Activity #5 – Mass per Nucleon
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Step 1: Review this video: https://wgu.hosted.panopto.com/Panopto/Pages/Viewer.aspx?id=7889fdcd-231a-45c9-b0ce-fe839aa5ec8b and read the excerpt from chapter 10 below (you don’t need to read all of chapter 10. The excerpt below is all that is needed).
Step 2:
Answer the following questions, in writing, in your words. 1. What is a nucleon? Protons and neutrons 2. What is fusion? Is two smaller atoms come together 3. How does mass per nucleon in a hydrogen atom compare to the mass per nucleon in a helium atom? 4. What happened to cause the mass per nucleon to change? When to two atoms comes together a pieces of mass is lost and converted into energy. 5. What is fission? Fission is when the nuclease of an atom splits into smaller pieces. 6. How does mass per nucleon change in a fission reaction? 7. How does the equation E=mc2 relate to mass per nucleon? It can find how much energy different amounts of mass can create. 8. Where does the energy released in a fusion or fission reaction come from? Is comes from the missing mass that was lost.

Step 3:
Submit your answers to a course mentor (naturalscience@wgu.edu) or come to a homework help session for feedback.

Reading excerpt:

10.8 The Mass–Energy Relationship—
Clearly, a lot of energy comes from every gram of nuclear fuel that is fissioned. What is the source of this energy? As we will see, it comes from nucleons losing mass as they undergo nuclear reactions.
Early in the early 1900s, Albert Einstein discovered that mass is actually “congealed” energy. Mass and energy are two sides of the same coin, as stated in his celebrated equation E=mc2. In this equation, E stands for the energy that any mass has at rest, m stands for mass, and c is the speed of light. The quantity is the proportionality constant of energy and mass. This relationship between energy and mass is the key to understanding why and how energy is released in nuclear reactions. The more energy associated with a particle, the greater the mass of the particle.
Is the mass of a nucleon inside a nucleus the same as that of the same nucleon outside a nucleus?
This question can be answered by considering the work that would be required to separate nucleons from a nucleus. From physics, we know that work, which is expended energy, is equal to force * distance. Think of the amount of force required to pull a nucleon out of the nucleus through a sufficient distance to overcome the attractive strong nuclear force. Enormous work would be required. This work is energy that is added to the nucleon that is pulled out.
According to Einstein’s equation, this newly acquired energy reveals itself as an increase in the nucleon’s mass. The mass of a nucleon outside a nucleus is greater than the mass of the same nucleon locked inside a nucleus. As discussed in Chapter 9, the nucleus of a carbon-12 atom, composed of six protons and six neutrons, has a mass of exactly 12.00000 atomic mass units (amu). Therefore, on average, each nucleon contributes a mass of 1 amu. However, outside the nucleus, a proton has a mass of 1.00728 amu and a neutron has a mass of 1.00867 amu. Thus, we see that the combined mass of six free protons and six free neutrons— (6 * 1.00728) + (6 * 1.00867) = 12.09570—is greater than the mass of one carbon-12 nucleus. The greater mass reflects the energy required to pull the nucleons apart from one another. Thus, what mass a nucleon has depends on where the nucleon is.
An important graph results from the plot of nuclear mass per nucleon inside each nucleus ranging from hydrogen through uranium (Figure 10.26). This graph is the key to understanding the energy associated with nuclear processes.
To obtain the average mass per nucleon, you divide the total mass of a nucleus by the number of nucleons in the nucleus. (Similarly, if you divide the total mass of a roomful of people by the number of people in the room, you get the average mass per person.) Note that the masses of the nucleons are different when they are combined in different nuclei. The greatest mass per nucleon occurs for the proton alone, hydrogen, because it has no binding energy to pull its mass down. Progressing beyond hydrogen, the mass per nucleon is smaller, and it is least for a nucleon in the nucleus of the iron atom. Beyond iron, the process reverses itself, as nucleons have progressively more and more mass in atoms of increasing atomic number. This continues all the way to uranium and the transuranic elements. From Figure 10.26, we can see how energy is released when a uranium nucleus splits into two nuclei of lower atomic number. Uranium, being towards the right-hand side of the graph, is shown to have a relatively large amount of mass per nucleon. When the uranium nucleus splits in half, however, smaller nuclei of lower atomic numbers are formed. As shown in Figure 10.27, these nuclei are lower on the graph than uranium, which means that they have a smaller amount of mass per nucleon. Thus, nucleons lose mass in their transition from being in a uranium nucleus to being in one of its fragments. When this decrease in mass is multiplied by the speed of light squared (in Einstein’s equation), the product is equal to the energy yielded by each uranium nucleus as it undergoes fission.
We can think of the mass-per-nucleon graph as an energy valley that starts at hydrogen (the highest point) and slopes steeply to iron (the lowest point), then slopes gradually up to uranium. Iron is at the bottom of the energy valley and is the most stable nucleus. It is also the most tightly bound nucleus; more energy per nucleon is required to separate nucleons from its nucleus than from any other nucleus.
All nuclear power today is produced by nuclear fission. A more promising long-range source of energy is to be found on the left side of the energy valley in a process known as nuclear fusion.

10.9 Nuclear Fusion
Notice, in the graphs of Figures 10.26 and 10.27, that the steepest part of the energy valley goes from hydrogen to iron. Energy is gained as light nuclei combine.
This combining of nuclei is nuclear fusion—the opposite of nuclear fission. We can see from Figure 10.28 that as we move along the list of elements from hydrogen to iron, the average mass per nucleon decreases. Thus, when two small nuclei fuse—say, a pair of hydrogen isotopes—the mass of the resulting helium nucleus is less than the mass of the two small nuclei before fusion. Energy is released as smaller nuclei fuse.

For a fusion reaction to occur, the nuclei must collide at a very high speed in order to overcome their mutual electric repulsion.* The required speeds correspond to the extremely high temperatures found in the Sun and other stars.
Fusion brought about by high temperatures is called thermonuclear fusion. In the high temperatures of the Sun approximately 657 million tons of hydrogen is converted into 653 million tons of helium each second. The missing 4 million tons of mass is converted to energy—a tiny bit of which reaches Planet Earth as sunshine.
Such reactions are, quite literally, nuclear burning. Thermonuclear fusion is analogous to ordinary chemical combustion. In both chemical and nuclear burning, a high temperature starts the reaction; the release of energy by the reaction maintains a high enough temperature to spread the fire. The net result of the chemical reaction is a combination of atoms into more tightly bound molecules.
In nuclear fusion reactions, the net result is more tightly bound nuclei. In both cases, mass decreases as energy is released.