CC2 Unit CC Notes: Nuclear ChemistryPage 1 of 9
CC-1 Reading: Changes in the Nucleus and Nuclear Stability
We have previously discussed chemical reactions, which result from the rearrangement of electrons of different atoms to form new compounds. The centers of atoms (nuclei) were not involved in chemical reactions. This unit discusses reactions that involve the nucleus of the atom. These are not chemical changes, but nuclear changes or reactions.
Scientists first learned about nuclear reactions when radioactivity was discovered. In 1895, Wilhelm Roentgen discovered X-rays, a type of high energy electromagnetic wave radiation. Other types of radiation have since been discovered by important researchers such as Henri Becquerel (he discovered radiation in uranium ore), and Pierre and Marie Curie (they isolated the radioactive elements radium and polonium).
The elements worked on by the Curies emitted very high radiation levels, so they were able to determine the properties of the radiation:
- While passing through air, the gas molecules became ionized so that air conducted electricity.
- Radiation caused phosphorescent substances (remember ZnS—zinc sulfide?) to glow brightly.
- When bacteria and other small organisms were exposed to this radiation, they died.
- The temperature near the surface of the radium was elevated.
All of these characteristics indicated that the radium was releasing energy.
Scientists later found that this radiation was produced when the nuclei of atoms changed, producing atoms of different elements. This change of one element into an entirely different element (or elements) is called transmutation. The radium atoms (atomic # 88) used by the Curies broke down to produce radon (atomic # 86) and helium (atomic # 2). Isotopes of the same element that are identified by their number of protons and different number of neutrons (and therefore mass number) are called nuclides. Particles that reside in the nucleus (like protons and neutrons) are called nucleons. Most nuclides found in nature are stable, but some are not.
Nuclear Stability
- One would think that the like charges of all the protons in an atomic nucleus would cause enormous repulsive forces that would force the nucleus apart. This doesn’t happen because the neutrons help to generate a strong nuclear attractive force that counteracts the repulsive force existing between the protons. All elements with atomic number 2 have neutrons. Stable nuclei up until atomic number 20 will have roughly equal numbers of protons and neutrons. As the number of protons increases beyond this number, a greater number of neutrons than protons are needed to maintain the nuclear force that keeps the nucleus stable. Figure 6 on page 646 shows the so-called belt, or band, of stability. It describes the ratio of neutrons to protons necessary to maintain a stable nucleus as the atomic number increases from 2 through 83 (bismuth). Above this atomic number, all nuclei are radioactive.
- There is also the concept of magic numbers. Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons are generally more stable than those that don’t have these numbers of nucleons. These numbers correspond to complete filling of nucleon energy levels. Scientists see this as analogous to the chemical stability imparted to elements with completely filled electron energy levels (noble gases).
- Additionally, nuclei with even numbers of both protons and neutrons tend to be more stable than those with odd numbers of nucleons. This observation is analogous to the idea that a completely filled electron orbital (2 electrons) is more stable than a half-filled orbital.
Unstable nuclei undergo spontaneous changes that change their number of protons and or neutrons, thus leading to the transmutation of one element into another. We can express the changes that occur through the use of nuclear reaction equations. These are actually pretty easy to deal with—all you have to remember is that the total of the atomic numbers (representing charges of the particles involved) and the total of the mass numbers (representing the sum of the number of protons and neutrons) must be equal on both sides of the equation:
Notice that all of the lower-left subscripts add up to 6 on each side, while all of the upper-left superscripts add up to 13 on each side. The equality of the superscripts indicates the conservation of mass, and the equality of the subscripts indicates the conservation of charge that must occur in every balanced nuclear reaction equation.
