Nuclear Chemistry

The energy needed to change an atomic nucleus is much greater than the energy needed to rearrange the valence electrons of atoms. While we are surrounded by many naturally occuring radioactive materials, the nuclear chemist must use accelerators and reactors to achieve the extreme energies needed for their reactions.

Fission is a nuclear reaction in which a very heavy nucleus is split into two approximately equal fragments. This process, known as a chain reaction, releases several neutrons which in turn split more nuclei. If it is not controlled, a nuclear explosion can occur. The photograph above is a ground explosion of twenty pounds of plutonium releasing the energy equal to 70 million pounds of TNT. WWW Click on the picture to see an air burst equal to 2,200 million pounds of TNT.

Fusion is a nuclear reaction in which two or more small nuclei are forced together to form one larger nucleus. The energy released during a fusion reaction is much greater than in a fission reaction. Transmutation - a change in the number of protons in the nucleus producing an atom with a different atomic number.

First controlled nuclear reaction, CP-1, December 2, 1942, at 3:25 p.m.
First atomic explosion, Trinity, July 16, 1945, at 5:29:45 a.m.
Hiroshima, August 6, 1945, at 8:15 a.m.
The Gadget and Fat Man each had an explosive yield of 21 kilotons of TNT. About 1 kg of the approximately 6.15 kg of plutonium in each of these bombs fissioned into lighter elements totaling almost exactly one gram less, after cooling. The heat, light and radiation carried the missing gram of mass.
First thermonuclear explosion, Mike, November 1, 1952, at 7:15 a.m.
Three Mile Island, March 28, 1979, at 4:00 a.m.
Chernobyl, April 26, 1986, at 1:23:40 a.m.

This is a nuclear chemical symbol:

The element is represented by its chemical symbol.
The top number is the mass number - total protons and neutrons.
The bottom number is the atomic number - number of protons, or positive charges.

Transmutation

, the conversion of one element into another, can be represented with a nuclear equation. The earliest artificial transmutation was performed by Lord Rutherford in 1911. Nitrogen-14 was bombarded with alpha particles, producing Oxygen-17 and protons. The nuclear equation for this reaction looks like this:

14
7

N +

4
2

He →

17
8

O +

1
1

The equation above is "balanced". The total mass number (top #) is 18 on both sides and the total charge (bottom #) is +9 on both sides. Changing the nucleus of an atom often turns it into another element. For this reason, you rarely have the same chemical symbols on both sides of balanced nuclear equations.

Rules For Balancing Nuclear Equations:

Mass number is conserved in a nuclear change.
The sum of the mass numbers before the change must equal the sum of the mass numbers after the change.
Electric charge is conserved in a nuclear change.
The total electric charge on subatomic particles and nuclei before and after the change must be equal.

Nuclear Particles and Symbols

While all particles produced by the decay of an atomic nucleus have the energy to penetrate substances, some particles have much more energy than others.

K-capture occurs when an atomic nucleus captures an electron from its own innermost energy level. When this happens, the atomic number is decreased by one and the mass number remains the same.

In an equation, K-capture looks like this:

100
44 Ru + 0
−1 e → 100
43 Tc

Nuclear Equations

Not all the isotopes of an element are equally stable.

A completely stable isotope is one whose nucleus will not spontaneously decay.
A completely unstable isotope is one whose nucleus will spontaneously decay completely.

Most isotopes fall somewhere in between these extremes.

It is possible to predict which isotopes will be the most stable using the following general rules:

The greater the binding energy per nucleon, the more stable the nucleus.

Nuclie of low atomic numbers with a 1:1 neutron to proton ratio are very stable.

The most stable nuclei tend to contain an even number of both protons and neutrons.
An Island of Stability

Half-life: The length of time it takes for one-half of the atoms of a radioactive isotope to disintegrate. The rate of disintegration was once measured in curies, a non-SI unit. The SI unit is now the becquerel, Bq.

