nuclear chemistry last revision: 100211 m. jones pisgah high school

Nuclear Chemistry

Last revision: 100211

M. Jones

Pisgah High School

Nuclear chemistry studies

1. Atomic theory2. Radioactivity3. Isotopes 4. Half-life 5. Decay equations6. Energy, fission and fusion

Atomic Theory

Atoms are the smallest particles of elements.

Atoms were first proposed by Democritus over 2000 years ago.

The idea of atoms was reintroduced in 1803 by John Dalton.

1. Atoms are tiny, discrete particles 2. Atoms are indestructible3. Atoms of the same element have the

same mass and properties4. Atoms combine in simple whole-

number ratios5. Atoms in different ratios produce

different compounds.

Dalton’s Atomic Theory

same mass and properties4. Atoms combine in simple whole-

number ratios5. Atoms in different ratios produce

different compounds.

We know that parts of Dalton’s atomic theory are no longer valid in today’s modern Quantum Mechanical model of the atom.

same mass and properties

We know that atoms are made up of smaller particles, and that there are slight differences between atoms of the same element - isotopes.

William Crookes

Used spectroscopy to discover thallium and used vacuums to measure its mass.

Invented the radiometer.

Improved vacuum systems. Used by Edison to make light bulbs.

The Crookes’ Tube

What we now call the cathode ray tube.

William Crookes

Used the cathode ray tube to to study electric fields in a vacuum and discovered rays, …

which were called “cathode rays” by Goldstein, since they came from the cathode, or negative electrode.

William Crookes

The shadow of the Maltese cross indicates that cathode rays travel in straight lines and can be stopped by a solid object.

He found that the cathode rays could be deflected by a magnet.

This suggested that the cathode rays might be a stream of

electrically charged particles.

William Crookes

Cathode Ray Tube

High voltageHigh voltage

Cathode Anode

Direction of cathode rays

Cathode Anode

Direction of cathode rays

Magnet

Cathode Ray Tube

Cathode Anode

Used by J. J. Thomson …to discover the

electron.

Cathode Ray Tube

J.J. Thomson and Cathode Rays

• Attracted to positive electrode• Thought might be atoms• Had same charge to mass ratio regardless of

metal in the cathode• The particle was much less massive than the

lightest element – H• Particle must be common to all matter, a

subatomic particle

He had discovered the electron.

In 1897 J. J. Thomson found that cathode rays are a basic building block of matter.

The term “electron” comes from George Stoney’s term for the “minimum electrical charge”.

Thomson concluded that this particle was the carrier of the minimum electrical charge and so the particle was later called an “electron”.

Even though Crookes and others observed cathode rays, Thomson is credited with the discovery of the electron because he recognized that it was a fundamental particle of nature as well as a sub-atomic particle.

Measured the charge to mass ratio, and found …

… that if this “minimum charge” was equal to the charge on a hydrogen ion, then the mass of the electron would be 1/1837th the mass of a hydrogen atom.

If that were the case, then the electron would be much smaller than the smallest atom ..… showing for the first time that

matter is made up of particles smaller than atoms.

Thomson tried to measure the fundamental charge on the electron.

Robert A. Millikan

Robert A. Millikan, an American physicist, set out to determine the charge on an electron.

From 1909 through 1910, he performed what is now called the

“Oil Drop Experiment”.

HighVoltage

Cast iron pot

Atomizer

Robert A. Millikan

Telescope

HighVoltage Telescope

Cast iron pot

Atomizer

Oil Drop

Parallel charged plates

Robert A. Millikan

Radiation stripped electrons from the oil droplets. The charged droplets fell between two electrically charged plates. By adjusting the voltage, he could change the rate of fall or rise of a single oil drop. After observing hundreds of drops, he calculated the charge on a single electron.

Robert A. Millikan

Charges on drops are multiples of 1.602 x 10-19 coulombs.

Robert A. Millikan

The fundamental charge on an electron is 1.602 x 10-19 coulombs.

With J. J. Thomson’s charge to mass ratio, and Millikan’s charge on the electron, we are able to compute the mass of an electron:

9.109 x 10-28 gram

Robert A. Millikan

He is to the atom what Darwin is to evolution, Newton to mechanics, Faraday to electricity and Einstein to relativity.

Ernest Rutherford

John Campbell http://www.rutherford.org.nz/biography.htm

He moved from New Zealand to Cambridge University in England (1895) where he pioneered the detection of electromagnetic waves, but was lured away by J.J. Thomson on work that would lead to the discovery of the electron. The invention of radio communications went to Marconi, instead. He later switched to working with radioactivity (1896) and discovered alpha and beta rays. He went to Montreal to teach at McGill University (1898) where he continued his work on radioactivity with Frederick Soddy, and others (1898-1907). He moved back to back to England to teach at Manchester (1907). He received the Nobel prize in chemistry in 1908 for his work on radioactivity in Canada.