Remember how we discussed in Unit C the Law of Conservation of Mass put forth by Antoine Lavoisier? It is often stated as “Matter is neither created nor destroyed.” We amended it in the modern atomic theory by stating that “Matter is neither created nor destroyed in a regular chemical process.” I implied in our discussion that matter can be converted to energy during nuclear processes, and Albert Einstein figured out the mathematical mass-energy relationship:
E = mc2, where m is the change in mass in kg, c is the speed of light (3.0 108), and E is energy in joules
This concept that mass can be gained or lost (indicated by m) during a nuclear reaction so that the law of conservation of mass doesn’t hold is called mass defect. The change in mass for a nuclear reaction is going to be extremely small, but you can see that the speed of light is very large, and the square of that value is gigantic, so even though the m is teensy, the energy change generated by a nuclear reaction can be substantial. The energy released or absorbed during a nuclear process is called the nuclear binding energy, because it is related to the energy involved with generating or breaking down the strong nuclear forces that hold the nucleus togetherp. This is the basis for our nuclear energy industry as well as atomic bombs.
CC-2 Reading: Radioactivity and Types of Radiation
The emission of radiation due to thespontaneous disintegration of a nucleus to form other lighter elements is called radioactivity. The process is called radioactive decay.
Natural radioactivity
The unstable, or radioactive, isotopes are called radioisotopes. The radioisotopes that are isolated from samples in nature exhibit natural radioactivity. Most isotopes of the lighter elements are stable, and therefore are not radioactive. Scientists are able to make radioactive isotopes of these and some heavier elements by bombarding (hitting) the nuclei of stable isotopes with high-energy particles. The decay of these resulting unstable isotopes produce induced radioactivity.
Artificial (Induced) Radioactivity:
We already talked about how Rutherford discovered the different types of emissions during his study of naturally radioactive elements. Once he characterized alpha particles, he discovered that he could make rare isotopes of elements by transmutating non-radioactive elements. He did this by bombarding stable nitrogen-14 atoms with alpha particles to produce radioactive oxygen-17 and emit protons:
You hopefully will recall that we mentioned that James Chadwick discovered the neutron in 1932. He did this using a transmutation experiment: He bombarded beryllium-9 with alpha particles to produce carbon-12 and an emitted neutron:
Both of the above transmutations produced stable elements, but it was only a matter of time before scientists started creating unstable (radioactive) elements. Irene and Frederic Joliot-Curie did this first by bombarding stable aluminum-27 with alpha particles to create radioactive phosphorus-30 and emitting a neutron. This unstable isotope then degraded to silicon-30 by positron emission:
Transuranium elements are elements with more than 92 protons in their nuclei, and they are all radioactive. Many have been made through artificial transmutation.
Do you remember the Rutherford Gold Foil experiments that used alpha (α) particles given off by a radioactive source? I mentioned that Rutherford had discovered them in his studies of the radiation emitted by radioactive substances. In fact, he discovered three types of radiation and characterized those using electrical plates similar to those used by J.J. Thomson when he concluded cathode rays were negatively charged. Rutherford concluded that α rays were positively charged because they were attracted toward a negatively-charged plate, that beta (β) rays were negatively-charged because they were attracted toward a positively-charged plate. The third type of radiation was gamma (γ) rays, and was not affected by charged plates. Although α and β rays have properties of both waves and particles, they are now usually called particles. Gamma rays are considered to be only electromagnetic radiation.
You can contrast and compare the types of radiation using the chart below:
Types of RadiationAlpha rays or particles / Beta rays or particles / Gamma rays
Nature / Sometimes behave like particles, sometimes like waves / Sometimes behave like particles; sometimes like waves / Electromagnetic waves of extremely short wavelength
Speed / About 1/10 the speed of light / Approaching light speed / Speed of light
Mass / 4 amu / 0.00055 amu / 0
Penetrating power / Relatively weak (can be stopped by a single sheet of paper) / Greater than alpha (can be stopped by a thin sheet of aluminum) / very penetrating (several centimeters of lead needed to stop them)
Ionizing ability / Ionizes gas molecules / Ionizes gas molecules / Ionizes the atoms in flesh, and causes severe damage to cells.
