Nuclear Chemistry

Nuclear Reactions VS Normal Chemical Reactions

 

Nuclear Reaction
  • involves the nucleus
  • the nucleus opens, protons and neutrons are rearranged
  • opening the nucleus releases a great amount of energy that holds the nucleus together called “binding energy”

 


Nuclear Reaction VS Chemical Reaction

Chemical Reaction

 

  • involves electrons, not protons and neutrons

Types of Radiation

 

  1. solar radiation
  2. cosmic radiation
  3. cosmogenic radionuclides
  4. inhaled radionuclides
  5. terrestrial radionuclides

 

Sources of Radioactivity

 

  • primordial – from before the creation of the earth
  • cosmogenic – formed as a result of cosmic rays interactions
  • human produced – enhanced or formed due to human actions

 

Where are the Sources of Radioactivity?

Manmade Sources:
Medical use of Radioactive Isotopes
Certain Consumer products –(eg Smoke detectors)
Fallout from nuclear testing
Emissions from Nuclear Power plants

Where are the Sources of Radioactivity?

 

Naturally Occurring Sources:
Radon from the decay of Uranium and Thorium
Potassium -40 – found in minerals and in plants
Carbon 14 – Found in Plants and Animal tissue


 

What does Radioactivity mean?

Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves.

There are numerous types of radioactive decay. 


The general idea:

An unstable nucleus releases energy to become more stable

What are three man decays?

 

  1. Alpha
  2. Beta
  3. Gamma

 

 

When is an Isotope Stable, or Why are Some Isotopes Radioactive?

 

 

“RULES”

A. All nuclei > 84 protons are unstable (the nucleus gets too big, too many protons)
B. Very Stable: Atomic Number 2, 8, 20, 50, 82 or 126
C. Isotopes with Proton=Neutrons are more stable

 

Half-Life Concept

 

The level of radioactivity for all radioactive samples decreases over time.
Radioactive decay shows a systematic progression.
If we start with a sample that has an activity of 1000 disintegrations per minute (dpm), the level will drop to 500 dpm after a given amount of time.  After the same amount of time, the activity will drop to 250 dpm.

 

Half-Life Concept
The amount of time for the activity to decrease by half is the half-lifet?.
Half-Life

 

After each half-life, the activity of a radioactive sample drops to half its previous level.
A decay curve shows the activity of a radioactive sample over time

 

Radioactive Waste

 

A sample of plutonium-239 waste from a nuclear reactor has an activity of 20,000 dpm.  How many years will it take for the activity to decrease to 625 dpm?
The half-live for Pu-239 is 24,000 years.
It takes 5 half-lives for the activity to drop to 625 dpm.

 

Half-Life Calculation

Iodine-131 is used to measure the activity of the thyroid gland.  If 88 mg of I-131 are ingested, how much remains after 24 days (t? = 8 days).
First, find out how many half-lives have passed:
Next, calculate how much I-131 is left:

Natural Radioactivity

There are three types of radioactivity:
Alpha particles, beta particles, and gamma rays
Alpha particles (a) are identical to helium nuclei, containing 2 protons and 2 neutrons.
Beta particles (b) are identical to electrons.
Gamma rays (g) are high energy photons.

Charges of Radiation Types

Alpha particles have a +2 charge and beta particles have a –1 charge.  Both are deflected by an electric field.
Gamma rays are electromagnetic radiation and have no charge, so they are not deflected.

Behavior of Radiation

Since alpha particles have the largest mass, they are the slowest moving type of radiation.
Gamma rays move at the speed of light since they are electromagnetic radiation.

Atomic Notation

A nuclide is the nucleus of a specific atom.
The radioactive nuclide strontium-90 has 90 protons and neutrons.  The atomic number is 38.

Nuclear Reactions

A nuclear reaction involves a high-energy change in an atomic nucleus.
For example, a uranium-238 nucleus changes into a thorium-231 nucleus by releasing a helium-4 particle, and a large amount of energy.
In a balanced nuclear reaction, the atomic numbers and masses for the reactants must equal those of the products.

Balancing Nuclear Reactions

1.The total of the atomic numbers (subscripts) on the left side of the equation must equal the sum of the atomic numbers on the right side.
2.The total of the atomic masses (superscripts) on the left side of the equation must equal the sum of the atomic masses on the right side.
3.After completing the equation by writing all the nuclear particles in atomic notation, a coefficient may be necessary to balance the reaction.

Alpha Emission

Radioactive nuclides can decay by giving off an alpha particle.
Radium-226 decays by alpha emission.
First, balance the number of protons: 88 = Z + 2, so Z = 86 (Rn)
Second, balance the number of protons plus neutrons: 226 = A + 4, so A = 222.

Beta Emission

 

Some radioactive nuclides decay by beta emission.
Radium-228 loses a beta particle to yield actinium-228.

Beta decay is essentially the decay of a neutron into a proton and an electron.

 

Gamma Emission

 

Gamma rays often accompany other nuclear decay reactions.
For example, uranium-233 decays by releasing both alpha particles and gamma rays.

Note that a gamma ray has a mass and a charge of zero, so it has no net effect on the nuclear reaction.

 

Positron Emission

A positron (b+) has the mass of an electron but a +1 charge.
During positron emission, a proton decays into a neutron and a positron.

Sodium-22 decays by positron emission to neon-22.

Electron Capture

A few large, unstable nuclides decay by electron capture.  A heavy, positively charged nucleus attracts an electron.
The electron combines with a proton to produce a neutron.

Lead-205 decays by electron capture.

Decay Series

Some heavy nuclides must go through a series of decay steps to reach a nuclide that is stable.
This stepwise disintegration of a radioactive nuclide until a stable nucleus is reached is called a radioactive decay series.
For example, uranium-235 requires 11 decay steps until it reaches the stable nuclide lead-207.

Radiocarbon Dating

A nuclide that is unstable is called a radionuclide.
Carbon-14 decays by beta emission with a half-life of 5730 years.
The amount of carbon-14 in living organisms stays constant with an activity of about 15.3 dpm.  After the plant or animal dies, the amount of C-14 decreases.

Radiocarbon Dating Continued

The age of objects can therefore be determined by measuring the C-14 activity.  This is called  radiocarbon dating.
The method is considered reliable for items up to 50,000 years old.

Uranium-Lead Dating

  • Uranium-238 decays in 14 steps to lead-206.  The half-life for the process is 4.5 billion years

 

 

The age of samples can be determined by measuring the U-238/Pb-206 ratio.
A ratio of 1:1 corresponds to an age of about 4.5 billion years.

Nuclear Chemistry

 

There are three types of natural radiation: alpha particles, beta particles, and gamma rays
Gamma rays are electromagnetic radiation.
Alpha particles are helium nuclei and beta particles are electrons.

 

What are the four processes of radioactive nuclide decay?

Radioactive nuclides decay by 4 processes:

Alpha emission
Beta emission
Positron emission
Electron capture


 

 

The time required for 50% of the radioactive nuclei in a sample to decay is constant and is called the half-life.  After each half-life, only 50% of the radioactive nuclei remain.
Artificial nuclides are produced by transmutation.


Geiger Counter

 

  • use to detect radioactive substances

 

Radiation Units

 

  • Roentgen: a unit for measuring the amount of gamma or x rays in the air
  • Rad: a unit for measuring absorbed energy from radiation
  • Rem: a unit for measuring biological damage from radiation

Effects of Radiation

  • 0-25 No effects
  • 26-50  small decrease in white blood cell count
  • 51-100 significant decrease in white blood cell count, lesions
  • 101-200 loss of hair, nausea
  • 201-500 hemorrhaging, ulcers, death in 50% population
  • >500 death

What is a radioactive decay series?
If a nuclide decays through the emission of radiation in more than one step, the overall process is called a radioactive decay series.
Nuclear Fission and Fusion

The splitting of a heavy nucleus into two lighter nuclei is nuclear fission.
The combining of two lighter nuclei into one nucleus is nuclear fusion.

Particles Penetrations

Alpha radiation is the weakest. It can be stopped by a piece of paper and posses no threat to any human tissue.

Beta radiation is strong, but still not lethal. It can be stopped by most clothing or fabrics. 

Gamma radiation is the strongest. It can only be stopped by lead. It posses great harm to tissue that it comes in contact with.

Alpha

Alpha – these are fast moving helium atoms. They have high energy, typically in the MeV range, but due to their large mass, they are stopped by just a few inches of air, or a piece of paper. 

Beta

Beta – these are fast moving electrons. They typically have energies in the range of a few hundred keV to several MeV. Since electrons are might lighter than helium atoms, they are able to penetrate further, through several feet of air, or several millimeters of plastic or less of very light metals. 

Gamma

Gamma – these are photons, just like light, except of much higher energy, typically from several keV to several MeV. X-Rays and gamma rays are really the same thing, the difference is how they were produced. Depending on their energy, they can be stopped by a thin piece of aluminum foil, or they can penetrate several inches of lead. 

Marie Curie

Marie Sklodowska Curie began studying radioactivity and completed much of the pioneering work on nuclear changes. Curie found that radiation was proportional to the amount of radioactive element present, and she proposed that radiation was a property of atoms (as opposed to a chemical property of a compound).Marie Curie was the first woman to win a Nobel Prize and the first person to win two (the first, shared with her husband Pierre and Becquerel for discovering radioactivity; the second for discovering the radioactive elements radium and polonium).

Chemists

The discovery of x-rays by William Conrad Roentgen in November of 1895 excited the imagination of a generation of scientists who rushed to study this phenomenon. Within a few months, Henri Becquerel found that both uranium metal and salts of this element gave off a different form of radiation, which could also pass through solids. By 1898, Marie Curie found that compounds of thorium were also “radioactive.” After pain-staking effort she eventually isolated two more radioactive elements[image]polonium and radium[image]from ores that contained uranium.

Rutherford

In 1899 Ernest Rutherford found that there were at least two different forms of radioactivity when he studied the absorption of radioactivity by thin sheets of metal foil. One, which he called alpha ([image]) particles, were absorbed by metal foil that was a few hundredths of a centimeter thick. The other, beta (� particles, could pass through 100 times as much metal foil before they became absorbed. Shortly thereafter, a third form of radiation, gamma ([image]) rays, was discovered that could penetrate as much as several centimeters of lead.

Particles

Alpha (?) – a positively charged helium isotope  – we usually ignore the charge because it involves electrons, not protons and neutrons
Beta (β) – an electron
Gamma (γ) – pure energy; called a ray rather than a particle

Other particles

Neutron
Positron – a positive electron
Proton – usually referred to as hydrogen-1
Any other elemental isotope

Wilhelm Conrad Roentgen

Wilhelm Conrad Roentgen on December 22 1895, “photographed” his wife’s hand, revealing the unmistakable image of her skeleton, complete with wedding ring. Roentgen’s wife had placed her hand in the path of X-rays which Roentgen created by beaming an electron ray energy source onto a cathode tube. Roentgen’s discovery of these “mysterious” rays capable of producing an image on a photographic plate excited scientists of his day, including Becquerel. Becquerel chose to study the related phenomena of fluorescence and phosphorescence. In March of 1896, quite by accident, he made a remarkable discovery.

Antoine Becquerel

Becquerel found that, while the phenomena of fluorescence and phosphorescence had many similarities to each other and to X-rays, they also had important differences. While fluorescence and X-rays stopped when the initiating energy source was halted, phosphorescence continued to emit rays some time after the initiating energy source was removed. However, in all three cases, the energy was derived initially from an outside source.

Becquerel
In March of 1896, during a time of overcast weather, Becquerel found he couldn’t use the sun as an initiating energy source for his experiments. He put his wrapped photographic plates away in a darkened drawer, along with some crystals containing uranium. Much to his Becquerel’s surprise, the plates were exposed during storage by invisible emanations from the uranium. The emanations did not require the presence of an initiating energy source–the crystals emitted rays on their own! Although Becquerel did not pursue his discovery of radioactivity, others did and, in so doing, changed the face of both modern medicine and modern science.
Ernest Rutherford
In 1911, Rutherford conducted a series of experiments in which he bombarded a piece of gold foil with positively charged (alpha) particles emitted by radioactive material. Most of the particles passed through the foil undisturbed, suggesting that the foil was made up mostly of empty space rather than of a sheet of solid atoms. Some alpha particles, however, “bounced back,” indicating the presence of solid matter. Atomic particles, Rutherford’s work showed, consisted primarily of empty space surrounding a well-defined central core called a nucleus.
Rutherford
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