Half-Life Table
Isotope	Half-life	Decay Type
⁶ ₂ He	0.802 seconds	Beta-minus
²²⁷ ₉₂ U	1.3 minutes	Alpha and Gamma
³ ₁ H	12.3 years	Beta-minus
¹⁴ ₆ C	5730 years	Beta-minus
²³⁵ ₉₂ U	7.1 x 10⁸ years	Alpha and Gamma

Nuclear half-lives conform to the same rate laws as first-order chemical reactions. The equation below is useful in half-life calculations:

N_t = amount of isotope remaining
N_o = original amount of isotope
#half-lives = elapsed time / half-life

Sample Problem: The half-life of the Pu-236 isotope is 87.74 years. Given 175 grams of Pu-236, how many grams will be left after 225 years?

Solution:

Many radioactive particles decay into other radioactive particles in a system known as a decay chain.

The final product of a decay chain will always be a stable substance.

Uranium-238 goes through a sequence of 14 individual steps to become Pb-206 in a decay chain shown on the right.

How many alpha particles are released?
How many beta particles are released?

All radiation produces a risk to living things. If the radiation has enough energy, it can penetrate living cells and disrupt their processes. This is particularly dangerous if DNA or RNA molecules are affected. Very small changes in this genetic material can cause mutations and cancer. Large amounts of radiation released into the upper atmosphere can quickly travel around the world.

Radiation absorption was once measured in non-SI units of rads, rems, and roentgens. Radiation absorption is now measured in the SI units grays, Gy. One gray is equal to the transfer of one joule of energy to one kilogram of living tissue.

Damage to living tissue by the absorbed dose of radiation is represented by the derived SI unit sievert, Sv. A sievert is equal to a gray multiplied by a "weighing factor", W_R, for the variables of radiation type and tissue characteristics.

We are always being exposed to radiation found naturally in our environment. This radiation is known as background radiation and is between 1 and 2.4 mSv/year.

Here is a general idea about radiation amounts:

A single medical X-ray produces about 0.2 mSv (0.0002 Sv).
Nuclear reactor workers are permitted to receive up to 0.05 Sv/year.
An exposure of 1 Sv/hour results in radiation poisoning.
Exposure to 3 to 5 Sv/hour results in death in 50% of the cases.

Here are some indications of the likely effects of a range of whole body radiation doses:

10,000 mSv (10 sieverts), as a short-term and whole-body dose would cause immediate illness, such as nausea and decreased white blood cell count, and subsequent death within a few weeks.
Between 2 and 10 sieverts in a short-term dose would cause severe radiation sickness with increasing likelihood that this would be fatal.
1,000 mSv (1 sievert), in a short term dose is about the threshold for causing immediate radiation sickness in a person of average physical attributes, but would be unlikely to cause death. Above 1000 mSv, severity of illness increases with dose.
If doses greater than 1000 mSv occur over a long period they are less likely to have early health effects but they create a definite risk that cancer will develop many years later.
100+ mSv, the probability of cancer (rather than the severity of illness) increases with dose. The estimated risk of fatal cancer is 5 of every 100 persons exposed to a dose of 1000 mSv.
50 mSv, conservatively, the lowest dose at which there is any evidence of cancer being caused in adults. It is also the highest dose which is allowed by regulation in any one year of occupational exposure.
Dose rates greater than 50 mSv/yr arise from natural background levels in several parts of the world but do not cause any discernible harm to local populations.
3 mSv/yr, the typical background radiation from natural sources in North America, including an average of almost 2 mSv/yr from radon in air.
2 mSv/yr, the typical background radiation from natural sources, including an average of 0.7 mSv/yr from radon in air. This is close to the minimum dose received by all humans anywhere on Earth.

Food Irradiation Center For Consumer Research Wikipedia

It has been found that food spoilage by microorganisms can be prevented by exposing it to gamma radiation from Cobalt-60. Depending on the dose level, irradiated food may last for weeks or even years without refrigeration and with no change in the taste or consistency of the food.

The U.S. Food and Drug Administration has approved cold pasteurization of some foods with dosages up to 10,000 grays.

Internationally, foods such as apples, strawberries, bananas, mangoes, papayas, avocados, onions, potatoes, spices, seasonings, meat, poultry, fish, and grains have been irradiated for many years.

Food irradiation has been endorsed by FAO, WHO, USDA, the American Medical Association (AMA), and the Institute of Food Technologists (IFT) as a safe and practical method for preserving a variety of foods and reducing the risk of foodborne disease.

Which of these has been irradiated?

Nuclear Chemistry