Symbol / or / or /
You will note that the symbols shown above have similar notation to those discussed in our atomic structure unit. The left superscript is the mass number of the particle, and the left subscript is the number of protons (in the case of alpha emission) or the charge of the emitted particle or radiation (as in beta and gamma emission).
Now, let’s talk about when each type of emission occurs:
Alpha particles () are high energy helium-4 nuclei () that are ejected by a very heavy unstable nucleus (atomic number 84). They are sometimes represented as . We can visualize the radioactive decay of uranium-238 (U-238) by the spontaneous emission of an particle with a nuclear equation:
Sometimes this process is called alpha decay. It’s important to note that the sum of the atomic numbers is the same on each side. Similarly, the sum of the mass numbers is also the same on both sides. Remember that the radioactive properties of an element are independent of their state of chemical combination. This means that radioactive nuclides of an element will chemically react with other elements in a manner that is identical to that of their non-radioactive counterparts of the same element. We exploit this property in medical diagnostic techniques and dating of artifacts.
Practice Exercise
What element undergoes alpha decay to form lead-208?
Explanation: You can see that the atomic number of the species on the left must be 82 + 2, or 84. If we look up the element with the atomic number of 84 on the periodic table, we see that it is polonium (Po). It must have a mass number of 208 + 4, or 212.
When a nuclide has a neutron : proton ratio that lies above the band of stability, high speed electrons are emitted by that unstable nucleus, and they are called beta particles. They are presented in nuclear equations as either OR . The zero superscript means that the mass of the electron is negligible compared to the mass of neutrons and protons. The 1 subscript indicates the charge of the particle, which is opposite that of a proton. See the sample -emission reaction below:
The above reaction shows the atomic number increasing by 1, so a -emission can be viewed as the conversion of a neutron into a proton and an electron, the latter of which is emitted:
THIS OCCURS ONLY DURING NUCLEAR PROCESSES—electrons don’t reside in the nucleus. You should also be able to see that the neutron : proton ratio has now been reduced, thus moving the new element formed closer to, or into, the band of stability.
Gamma radiation (gamma rays) are high energy photons (short wavelength EM radiation). It doesn’t change the atomic number or the mass number of a nucleus, so it is represented as or . It is present along with most other forms of radiation because it represents the energy lost when the remaining nucleons reorganize into more stable configurations.
A positron, e, is a particle with the same mass as an electron, but an opposite charge. It doesn’t last very long, as it is destroyed, or annihilated, when is collides with an electron: e + e 2. Positrons are sometimes called “anti-electrons,” as they are the first type of “anti-matter” that was found. Positron emission is demonstrated by the decay of carbon-11:
You can see that the neutron : proton ratio has increased with the formation of boron-11, so this form of decay often occurs in unstable nuclides with neutron : proton ratios that lie below the belt of stability. Because the number of protons is reduced, the atomic number decreases by 1, so we say that a proton has been converted into a positron and a neutron:
Electron capture (sometimes called “K-capture”) involves the trapping of an inner shell electron from the electron cloud outside the nucleus:
Rb + e (orbital electron) Kr
Notice that the mass number stays the same, but the atomic number decreases by 1, so this can be viewed as the conversion of a proton into a neutron:
p + e n
It appears to be the reverse of the beta emission reaction, but it also acts to reduce the neutron : proton ratio for unstable nuclides that are above the band of stability.
Radioactive decay and balancing nuclear equations:
When unstable isotopes decay by emitting (giving off) an alpha particle, scientists say that alpha emission is occurring. Because alpha particles contain two protons and two neutrons, the atomic number of the radioactive atom that decayed will transmutate into an isotope of a different element (perhaps radioactive, perhaps not). The decay of uranium 238 by alpha emission can be expressed as a word nuclear equation or one with symbols:
Uranium-238 → thorium-234 + alpha particle
OR
The production of a different element where it was not previously present due to radioactive decay or emission is a nuclear reaction. It is important to note in the above equation that the sums of the superscripts and subscripts on the right side are equal to the mass number and atomic number (# of protons) of uranium-238 respectively to the left of the arrow.
