles with a mass roughly 4x that of the proton) β-rays - beta particles (electrons) γ-rays - gamma rays (form of light with very high energy)

Slide 48 / 141

15 Of the three types of radioactivity characterized by Rutherford, which are particles? A α-rays, β-rays, and γ-rays B γ-rays C α-rays and γ-rays D α-rays and β-rays

Slide 49 / 141

16 Beta-particles are attracted to a ________ charged plate, indicating they are __________ charged. A positively, negatively B negatively, positively C neutrally, negatively D neutrally, positively

Slide 50 / 141

17 Alpha particles are __________ charged. A negatively B positively C neutrally D unknown

Slide 51 / 141 Rutherford's Gold Foil Experiment

Physicists Geiger and Marsden under the direction

• f 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

Slide 52 / 141 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.

Slide 53 / 141 Discovery of the Nucleus

What actually happened was very surprising. Most of the particles flew right through the foil with no deflection at all.

Slide 54 / 141

18

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While most particles went straight through some bounced back...totally unexpected? What does this indicate about the location of protons in an atom?

Slide 55 / 141 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

Slide 56 / 141 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

• f 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.

• Rutherford

Slide 57 / 141

19 The gold foil experiment performed in Rutherford's lab __________. A confirmed the plum-pudding 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

Slide 58 / 141

20 In the Rutherford nuclear-atom 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

Slide 59 / 141

.

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 Slide 60 / 141

.

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.

Slide 61 / 141

.

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 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

1 u = 1.66053892x10-27 kg

Slide 62 / 141

.

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

Slide 63 / 141 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.

10-4 A

• 1-5A
• Nucleus containing

protons and neutrons Volume occupied by by electrons

10 A = 1 nm

• scale:

A = 10

• 10 m
• Click here to see Atom animation

Slide 64 / 141

21 What is the mass of an element that has 10 protons and 11 neutrons (in u)?

Slide 65 / 141

22 How many neutrons are present in an oxygen atom with a mass of 18 u?

Slide 66 / 141

23 How many protons are present in atom with a mass of 13 u if it has 7 neutrons?

Slide 67 / 141

24 What is the mass of an element with 18 protons, 18 electrons, and 22 neutrons?

Slide 68 / 141 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.

H 1 1.0079

Hydrogen

U 92 238.029

Uranium

Atomic Number Atomic Symbol and Name Atomic Mass

Slide 69 / 141 Nomenclature

Together, 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.

Slide 70 / 141

.

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 X

A Z

Ag - 107 Ag

107 47

Slide 71 / 141

25 How many neutrons are present in a neutral atom of Sr-80? A 32 B 38 C 80 D 42

Slide 72 / 141

26 Find the mass number.

Na

23 11

Sodium Atom

Slide 73 / 141

27 How many protons does this element have?

Na

23 11

Sodium Atom

Slide 74 / 141

28 How many electrons does this element have?

Na

23 11

Sodium Atom

Slide 75 / 141

29 How many neutrons does this element have?

Na

23 11

Sodium Atom

Slide 76 / 141

30 How many neutrons does this element have?

Br

80 35

Bromine Atom

Slide 77 / 141

31 Which of the following has 45 neutrons?

A

80Kr

B

80Br

C

78Se

D

103Rh

Slide 78 / 141

Formation of the Elements

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• f Contents

Slide 79 / 141 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?

Slide 80 / 141 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

Deuterium Hydrogen-3 Tritium

Slide 81 / 141 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. Eb = Δmc2

+ +

Energy

• Helium-4

+ +

• Slide 82 / 141

Binding Energy and Mass Defect *

To calculate the binding energy we start by converting Atomic mass units to kilograms. Then use the energy-mass equivalence to solve for binding energy. The binding energy is measured in Joules. #m = 0.069513u x

1.6605 x 10-27 kg 1u = 1.1543 x 10-28 kg E = #mc2 E = #mc2 = 1.1543 x 10-28kg)(3 x 108 m/s)2 = 1.0388 x 10-11 J

Slide 83 / 141 Binding Energy and Mass Defect

For example, if we want to calculate the mass defect and binding 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 1.008665u H: 5 x 1.007825u B: 10.012937u

1 5 10 1 1

#m = (5 x 1.008665u) + (5 x 1.007825u) -

(10.012937u)

#m = 5 x mass(neutron) + 5 x mass(proton) - mass(Boron)

* Slide 84 / 141

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.

*

Slide 85 / 141

33 What is the mass defect of ?

12 6C: 12.000000u 12 6C 1 0n: 1.008665u 1 H: 1.007825u 1

* Slide 86 / 141

34 What is the binding energy (in Joules) of ?

12 6C 12 6C: 12.000000u 1 0n: 1.008665u 1 H: 1.007825u 1

* Slide 87 / 141

35 What is the mass defect of U?

238 92U: 238.05078826u 1 0n: 1.008665u 1 H: 1.007825u 1 238 92

* Slide 88 / 141

36 What is the binding energy (in Joules) of U?

238 92U: 238.05078826u 1 0n: 1.008665u 1 H: 1.007825u 1 238 92

* Slide 89 / 141 Nuclear Fusion

Making 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++ v

1 1 1 1 2 1

Slide 90 / 141 Nuclear Fusion

Producing a Helium-3 atom and a gamma ray.

H + H He + γ

1 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...

Slide 91 / 141

Producing a Helium-4 atom and two hydrogen atoms.

H + H

1 1 1 1 4 2

He +

3 2 3 2

He + He

Nuclear Fusion

Making Helium occurs in 3 steps in the core of the star. Step 3: Two helium-3 atoms fuse... Note: Steps 1 & 2 must occur twice to produce the required helium-3 atoms.

Slide 92 / 141 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

• f nucleons stays constant.

4 H He + 2e+ + 2v + 2γ

1 1 4 2

Slide 93 / 141

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

Slide 94 / 141

38 In the following fusion reaction, how many nucleons are in the unknown nucleus? C + H X + γ

12 6 1 1

Slide 95 / 141

39 Identify the unknown element in the nuclear reaction. A Boron B Carbon C Nitrogen D Oxygen C + H X + γ

12 6 1 1

Slide 96 / 141

40 In the following fusion reaction, how many nucleons are in the unknown nucleus? H + H X + n

2 1 3 1 1

Slide 97 / 141

41 Identify the unknown element in the nuclear reaction. A Hydrogen-1 B Hydrogen-2 C Helium-3 D Helium-4 H + H X + n

2 1 3 1 1

Slide 98 / 141 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 Uranium-235 is: Note: Q represents energy released.

U + n

235 92 1

U* X + Y + neutrons + Q

236 92

Slide 99 / 141 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 + Q

235 92 1 1 236 92 141 56 92 36

U + n U* Xe + Sr + 2 n + Q

235 92 1 1 236 92 140 54 94 38

Slide 100 / 141

42 Identify the missing element in the following fission reaction. A Kr B Sr C U D Pu U + n U* Ba + __ + 3 n + Q

235 92 1 1 236 92 141 56

Slide 101 / 141

43 Identify the missing element in the following fission reaction. A Kr B Zr C Pd D Bk U + n ___ + Te + 2 n

235 92 1 1 137 52

Slide 102 / 141

44 Identify the missing element in the following fission reaction. A Rb B Np C Cf D Cm U + n ___ + Cs + 3 n

235 92 1 1 133 55

Slide 103 / 141 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/nuclear-fission

Slide 104 / 141 Nuclear Reactions

First fill in the missing component: Next, find the mass defect: Find the reaction energy:

H + N He + ___?

2 1 14 7 3 2

m = 2.014102u+14.003074u-3.016029u-13.003355u = -0.002207u E = #mc2 = -0.002207u x 1.6605 x 10-27 kg 1u = 2.9979 x 108 m/s2 E = -3.294 x 10-13 J

* Slide 105 / 141

45 Compute the Q value of the reaction. H + H n + He?

2 1 3 1 4 2 1

H: 2.014u H: 3.016u He: 4.003u

2 1 3 1 4 2

* Slide 106 / 141

46 Compute the Q value of the reaction. U + n Sr + Xe + 2 n

235 92 1 94 38 1 140 54 235 92 U: 235.044u

Sr: 93.9154u

38 94

Xe: 132.9059

140 54

* Slide 107 / 141 Nuclear Fission

This is a schematic of a nuclear power plant. The fission process

• ccurs 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 108 / 141 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.

*

Slide 109 / 141

Isotopes

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• f Contents

Slide 110 / 141 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.

C-12 C-14 protons neutrons electrons 6 6 6 6 8 6

Note: Isotopes of an element will always have the same number of protons but differing masses due to the differing numbers of neutrons.

Slide 111 / 141

Write the complete symbol for each of these isotopes.

Neon 20 10 protons 10 neutrons 10 electrons Neon 21 10 protons 11 neutrons 10 electrons Neon 22 10 protons 12 neutrons 10 electrons

Ne Ne Ne

Isotopes Slide 112 / 141

47 Which pair of atoms constitutes a pair of isotopes of the same element? X

14 7 14 6

X A B C D X

6 12

X

14 6

X

8 17

X

17 9

X

9 19

X

19 10

Slide 113 / 141

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

Slide 114 / 141

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

Slide 115 / 141

50 Which species is an isotope of

39Cl?

A

40Ar+

B

34S2-

C

36Cl -

D

39Ar

* Slide 116 / 141 Isotopes and Atomic Masses

Not all isotopes are found in the same abundances in nature.

Neon 20 10 protons 10 neutrons 10 electrons Neon 21 10 protons 11 neutrons 10 electrons Neon 22 10 protons 12 neutrons 10 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)

Slide 117 / 141 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

Slide 118 / 141 Calculating Atomic Masses

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.

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

• f 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 decimal
• 2. Add the products together

20(.9048) + 21(0.0027) + 22(0.0925) = 20.18 amu

Slide 119 / 141 Example: Calculate Atomic Mass

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 then... 12(.9889) + 13(.011) = 12.01 amu move for answer

Slide 120 / 141

51 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. (liquid

• xygen)

*

Slide 121 / 141

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.

Slide 122 / 141

53 Sulfur has two stable isotopes: S-32 and S-34. Using the average atomic mass on the periodic table, which

• f the following best approximates the natural

relative abundances of these isotopes of sulfur? A 50% S-32 and 50% S-34 B 25% S-32 and 75% S-34 C 75% S-32 and 25% S-34 D 95% S-32 and 5% S-34

Slide 123 / 141

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.

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 N-15/N-14 where the normal ratio is 0.0034 N-15/ N-14?

Slide 124 / 141

Radioactive Decay

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• f Contents

Slide 125 / 141 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.

Slide 126 / 141 Radioactivity

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.

Slide 127 / 141 Radioactivity Stopping Power

Alpha particles are stopped by a sheet of paper. Beta particles are stopped by a thin sheet of aluminum. Gamma rays are the most penetrating and are stopped by several meters of lead.

Slide 128 / 141 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: 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 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 - 4 Z - 2

X + Y + e- + v

A Z 4 Z + 1

X + Z + e+ + v

A Z 4 Z - 1

X* X + γ

A Z A Z

Slide 129 / 141 Alpha 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). Another example is Radium 218.

Po Pb + He

2 4 208 82 212 84 218 88Ra Rn + He 214 86 4 2

? ?

Slide 130 / 141 Beta Decay

Here are two examples of Beta Decay. Electron & Anti-neutrino Positron & Neutrino

Be B + e

11 4 11 5

• 1

?

Na B + e

22 11 22 10

• 1

?

Slide 131 / 141

54 Which type of radiation is the hardest to shield a person from? A Alpha particles. B Beta particles. C Gamma rays. D X-rays.

Slide 132 / 141

55 Which type of radiation is stopped by the shirt you wear? A Alpha particles. B Beta particles. C Gamma rays. D X-rays.

Slide 133 / 141

56 What is the missing component?

Students type their answers here

B C + ?

12 5 12 6

Slide 134 / 141

57 What is the missing component?

Students type their answers here

Po He + ?

190 84 4 2

Slide 135 / 141

58 What is the missing component?

Students type their answers here

U Th + ?

238 92 234 90

Slide 136 / 141

Nuclear Half-life

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• f Contents

Slide 137 / 141 Nuclear Half-life

A 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 half-life. The isotopes of a specific element have very different half-lives; ranging from μseconds to never decaying at all.

Slide 138 / 141 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.

Slide 139 / 141 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. 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 2n 23

Slide 140 / 141

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?

Slide 141 / 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?