Ernest Rutherford

In 1907, he and a student, Hans Geiger, developed what would later become the “Geiger counter”. While at McGill, Rutherford discovered that after alpha rays passed through a thin film of mica, the image formed on a photographic plate was “fuzzy”. He and Geiger began a project to investigate the scattering of alpha particles by thin films. Rutherford later gave Ernest Marsden, an undergraduate, his own research project which was to look for evidence of the backscatter of alphas (1909). To their surprise, Marsden found that some alpha particles were scattered backwards from thin films of lead, platinum, tin, silver, copper, iron, aluminum, and gold.

Ernest Rutherford

Rutherford remarked that it was like firing a navel gun at a piece of tissue paper and the shell bouncing back and hitting you. By 1910, Hans Geiger had finished his research on the forward scattering of alpha particles but he could not reconcile it with Marsden’s observations of the backscatter of alphas. The problem was passed on to Rutherford, who came up with the answer, and the astounding results were published in 1911.

Ernest Rutherford

Rutherford had discovered a new piece to the atomic puzzle, the nucleus. According to Rutherford, the positively charged alpha particles were encountering a tiny, positively charged particle within the atoms of the metal and were being repelled. The atoms themselves appeared to mostly empty space. It was the repulsion of two positively charged particles which caused the scattering observed by Geiger and Marsden. Rutherford had found that atoms are mostly empty space with a small, dense, positively charged nucleus.

Ernest Rutherford

Alpha scattering

Apparatus for investigating alpha scattering.

What some textbook authors call the “gold foil experiment.”

+Most of the alpha particles pass through undeflected.

Alpha scattering source

+Some positive alpha particles are repelled by the small, dense, positively charged nucleus.

sourceAlpha scattering

Alpha scattering source

Some positive alpha particles are repelled by the small, dense, positively charged nucleus.

Alpha particles are repelled by a small, dense, positively charged nucleus.

Almost all the mass of an atom is in the nucleus. Atoms are mostly empty space.

Electrons are located outside the nucleus.

Published results in 1911.

Alpha scattering

Rutherford, during the First World War, worked on developing SONAR and submarine detection, but still found time to tinker with alpha radiation. In 1917 he bombarded nitrogen gas with alpha particles and discovered that oxygen and hydrogen were produced. Rutherford had resorted to alchemy and accomplished the first transmutation of one element into another. He had also indirectly discovered the proton.

Ernest Rutherford

N + O + H

7 protons

2 protons

1 proton

8 protons

9 protons9 protons

Ernest Rutherford

We now know…

Rutherford concluded that the nucleus must contain the positively charged protons in a number equal to the negative charge from the electrons, but this did not account for all of the mass of the atom. He, along with James Chadwick, rejected the idea that there must be additional protons and electrons in the nucleus, and concluded that there must be a neutral particle in the nucleus that accounted for the additional mass. In 1932, Chadwick confirmed the existence of the neutron.

Ernest Rutherford

Radioactivity

Demonstrations with radioactivity

Investigate the properties of Alpha, Beta and Gamma

Radiation

Mica window (fragile)

Wire (+ side of circuit)

Metal shield (- side)

Low pressure Ar gas

Counter 2435

Geiger-Mueller Tube

Rays leave the source

Some hit the GM tube

Most do nothing

One ray may cause a discharge…

Source and the detector clicks

Geiger-Mueller Tube

• Filled with low pressure argon gas

• About 1% efficiency

• About 1 in 100 rays causes an electric spark between the case and the wire

• Each spark registers as a count or click on the counter

Geiger-Mueller Tube

Radioactivity

• Alpha particles • Beta particles • Gamma rays

- helium nuclei- electrons - high energy

electromagnetic energy - similar to light, but higher in energy.

Alpha particles

An unstable nucleus splits to form a more stable nucleus an an alpha particle.

An alpha particle is the nucleus of a helium atom.

Two protons and two neutrons.Has a +2 charge.

Radioactivity

Beta particles

Ejected from the nucleus when a neutron decays.

A beta particle is identical to an electron

Has a -1 charge.

Radioactivity

Gamma rays

Emitted by an unstable nucleus as it becomes more stable

Electromagnetic energy with short wavelengths and high energy.

Radioactivity

Has no charge.

- comes from the natural decay of unstable atoms.

- can be detected by photographic film, scintillation detector or a Geiger counter.

- is “ionizing radiation”. Causes cell damage and mutations – cancer.

- is protected against by shielding and distance.

Radioactivity

Mass number /Atomic number

Mass number

Symbol of Element

Atomic number

protons + neutrons

Protons in nucleus

Mass number

Symbol of Element

Atomic number

protons + neutrons

Protons in nucleus

Mass number

Alpha () particles are the nuclei of helium atoms and have the symbol

What is the atomic number of an

particle?2 He4

Radioactivity

What is the mass number of

an particle?2 He4

Radioactivity

How many times heavier is an alpha particle than a

hydrogen atom?

Radioactivity

Beta () particles are high speed electrons ejected from the nuclei of atoms and have the symbol -1e0.

What is the mass number of a particle? -1e0

Radioactivity

No protons or neutrons in an electron. -1e0

Radioactivity

NoneWhat is the difference between a particle and a “regular” electron?

Radioactivity

LocationLocationLocation

What is the difference between a particle and a “regular” electron?

Radioactivity

Gamma () rays are high energy electromagnetic waves, not particles.

No protons, neutrons or electrons.

Gamma rays have short wavelengths, high energies and travel at the speed of light.

Radioactivity

Gamma rays have short wavelengths

… and high energies.

Increasing energy

Alpha, Beta, Gamma

Radioactive Source

- - - - - - - - -

+ + + + + + + +

Electric field from electrically charged plates

What is the effect of an electric field on

Alpha, Beta, Gamma

Radioactive Source

- - - - - - - - -

+ + + + + + + +

Alpha, Beta, Gamma

Radioactive Source

- - - - - - - - -

+ + + + + + + +

Are , and rays deflected by magnetic fields?

Radioactive Source

Aluminum foil

Alpha, Beta, Gamma

Radioactive Source

Aluminum foil

Alpha, Beta, Gamma

Radioactive Source

Aluminum foil

Alpha, Beta, Gamma

Radiation Project

Create a table listing information for each of the three kinds of radiation:

Alpha, beta and gamma

Properties to include in your table:

(1) Greek letter

(2) symbol

(3) actually is

(4) atomic number

(5) mass number

(6) relative mass

(7) relative. charge

(8) penetrating ability

(9) shielding

Nuclear Properties TableProperty Alpha Beta Gamma

Greek Letter

Symbol

Actually is…

Atomic number

Mass number

Relative mass

Relative charge

Penetrating

Shielding

Stop!Complete the chart on notebook paper,

then continue.

Greek Letter

Symbol

Actually is…

Atomic number

Mass number

Relative mass

Relative charge

Penetrating

Shielding

Greek Letter Symbol

Actually is…

Atomic number

Mass number

Relative mass

Relative charge

Penetrating

Shielding

Greek Letter Symbol

2He4-1e0 NA

Actually is…

Atomic number

Mass number

Relative mass

Relative charge

Penetrating

Shielding

Greek Letter Symbol

2He4-1e0 NA

Actually is… He nucleus electron EM energy

Atomic number

Mass number

Relative mass

Relative charge

Penetrating

Shielding

Greek Letter Symbol

2He4-1e0 NA

Atomic number 2 -1 NA

Mass number

Relative mass

Relative charge

Penetrating

Shielding

Greek Letter Symbol

2He4-1e0 NA

Mass number 4 0 NA

Relative mass

Relative charge

Penetrating

Shielding

Greek Letter Symbol

2He4-1e0 NA

Mass number 4 0 NA

Relative mass 4 1/1837NA

Relative charge

Penetrating

Shielding

Greek Letter Symbol

2He4-1e0 NA

Mass number 4 0 NA

Relative charge +2 -1 NA

Penetrating

Shielding

Greek Letter Symbol

2He4-1e0 NA

Mass number 4 0 NA

Penetrating Low Medium High

Shielding

Greek Letter Symbol

2He4-1e0 NA

Mass number 4 0 NA

Penetrating Low Medium High

Shielding 2.5 cm of air;anything else

Metal, plastic or wood

Lead or concrete

Protection from radiation1. Shielding 2. Distance

How do you protect yourself from …

2.5 cm of air, paper, skinaluminum, lead, other metals, wood, plastic, etc.up to a foot or two of lead, many feet of concrete

There are some kinds of radiation you can not

protect your self from.

Radiation

Gamma rays and high energy cosmic particles from space.

But there is one kind of radiation hazard that you

can protect against.

Radiation

That hazard comes from the uranium beneath your feet.

Uranium in the ground decays according to …

Uranium-238 decays through

many steps to make stable

lead-206

The uranium decay series

http://library.tedankara.k12.tr/chemistry/vol1/nucchem/trans90.htm

The uranium decay series

Radon is the only gas in the series.

http://library.tedankara.k12.tr/chemistry/vol1/nucchem/trans90.htm

Hazards from radon

Since radon is the only gas in the decay series of uranium …

…it can work its way up through the ground and into your

basements and crawl spaces.

You breathe radon into your lungs.

Hazards from radon

And when radon is in your lungs…

…it can decay and release an alpha particle …

…which travels only a short distance before it is absorbed by your lungs, and transfers its energy.

Hazards from radon

This ionizing radiation in your lungs can cause lung cancer.

Smoking cigarettes and breathing radon really increases your

chances of getting lung cancer.

Protecting against radon

Get a test kit to see if there is a problem. Charcoal canisters, which are sent off for analysis.

Abatement:Seal places where gas gets in.

Ventilation – bring in fresh air.

Atomic Theory

We know that atoms are made up of protons, neutrons and electrons.

Protons and neutrons are located in a small, dense, positively charged nucleus.

We know that atoms are mostly empty space.

Atomic Theory

We know atoms are mostly empty space and that protons and neutrons

are located in a small, dense, positively charged nucleus because

of Rutherford’s explanation of Geiger and Marsden’s work in alpha

scattering (gold foil experiment ).

Atomic Theory

We know that electrons are outside the nucleus in an “electron cloud”.

Electrons exist in specific energy levels, which explains the line

spectra of the elements.

Started with the Bohr model.

Atomic Theory

We now use the Quantum Mechanical Model of the atom.

Quantum Theory describes the nature of electrons and their

interactions with the electrons of other atoms in chemical reactions.

Atomic Theory

The subatomic particles that make up atoms have known properties like mass and electrical charge.

Our understanding came through the efforts of a number of

scientists like Thomson, Millikan, Rutherford, and Chadwick.

Mass number

Symbol of Element

Atomic number

protons + neutrons

Protons in nucleus

Mass number

Subatomic particles

n10proton

neutronelectron

What do the numbers represent?

Property Proton Neutron Electron

Symbols

Location

Rel. mass

Mass (amu)

Mass (g)

Rel. charge

Charge (C)

Fill in the chart with the correct information.

Symbols p+ and 1H1 n0 and 0n

1 e- and -1e0

Location

Rel. mass

Mass (amu)

Mass (g)

Rel. charge

Charge (C)

1 e- and -1e0

Location nucleus nucleus cloud outside nucleus

Rel. mass

Mass (amu)

Mass (g)

Rel. charge

Charge (C)

1 e- and -1e0

Rel. mass 1 1 1/1837

Mass (amu)

Mass (g)

Rel. charge

Charge (C)

1 e- and -1e0

Rel. mass 1 1 1/1837

Mass (amu) 1.0073 amu 1.0087 amu 0.00549 amu

Mass (g)

Rel. charge

Charge (C)

1 e- and -1e0

Rel. mass 1 1 1/1837

Mass (g) 1.673x10-24 1.675x10-24 9.11x10-29

Rel. charge

Charge (C)

1 e- and -1e0

Rel. mass 1 1 1/1837

Mass (g) 1.673x10-24 1.675x10-24 9.11x10-29

Rel. charge +1 0 -1

Charge (C)

1 e- and -1e0

Rel. mass 1 1 1/1837

Mass (g) 1.673x10-24 1.675x10-24 9.11x10-29

Rel. charge +1 0 -1

Charge (C) +1.6x10-19 C 0 -1.6x10-19 C

Subatomic particles1. Protons and neutrons are located in

the nucleus.2. Protons and neutrons have almost

the same mass. Neutrons heavier.3. Electrons are outside the nucleus and

much lighter than proton or neutron.4. Protons and electrons have the same

charge but opposite polarity. 5. Neutrons have no charge.

Subatomic particles6. Protons and neutrons are each made

of smaller particles called quarks.7. Quarks are elementary particles just

like electrons. They are not composed of smaller particles.

8. There are six kinds of quarks: “up”, “down”, “top”, “bottom”, “charm” and “strange”.

Subatomic particles9. Protons are composed of two “up

quarks” and one “down quark”.10. Neutrons are composed of two

“down quarks” and one “up quark”.11. Quarks are held together to make

protons and neutrons by the strong force, the strongest of the four fundamental forces in nature. Gravity, electromagnetism, weak and strong.

Isotopes

Isotopes …

Therefore, isotopes of the same element have different masses.

…of the same element have the same number of protons and electrons but different numbers of neutrons.

Isotopes …

…don’t have to be radioactive. Some isotopes are unstable and

decay, releasing alpha or beta particles, or gamma rays.

But, there are many stable isotopes that don’t decay.

Isotopes …

Mass number - the sum of the protons and neutrons in the nucleus.

Atomic number - the number of protons in the nucleus of an atom.

…have different mass numbers but the same atomic number.

Symbols for Isotopes

Symbol of Element

Mass number

Atomic number

A is the symbol for mass number

Z is the symbol for atomic number

Symbol of Element

Mass number

Atomic number

An isotope of uranium

Mass number

Symbol of Element

Atomic number

An isotope of uranium

This form solves the word processor dilemma.

Symbol of Element

Mass number

How do you know the atomic number?

Find U in the periodic table.

Z = 92

Some elements have several Isotopes

Lead has four naturally occurring isotopes, Pb-204, Pb-206, Pb-207, and Pb-208; but there are 23 man-made isotopes of lead.

Some elements have several Isotopes

Bismuth has only one naturally occurring isotope,

Bi-209, but there are 22 man-made isotopes of bismuth.

Finding the number of Protons, Neutrons, and Electrons

The number of electrons in a neutral atom equals the

number of protons.

The atomic number is the number of protons in the nucleus.

neutrons = A - Z

The number of neutrons is the difference between the mass number and the atomic number.

Look at the periodic table and find the element by using the symbol.

A = 235protons + neutrons = 235

Z = 92protons = 92electrons = 92

A = 235protons + neutrons = 235

How many neutrons are in a U-235 atom?

235 – 92 = 143 neutrons

How many neutrons are in a U-235 atom?

Q. Find the number of neutrons in the Ba-137 isotope.

A. In the Ba-137 isotope …… Z = 56 and A = 137137 – 56 = 81 neutrons

Copy the following table on notebook paper, and

fill in the blanks.

Element Symbol Z A #p #n #e

Zinc 66

82 210

Rn 136

Zinc 66

82 210

Rn 136

Stop!Complete the table, then go

Zinc 66

82 210

Rn 136

Zinc Zn 30 66 30 36 30

82 210

Rn 136

Zinc Zn 30 66 30 36 30

Indium In 49 117 49 68 49

82 210

Rn 136

Zinc Zn 30 66 30 36 30

Indium In 49 117 49 68 49

Strontium Sr 38 85 38 47 38

82 210

Rn 136

Zinc Zn 30 66 30 36 30

Indium In 49 117 49 68 49

Lead Pb 82 210 82 128 82

Rn 136

Zinc Zn 30 66 30 36 30

Indium In 49 117 49 68 49

Lead Pb 82 210 82 128 82

Radon Rn 86 222 86 136 86

Zinc Zn 30 66 30 36 30

Indium In 49 117 49 68 49

Lead Pb 82 210 82 128 82

Radon Rn 86 222 86 136 86

Bromine Br 35 82 35 47 35

Atomic mass is the weighted average of all the isotopes of an

element

Boron has two isotopes:B-10 19.8% 10.01 amuB-11 80.2% 11.01 amu

0.198 x 10.01 + 0.802 x 11.01 = 10.81 amu

element

Determine the atomic mass of silicon:Si-28 92.23% 27.977 amuSi-29 4.67% 28.976 amuSi-30 3.10% 29.974 amu

0.9223 x 27.977 + 0.0467 x 28.976 + 0.0310 x 29.974 = 28.086 amu

Consider the two isotopes of chlorine. Which isotope is more abundant?Cl - 35 ??.?? % 34.97 amuCl - 37 ??.?? % 36.97 amu

The average atomic mass is 35.453 amu.

element

Consider the two isotopes of chlorine. Which isotope is more abundant?Cl - 35 75.85% 34.97 amuCl - 37 24.15% 36.97 amu

The average atomic mass is 35.453 amu.

element

Which isotope of neon is more abundant? Ne-20 or Ne-22

element

Ne-20 90%Ne-22 10%

How are isotopes of the same element alike and different?

Alike:

1. Number of protons and electrons

2. Atomic number

3. Chemical properties

Different:

1. Number of neutrons

2. Mass Number

3. Atomic mass of the isotopes

Which of the following is the same for the three isotopes of magnesium?

1. The atomic number of 122. The number of protons and electrons3. The number of neutrons4. The atomic weight of 24.986 AMU5. The reaction with hydrochloric acid6. The speed of gaseous Mg atoms

1. The atomic number of 12

All three isotopes of magnesium have the same atomic number.

2. The number of protons and electrons

All isotopes of the same element have the same number of protons in the nucleus, and electrons outside the nucleus.

3. The number of neutrons

The number of neutrons varies with the isotope. Different isotopes have different numbers of neutrons.

Not the same

4. Atomic weight of 24.986 AMU

Mg-24 23.985 AMU

Mg-25 24.986 AMU

Mg-26 25.983 AMU

Not the same

5. The reaction with HCl

All isotopes of the same element react the same chemically.

The number and arrangement of electrons is the same for each isotope.

6. The speed of gaseous Mg atoms

The speeds of atoms depend on mass.