The Uranium-238 Decay Series and Beta Emission
Sometimes after emitting an alpha particle, the nucleus of an isotope is still not stable. The thorium-234 produced above is one of those types of nuclides. It continues to decay via beta emissions and alpha emissions. If the next step in the decay is a beta emission, let’s figure out how this would work:
In order to balance charge and mass number on each side of the equation, the missing item (?) must have an atomic number of 91, and a mass number of 234:
The element that has an atomic number of 91 is protactinium, Pa, so the final balanced nuclear equation is:
Don’t worry about memorizing a decay series—just realize that you can predict the final products by knowing the starting nuclide, the end nuclide, and the number of each type of emission.
CC-3 Reading: Fission and Fusion
Nuclear Fission
Fission means splitting, so nuclear fission refers to the breaking apart of an atomic nucleus into 2 or more pieces of smaller mass that are more stable. As mentioned in the previous section, breaking apart a nucleus liberates a great deal of energy. We use nuclear fission reactors to carefully control the splitting reaction, and use the energy given off to do work. An example of a nuclear fission reaction that scientists have studied extensively is the bombardment of uranium-235 by slow neutrons to produce barium-140, krypton-93, more neutrons and a large quantity of energy:
A picture of this reaction appears in Figure 11 on page 654. The neutrons produced can collide with other U-235 nuclei to split successively more and more uranium-235 atoms and release more energy. It is easy to see that a chain reaction rapidly develops that perpetuates itself until too little uranium is left to be split.
If the starting mass of U-235 is too small, the neutrons produced by the first fission reaction will escape without striking other U-235 nuclei. A chain reaction therefore will not occur. As the size of the U-235 sample is increased, a point is reached at which enough of the neutrons are captured to keep the chain reaction going. This sample amount is called the critical mass. A very small additional increase in the amount of U-235 beyond this point will lead to a rapid build-up in the rate of fission, and to the generation of so much heat energy that an explosion will occur. Explosions from the fission of U-235 and plutonium are used to make atomic bombs. In this case, neutron bombardment is initiated when two or more portions of fissionable material (U-235 or plutonium) are rapidly brought together. The mass of each separate sample is less than critical mass, but the two samples together have a combined mass that is slightly larger than the critical mass required to generate the chain reaction.
Our society has found a way to harness the heat energy generated during a nuclear reaction to produce electricity. We use shielding (radiation-absorbing materials) to contain radioactive emissions that occur. Control rods absorb neutrons, thereby interfering with the chain reaction when they are inserted into the fissionable material (usually uranium-235). If the neutrons are moving too quickly, a chain reaction cannot be initiated, so moderators are inserted into the uranium fuel to slow them down. Make sure you look at Figure 12 on page 655 for a diagram of a nuclear fission power plant.
Fusion Reactions
When energy is released during a nuclear reaction between two or more light nuclei, to form one or more nuclei of smaller total mass, we say that a fusion reaction occurs. Fusion reactions must give off energy if the total mass has been reduced.
The fusion of hydrogen isotopes to form helium nuclei is responsible for two things that are part of our daily life: the sun and thermonuclear devices such as the hydrogen bomb. Because hydrogen nuclei are both positively charged, they normally repel each other. In order to overcome this repulsive force, the hydrogen nuclei must have a lot of kinetic energy (TRANSLATE THAT AS MOVE VERY FAST). The only way nuclei can reach such large kinetic energies is at very high temperatures (~2 107 Kelvins), like those found on the sun and other stars. Because they occur at very high temperatures, fusion reactions are often called THERMONUCLEAR REACTIONS. The chief source of light given off by the sun is likely due to the energy released during the conversion of hydrogen nuclei to helium nuclei. It is a multi-step process with a net reaction of: