1. Describe both fission and fusion, and relate these processes to the curve of binding energy per nucleon versus mass number.
2. Compare the energy released by fission and fusion with that of an ordinary chemical reaction.

Nuclear Fission. The curve of binding energy per nucleon versus mass number ( Figure 23.2) shows that the most stable nuclei are those with masses close to iron-56, which is the most stable nucleus. During fission a heavy nucleus of mass greater than about 230 amu splits into two lighter nuclei whose masses are usually between 80 and 160 amu. Since the two smaller nuclei are more stable than the larger nucleus, energy is released in the process.

Although many nuclei heavier than uranium can undergo fission, the most important ones are U-233, U-235, and Pu-239. These isotopes undergo fission upon capture of a neutron. It is important to realize that naturally occurring uranium consists of two isotopes, U-235 and U-238, but that only U-235 is fissionable with thermal neutrons. The term thermal neutrons refers to those existing at temperatures around 25°C. Since the nucleus does not repel a neutron, as it does an alpha particle, for instance, neutron-induced fission will occur at ordinary temperatures.

The reaction is quite complex because the same two products are not formed by all fissioning nuclei. The two reactions below show just two out of many possibilities.

The actual distribution of product yields is shown in Figure 23.7 of the text.

A significant feature of fission is that on the average 2 to 3 neutrons are released per fission event. Since neutrons are required to initiate fission, and because neutrons are also products of fission, a nuclear chain reaction is possible.

The energy released during nuclear fission depends somewhat on just what products are formed. The energy released from the fission of one mole of U-235 atoms can be calculated from the equation E = mc2. The calculation shows that about 2.0 x 10¹⁰ kJ are released per mole of uranium. This amount of energy is 70 million times the amount released in the exothermic chemical reaction in which 1 mol of H₂ reacts with 1/2 mol of O₂ to form 1 mol of water!

Nuclear Fusion. Radioactivity and nuclear fission are processes in which matter "comes apart." The energy releasing processes that occur on the sun are ones in which matter is fused. Nuclear fusion is the combining of small nuclei, such as hydrogen, to form a larger, more stable nucleus. Such a nucleus will have a higher average binding energy per nucleon, and fusion reactions will be exothermic. Because all nuclei are positively charged they must collide with enormous force in order to combine (fusion). This means that the atoms that will undergo fusion must be heated to millions of degrees. Fusion reactions are called thermonuclear reactions because they occur only at very high temperatures, such as those in the sun.

The reaction that accounts for the tremendous release of energy by the sun is believed to be the stepwise fusion of four hydrogen nuclei to produce one helium nucleus. The net process is

One gram of hydrogen upon fusion releases the energy equivalent to the combustion of 20 tons of coal. The fusion of four moles of H atoms by the preceding equation releases 2.6 x 10⁹ kJ of energy.

Our sun is made up of mostly hydrogen (90 percent) and helium (9 percent). In its interior the temperatures are estimated to reach 15 million °C. At these temperatures hydrogen will fuse to form helium; but helium, with its greater nuclear charge, will not fuse to form the heavier elements. The heavier elements up to iron are formed by fusion reactions that occur in exploding stars, called nova and supernova, where the temperatures can reach 2 billion °C.