Heavier atoms move more slowly, and lighter atoms move faster.

Not the same

How did knowing about Graham’s Law allow the United States to win World War II?

Who were the two guys responsible for winning World War II?

Fat Man, and … Little Boy

Atomic bombs dropped on Hiroshima and Nagasaki

Hiroshima

Nagasaki

Manhattan Project

Oak Ridge, TN

Gaseous diffusion

Graham’s law

Enriched uranium

Manhattan Project

Less than 1% of naturally occurring uranium is U-235

Manhattan Project

Naturally occurring uranium is mostly U-238

To sustain a nuclear chain reaction, uranium must be at least 4% U-235.

Manhattan Project

Bomb grade uranium is over 90% U-235

The process of increasing the percentage of U-235 is

called enrichment.

The uranium for a nuclear reactor is around 4% U-235.

Manhattan Project

Uranium ore is reacted with fluorine to make gaseous UF6.

Then the gaseous UF6 is introduced into chambers with porous disks in the ends.

Manhattan Project

The lighter UF6 molecules containing U-235 effuse

through the holes in the disk faster. There is more U-235

on the other side of disk.

Manhattan Project

As the UF6 continues to move through many, many disks, the percentage of U-235 atoms in the gas increases, resulting in

enrichment.

Manhattan Project

Graham’s Law says that gas molecules which weigh less, will move faster than molecules which weigh more.

Manhattan Project

The enriched UF6 containing a much higher percentage of U-235 atoms, is reacted with water to make uranium oxide and HF. The uranium oxide is dried and made into fuel pellets.

Manhattan Project

Uranium Pellet

Fuel rod assembly

Only one element has unique names for its isotopes …

tritium H

deuterium H

hydrogen H

Deuterium and tritium are used in nuclear reactors and fusion research.

Some isotopes are radioactive

Radioactive isotopes are called radioisotopes.

Radioisotopes can emit alpha, beta or gamma

radiation as they decay.

Man-made Isotopes

Cobalt-59 occurs naturally. When a neutron “sticks” to the nucleus,

cobalt-60 is formed.

Man-made isotopes are usually made by bombarding atoms with protons or neutrons.

Uses for Isotopes

Radioisotopes are used as tracers in chemical reactions.

Radioisotopes are used in “imaging” living and nonliving systems.

Radioisotopes are used to kill cancer cells. (Co-60, Bi-212)

Half life

What is half life?

Half life is the time needed for one half of a radioisotope to decay.

Suppose you start with 100.0 grams of a radioisotope that has a half life

of exactly 1 year.

What is half life?

How much will be left after 1 year?

of exactly 1 year.

What is half life?

After one year there will be 50.0 g left.

of exactly 1 year.

After a second year there will be 25.0 g left.

What is half life?

After a third year there will be 12.5 grams left.

After one year there will be 50.0 g left. After a second year there will be

25.0 g left.

After a fourth year there will be 6.25 grams left.

Half life project1. Pick a mass between 10g and 50g. 2. Decide on a half life – any time.3. Scale your graph – mass on y-axis

and at least six (6) half-lives on the x-axis.

4. Plot the masses after intervals of one half-life.

Half life project5. What shape is the graph?6. When will the mass of the

radioisotope fall to zero?7. When is the radioactivity no

longer a problem? 8. What mathematical function

describes radioactive decay?

Half life projectm

t1/2 t1/2 t1/2

Half life projectm

t1/2 t1/2 t1/2

Half life project

t1/2 t1/2 t1/2Act

Exponential decay

A = A0e-kt

Half life project

t1/2 t1/2 t1/2Act

background

Radiation is “not a problem” when it falls below background level.

Half life projectQuestions:

1. A radioisotope has a half-life of 100 years. How long will it take for the radiation to decrease to 1/16 of its original value?

400 years

Half life projectQuestions:

2. A radioisotope has an activity of 560 counts per minute. After 16 hours the count rate has dropped to 35 counts per minute. What is the half life of the radioisotope?

4 hours

Decay equations

Alpha decay

In alpha decay, an alpha particle (2He4) is released from the nucleus.

The alpha particle carries away two protons and two neutrons.

Alpha decay

92U238 2He4 + 90Th234

alpha particle

decay product

Alpha decay

92U238 2He4 + 90Th234

The atomic number decreases by 2.

The mass number decreases by 4.

Alpha decay

These must add up to 238

These must add up to 92

92U238 2He4 + 90Th234

Alpha decay

86Rn220 2He4 + ???

Radon-220 decays by alpha emission. What is the decay product?

84Po216

Alpha decay

Write the alpha decay equations for:

1. 95Am241

2. 84Po216

3. 88Ra226

2He4 + 93Np237

2He4 + 82Pb212

2He4 + 86Rn222

Beta decay

Neutrons are a little more massive than protons; neutrons are neutral.

What does this suggest about the composition of neutrons?

Beta decay occurs because of the instability of a neutron.

Beta decayScientists used to think that neutrons might be a combination of a proton and an electron.

We know that neutrons decay into protons, which stay in the nucleus,

and electrons, which are ejected from the nucleus as beta particles.

Beta decayThe conversion of a neutron to a proton involves the “weak” force. An “up” quark flips to become a “down” quark. When this occurs a high energy electron (beta) and an antineutrino are produced, both of which leave the nucleus.

Beta decay

0n1 1H1 + -1e0

neutron proton electron

The electron ejected from the nucleus is a beta particle.

Decay of a neutron:

Beta decay

0n1 1H1 + -1e0 + 00

Technically, the decay of a neutron also involves a neutrino.

anti-neutrino

Beta decay

0n1 1H1 + -1e0 + 00

Actually, an anti-neutrino.

anti-neutrino

The word “neutrino” comes from Enrico Fermi, meaning “little neutral one” in Italian.

Beta decay

0n1 1H1 + -1e0 + 00

A neutrino is a particle with no charge and almost no mass.

anti-neutrino

Beta decay

0n1 1H1 + -1e0 + 00

A neutrino carries off some of the energy in the decay of the neutron.

anti-neutrino

Beta decay

0n1 1H1 + -1e0 + 00

When predicting the products of beta decay we will ignore neutrinos.

anti-neutrino

Beta decayStart with a Li atom with

3 protons and 4 neutrons.

Suddenly a neutron decays!

Now there are 4 protons

and 3 neutrons.

A beta particle goes zipping out of

the nucleus.

Beta decay

The number of neutrons

The number of protons

The mass number

The atomic number

A neutron decays to make a proton.

decreases by 1

increases by 1

stays the same.

increases by 1

Beta decay

6C14 7N14 + -1e0

beta particle

decay product

Beta decay

6C14 7N14 + -1e0

The atomic number increases by 1.

The mass number stays the same.

Beta decay

6C14 7N14 + -1e0

Notice that these add up to 6

These add up to 14

Beta decay

Zn-69 decays by beta emission. What is the decay product?

30Zn69 -1e0 + ??? 31Ga69

Beta decay

Write the beta decay equations for:

1. 82Pb214

2. 27Co62

-1e0 + 83Bi214

-1e0 + 28Ni62

3. ??? -1e0 + 48Cd11347Ag113

Gamma rays

Gamma radiation is often emitted along with alpha and beta radiation.

When a decay event occurs, “extra” energy is sometimes

left in the nucleus.

Gamma rays

The “extra” energy in the decay product is released as gamma radiation. This lowers the energy of the nucleus and makes it more stable.

Review: decay equationsAlpha:

Go down two on periodic tableAtomic number decreases by 2Mass number decreases by 4

Beta:Go up one on periodic tableAtomic number increases by 1Mass number stays the same

What holds the nucleus together?

Did you ever wonder ...Why the nucleus stays together with all those positively charged protons in such a small space?

Protons have a positive charge and objects with like charges

repel each other.

Why do they look like this?

Each hair has the same charge.

…the nucleus shouldn’t even exist!

Did you ever wonder ...

Because of the electrostatic repulsion…

The strong force.

There must be a force that is stronger than the electrostatic repulsion.

The strong force is the force that holds the quarks together to make

protons and neutrons.

The residual strong force extends from the quarks in a proton or neutron to the quarks in an adjacent proton or neutron and holds the nucleus together.

There is a closely related mystery.

Here’s a mystery

Consider the iron-56 isotope.

It has a mass of 55.935 amu.

How many protons, neutrons and electrons? 26 protons

30 neutrons 26 electrons

Here’s a mysteryCalculate the mass of the Fe-56 atom in

amu from the sum of the parts:Protons: 26 x 1.0073 = 26.189

Electrons: 26 x 0.000549 = 0.014Neutrons: 30 x 1.0087 = 30.261

Total mass = 56.465But! The actual mass is 55.935

Here’s a mystery

The actual mass of an isotope can be found using a device called a mass spectrometer.

The actual mass is 55.935

Mass spectrometer

http://www.chemistry.ccsu.edu/glagovich/teaching/472/ms/instrumentation.html

magneticfield

Magnetic field makes charged atoms curve.

http://www.chemistry.ccsu.edu/glagovich/teaching/472/ms/instrumentation.html

magneticfield

Here’s a mystery

56.465 – 55.935 = 0.530 amu

The sum of the protons, neutrons and electrons is 56.465 amu.

The actual mass is 55.935 amu.

