Objective: To become familiar with the use of the Chart of Nuclides
Equipment: The Chart of Nuclides
Theory:
1. Radioactivity
A nucleus with a given number of protons and neutrons is called a nuclide. The number of protons in a nucleus is denoted by Z, called the atomic number. The number of neutrons is denoted by N, called the neutron number. The total number of nucleons (protons and neutrons) in a nucleus is denoted by A, called the mass number. Note that A = Z + N. A nuclide is represented by
where X represents the chemical symbol of the element. For example, some of the nuclides of the first few elements of the periodic table (H, He, Li, Be, B, C, N, and O) are:
The chart of nuclides arranges the known nuclides on a Z versus N basis. There are thousands of known nuclides, many of which are radioactive. Radioactivity refers to the emission of particles or electromagnetic radiation from a nucleus.
The protons in a nucleus all repel each other by the electrostatic force given by Coulomb’s law. A nucleus is held together by a short-range, strong nuclear force between the nucleons (protons and protons, neutrons and neutrons, and protons and neutrons).
For very light elements, the nuclei tend to be stable when the number of neutrons is about the same as the number of protons. For heavier elements, more neutrons are needed in order to maintain the stability of the nucleus. For heavy elements, the N/Z ratio of stable nuclei approaches about 1.5. If a nucleus has too few or too many neutrons, it tends to be radioactive. There are far more radioactive nuclides than stable ones.
2. Isotopes of an Element
Every element is identified by the number of protons (Z) in its nucleus. For example, any atom having exactly 8 protons in its nucleus is known as oxygen. Oxygen nuclei can contain different numbers of neutrons. As long as an atom has 8 protons in its nucleus, it is oxygen and behaves chemically as oxygen. The nuclides of a few of the isotopes of oxygen are listed below:
Definition: Isotopes of an element have the same number of protons but different numbers of neutrons.
As another example, the following are the nuclides of some of the isotopes of uranium, the 92nd element in the periodic table. Uranium is the heaviest naturally found element in nature.
3. Types of Radiation
Three important types of radiation are alpha particles, beta particles, and gamma rays.
Alpha particles (α particles) are helium nuclei that eject from some radioactive isotopes that undergo spontaneous decay. Therefore an α particle is made of 2 protons and 2 neutrons.
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When a nucleus undergoes an α-decay, the daughter element has 2 fewer protons, and therefore it moves 2 elements lower in the periodic chart. |
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Beta particles (β particles) are fast moving electrons (or positrons). (A negatively charged beta particle (β-) is an electron, and a positively charged beta particle (β+) is a positron.
| For an electron to come out of a nucleus, a neutron turns into a proton, an electron, and an antineutrino, as shown on the right. The resulting element gains an extra proton while losing a neutron |
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An example is shown on the right.
This means that the number of protons increases by 1, and the daughter element becomes one element higher in the periodic table. |
 Nitrogen, the daughter, is a higher element than carbon in the periodic table.
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Gamma rays (γ rays) are highly energetic electromagnetic radiation. Gamma rays are very penetrative and therefore dangerous due to their extremely small wavelengths. Alpha rays can be stopped by putting on suitable clothing and even by human skin if they are not very energetic. Beta rays can be stopped by a few millimeters of aluminum. The radiation concerns of nuclear reactors are mainly for gamma ray radiation.
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When a nucleus is in an excited state, it sometimes emits high energy electromagnetic radiation (a γ ray) to get out of that state. The excited state is usually shown by a star. An example is:
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4. Information Given in the Squares of the Chart of Nuclides
Guidelines for understanding the nuclides in the chart are given below:
1. Gray shaded square: (Stable Nuclide)
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Isotopes that occur in nature and are classified as stable
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2. White or "color" square: ( Artificially Produced Radioactive Nuclide)
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Artificially
produced radioactive isotopes. Some charts have color coding for the
range of half-lives and neutron absorption properties.
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3. Black rectangles across the top of square:
a. On gray-shaded square: Radioactive nuclide with long half life (Considered Stable)
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Radioactive nuclide found in nature with very long half-life.
An example is Ce-142 T1/2 =5x1015 years.
Such long half-life is considered to be stable. |
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b. On white square: Radioactive nuclide found in nature with relatively short half life
4. Smaller black rectangle near top of square:
Nuclide is a member of a natural radioactive decay chain.
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Nuclide is a member of a natural radioactive decay chain.
The historic symbol is inserted in the black area.
For example, Ra A for Po-218, and UX1 for Th-234. |
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5. Black triangle at bottom corner of square: Refer to item 1 above.
This indicates nuclide is formed by fission of U-235 or Pu-239.
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For example, Xe-140 and Sr-94 in the induced fission reaction of U-235 |
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6. Vertically divided square:
This is indicative of a nuclide with isomeric or metastable states. The nuclide possesses different states with different radioactive properties. The nuclei of the different states of a particular nuclide are called “nuclear isomers” (same Z and N numbers, but different radioactive properties). If two isomers exist, the higher energy state is shown on the left. If three isomers exist, the higher energy state is shown on the left with the lower energy state below it or to the right of it, and the ground state (the lowest energy level) to the right of both or below them.
Two isomeric states, one stable:
Two isomeric states, both radioactive:
The arrangement of nuclides in the chart is such that the nuclear processes can be understood by examining the chart carefully. Following is the explanation of two such processes.
1. Induced Reactions
An induced reaction is done by bombarding a target nucleus by a particle, a neutron, a proton, or an alpha particle, for example. There is an “in” particle colliding with the target nucleus, and an “out” particle could result in addition to the altered target nucleus.
When (9, 4)Be is bombarded by an alpha particle (4, 2)He, the compound nucleus (13, 6)C* is located two squares diagonally upward to the right on the chart of nuclides. This excited nucleus then releases a neutron, and the product nucleus (12, 6)C is located one square to the left of the excited nucleus (13, 6)C*.
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For example, Xe-140 and Sr-94 in the induced fission reaction of U-235 |
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The following two diagrams are useful for determining the relative chart locations of the products of
various nuclear processes.
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Displacements caused by nuclear bombardment reactions:
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2. Radioactive Decay:
In this case, there is no bombardment of a target nucleus. A radioactive nuclide spontaneously emits radiation and normally moves toward becoming more stable. The daughter nucleus may be obtained from the diagram that contains the “in” and “out” particles. Two spontaneous decays are shown below that follow the rules in the diagram that has “in” and “out” particles in it.
Procedure:
Use the information and guidelines you learned under “Theory” to answer the following questions:
Seconds
Minutes
Hours
Days
Years
For (e), list the energies of the emitted alpha particles in the decay process beside the directional arrows.
Compound nucleus Product
(a) 10 B (n, α) -------------------------- --------------------------
(b) 16 O (n, p) -------------------------- --------------------------
(c) 7 Li (p, γ) -------------------------- --------------------------
(d) 17 O (γ, n) -------------------------- --------------------------
(e) 32 S (n, p) -------------------------- --------------------------
(f) 3 H (d, n) -------------------------- --------------------------
(g) 2 H (t, n) -------------------------- --------------------------
Data:
Given: The Chart of Nuclides
Measured: N/A
Conclusion: To be explained by students
