Radiation Chemistry

Written by tutor Fatima I.

Radiation chemistry involves the study of nuclear reactions. There are clear differences between a “chemical reaction,” and a “nuclear reaction:” a chemical reaction involves electrons of an atom (which orbit the nucleus), while a nuclear reaction involves a reaction within the nucleus of an atom (which consists of protons and neutrons). Marie Curie discovered “radioactivity,” which is the spontaneous disintegration of the unstable nuclei of some elements, or isotopes of elements, into more stable nuclei.

Recall that isotopes are different forms of an element which contain the same number of protons as the element on the periodic table, but each isotope of the element contains a different number of neutrons (thus isotopes have varying mass numbers). Generally, all isotopes with an atomic number over 83 are considered radioactive. Because a great amount of energy holds the nucleus together, in a nuclear reaction, a great amount of energy is released as the nucleus is disintegrated: this energy is called the “binding energy.” The term “radiation” refers to the particles and energy that are released as a result of the radioactive reaction.

It is important to note that in a radioactive reaction, as in almost all reactions, the law of conservation of mass applies, which means that the total mass BEFORE the reaction has to equal the total mass AFTER the reaction occurs. This concept is key in understanding radioactive reactions.

To understand the notation of nuclear reactions, it is helpful to review what “mass number,” and “atomic number” refer to, and how they are represented along with the chemical symbol of an atom. Mass number refers to the total number of neutrons and protons in a nucleus, and atomic number refers to the total number of protons in the nucleus. Given a nucleus “X,” mass number and atomic number will be denoted as follows:

Also, an isotope of element “X” with mass number of 50, for example, may be written as “X-50.”

There are three primary types of nuclear reactions: alpha, beta, and gamma radiation. These three main types of radiation are outlined below:

1) Alpha Decay:

In alpha decay, an unstable radioactive nucleus releases an “alpha particle,” which is essentially a helium nucleus, and therefore the alpha particle has a mass number of 4 and an atomic number of 2, and can be denoted in either one of the two ways shown below:

Because of the law of conservation of mass, the new nucleus has a mass number that is four less than that of the initial nucleus, and an atomic number that is decreased by two. In layman’s terms, if you add the mass numbers on one side of the equation, they should equal the mass number on the other side of the equation, and the same goes for atomic numbers.

In an alpha radiation problem, often you are given only the initial unstable nucleus and asked to figure out the resulting nucleus. You know an alpha particle is released, so you use the law of conservation of mass to figure out the missing nucleus. Once you have found the missing nucleus, you can then use its atomic number to identify the element (referencing the periodic table):

Mass number of unknown nucleus: 227 – 4= 223
Atomic number of unknown nucleus: 88 – 2= 86
If we look on the periodic table, Radon (Rn) has an atomic number of 86
Therefore, our complete reaction is:

2) Beta Decay:

In beta decay, an unstable radioactive nucleus releases a “beta particle,” which is an “electron.” An electron has no mass, thus it has no mass number, and essentially a -1 “atomic number,” due to the -1 charge of an electron:

In much the same way as we solved the alpha decay reaction, we can solve for the new nucleus in beta decay (Hint: remember law of conservation of mass!):

Mass number of unknown nucleus: 216 –0= 216
Atomic number of unknown nucleus: 84 – (-1)= 85
If we look on the periodic table, Astatine (At) has an atomic number of 85
Therefore, our complete reaction is:

3) Gamma Decay:

In gamma decay, a high-energy photon is released. Radioisotopes, used in medicine for uptake in a person’s body to detect key organs and glands, are frequently made by gamma reactions. A gamma particle is:

Thus, in this type of radiation, there is no change in mass number or atomic number- the nucleus stays the same, and there is just a release of energy. Here is an example of a gamma reaction, again following the law of conservation of mass:

Other nuclear particles (besides the alpha, beta and gamma particles) seen in nuclear reactions include:

Uses of Radiation:

One use of radiation is in dating fossils, or any specimen that you want to find the age of. The “half life” is the time required for half of the nuclei in a specific element to undergo radioactive decay. For instance, let’s say:

The half life of I-123 is 20 hours. How much of a 100 mg sample of I-123 is left after 60 hours?
1) First, you want to figure out “how many 20 hrs are in 60 hours?” This would tell you how many half-lives the specimen goes through:
60 ÷ 20= 3 half lives

2) So this means, that you have to start with 100 mg, and divide that in half three times, so:
100 mg becomes 50 mg
50 mg becomes 25 mg
25 mg becomes 12.5 mg.

So 12.5 mg of I-123 is left after 60 hours.

“Fission reactions” refer to the splitting of an atomic nucleus into approximately equal parts with a release of energy. Each subsequent nuclei that results can continue to be split in a type of “fission chain reaction,” generating a great amount of energy. This is precisely how nuclear power plants generate a large amount of energy. About 15-20% of the world’s energy comes from these nuclear power plants. “Fusion reactions” refer to reactions where multiple nuclei, combine to form a heavier nucleus (the opposite of fission reactions). The sun and stars undergo fusion reactions.

Also, irradiation of food by gamma rays from certain isotopes also increases shelf life of the food. For instance, irradiation of milk gives it a shelf life of three months without refrigeration.

Dangers of Radiation:

Radiation can ionize atoms in our tissues, and cause reactions similar to decomposition or combustion in our skin. For instance, radiation can mutate nucleotide sequences in our DNA, potentially causing cancer.

The dangers of the specific type of radiation- alpha, beta or gamma- depend on each of their penetrative abilities. Alpha particles are not very dangerous because they cannot penetrate the barrier of the skin. Beta particles can penetrate about one centimeter deep in our skin. The most dangerous are gamma rays, which can penetrate through the skin very effectively.

Units called “REMs” are used to measure the effective biological damage that radiation causes. Some natural sources of radiation include cosmic radiation, the earth’s radiation, and even the radiation from elements found naturally in human tissue. Medical sources of radiation include X-rays, and radiotherapy, for instance in cancer treatment. Artificial sources of radiation include the nuclear power industry, and fallout from nuclear tests. However, the average exposure for an individual with all of these sources combined is only about 0.2 REMs/year. Here is a review of the extent of damage with greater exposure, due to living by a nuclear power plant, or a nuclear meltdown, for instance:

REMs	Damage
25	Changes in blood cell components
200	In a short period of time, you will have “radiation sickness:” nausea, vomiting, decrease in white blood cell count, dehydration, diarrhea, hemorrhaging and loss of hair
400 (anyone near the Chernobyl plant when it melted received this amount of damage almost immediately)	Half of the exposed population will be dead in sixty days
600	All exposed to this level will be dead in a week