Here’s a mystery

Where is the missing mass?

56.465 – 55.935 = 0.530 amuSum of parts: p+, n, e-

actual isotope

mass ?

The solution

What does it tell us?

Recall Einstein’s famous equation:

E = mc2

Matter and energy are equivalent.

The solution

Matter can exist as energy and …… energy can exist as matter.

All calculated from E = mc2

They are both the same “thing”.

The solution

The difference between the mass of the parts (p+, n and e-) and the actual mass is called the “mass defect” and equals the mass of nuclear material that “exists as energy”.

The solution

The energy from the missing mass is the binding energy of the nucleus.

The binding energy is derived from the strong force which does

hold the nucleus together.

The solution

The binding energy is the energy required to “take apart” the nucleus to form nothing but individual protons and neutrons.

Is this binding energy related to nuclear

energy?

Nuclear energy

All have enough energy to ionize atoms.

Gamma rays are electromagnetic energy.

Alpha and beta particles have high kinetic energies.

All nuclear decay is accompanied by a release of energy.

Nuclear energy

This can result in damage to your body.

Ionization occurs when electrons are removed from

atoms by or radiation.

An ion is a “charged atom” or group of atoms.

cancer

Nuclear energy

Forms of ionizing radiation are:

Alpha Beta Gamma X-rays

Cosmic rays

Ultraviolet light (UV) can cause cancer, but it is not ionizing radiation.

Neutrons Positrons

There’s even more!

But there is an even greater release of energy when the atom splits apart …

Some of the energy that holds the nucleus together is carried away by the alpha, beta and gamma radiation.

Nuclear Fission

Nuclear fission

Fission – the splitting of an atom after the nucleus absorbs a neutron.

Nuclear fission

The mass number of the atom increases and the nucleus

becomes unstable.

A neutron collides with a nucleus and is absorbed.

Nuclear fission

The neutrons strike other atoms causing more fission.

Plus, two or three neutrons are released along with a great deal of energy.

The unstable nucleus splits into two or more fission fragments.

U-235U-235

Nuclear fission

NeutronNeutrons

Fission fragment

Nuclear fission

Neutrons

Fission fragment

These U-235 atoms can split when hit by neutrons, and

release more neutrons, starting a

chain reaction.

Nuclear fission

To picture a chain reaction, imagine 50 mousetraps in a wire cage.

And on each mousetrap are two ping-pong balls.

Now imagine dropping one more ping-pong ball into the cage …

Detail of ping-pong balls on mousetraps.

http://www.physics.montana.edu/demonstrations/video/modern/demos/mousetrapchainreaction.html

Nuclear fission

Billions of splitting atoms releases a huge amount of heat energy.

This energy originally held the nucleus together.

As the chain reaction proceeds, energy is released as heat energy.

Nuclear fission

This heat energy can be harnessed to boil water, creating steam,

that can spin a turbine,

that can turn a generator,

creating electricity.

Nuclear reactor

Nuclear reactorReactor core

Containment building

Heat exchangerSteam generator

Water circulates in the core

Steam to turbine

Water from cooling lake

Steam to turbine

Cadmium control rods – absorb neutrons

Steam to turbine

The water in the core serves two functions.

(1) The water cools the core and carries away heat.

(2) Water is a moderator. The water slows the

neutrons so that they can cause fission. Fast

neutrons do not cause fission.

Steam to turbine

From nuclear energy to…

Steam to turbine

Water from cooling lake Cooling towers

or lake

turbine generator

Transmission wires

Condensed steam

Steam to turbine

or lake

turbine generator

Transmission wires

Condensed steam

Electrical energy

Steam to turbine

or lake

turbine generator

Transmission wires

Condensed steam

Electrical energy

This part of the system is the same regardless of how the steam is produced. The heat can come from nuclear energy or by burning coal, natural gas or fuel oil.

Electrical energy

In fact, the only purpose of a nuclear reactor is to boil water.

Pros and cons

Cheap, plentiful power, no CO2, nuclear waste, terrorist attack, running out of oil and coal, on-site storage, breeder reactors, transportation of spent fuel, “not in my backyard”, …

What about fusion?

Nuclear fusion

is like a day without fusion.

A day without sunshine

Nuclear fusion

Is nuclear fusion an alternative to fission for producing electricity?

Fusion occurs when hydrogen atoms combine to make helium,

and release energy.

Nuclear fusion powers the sun.

Nuclear fusion

Now consumes more energy than it releases.

Magnetic bottle. Control problems.

Occurs at very high temperatures which nothing can withstand.

Fusion not now technically feasible.

Nuclear Chemistry

Developed by Mike JonesPisgah High School

Canton, NC

nuclear chemistry last revision: 100211 m. jones pisgah high school

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