Nuclear Reactions

　　All of the processes discussed in this section are examples of nuclear reactions, which are different from ordinary chemical reactions. Ordinary chemical reactions involve the exchange and sharing of electrons, while nuclear reactions involve alterations in the very core of an atom; that dense nucleus made up of protons and neutrons. 　　Radioactivity　　You will need to be familiar with several types of nuclear reactions and terms related to them to be fully prepared for the SAT II Chemistry test, and in this section we’ll review everything you’ll need to know. The first concept we discuss is radioactivity. Strictly speaking, radioactivity is the spontaneous disintegration of an unstable atomic nucleus and the subsequent emission of radiation. But what makes atoms radioactive to begin with, and what makes them undergo radioactive decay? It turns out that there is a stable ratio of protons to neutrons for each element; for the first 20 elements on the periodic table (hydrogen through calcium), this ratio is 1 proton to 1 neutron, for example. Protons and neutrons in excess of this stable number can be emitted radioactively. Below we have listed examples of the important types of radioactive decay. 　　Alpha decay occurs when the nucleus emits an alpha particle. Alpha particles have a positive charge and are equivalent in size to a helium nucleus, and so they are symbolized as . Alpha particles are the largest radioactive particle emitted. This type of radioactivity results in a decrease in the atomic number by 2 and a decrease in the atomic mass by 4. The equation below shows uranium-234 undergoing alpha decay:　　Beta decay occurs when the nucleus emits a beta particle. Beta particles have a negative charge and are much smaller than alpha particles. They’re equivalent to high-speed electrons and are symbolized by　or . This type of radioactivity causes an increase in the atomic number by 1 but no change in mass number. The equation below represents uranium-233 undergoing beta decay.　　How does a nucleus, which is composed of only protons and neutrons, eject an electron? A neutron is composed of a proton and an electron fused together. In beta emission, the electron is emitted from the nucleus, while the proton part remains behind, thus increasing the atomic number by 1.　　Example　　Complete the balanced equation by determining the missing term.　　Explanation　　Remember, the sum of the atomic numbers and the mass numbers must be equal on both sides of the equation. We are looking for a component that has mass number of 80 and an atomic number of 34 (34 protons). Using this information and the periodic table, we can identity the element produced by this beta decay as Se, or selenium. The missing term is Se. And the completed equation is:　　Gamma decay consists of the emission of pure electromagnetic energy; no particles are emitted during this process, and it is symbolized by equation;00g. After beta, positron, or alpha decay, the nucleus is left in a high-energy state, and at this point it will often emit gamma rays, which allows it to relax to its lower-energy ground state. Since gamma rays do not affect charge or mass, they are often not included in nuclear equations.　　Positron emission occurs when an atom becomes more stable by emitting a positron01e, which is the same size and mass as an electron but has a positive charge. This process converts a proton into a neutron; the positron is emitted and the neutron remains behind in the nucleus, decreasing the atomic number by 1.　　Often the emission of an alpha or a beta particle creates another radioactive species, which undergoes further radiation/emission in a cascade called a radioactive series. Notice that in the course of all of these types of radioactive decay, neither protons nor neutrons are either created or destroyed: this is due to what’s known as the law of conservation of matter, which states that mass is neither created nor destroyed. So when you see radioactivity equations on the SAT II Chemistry test, one of the most important things to remember is that the sum of the mass numbers and the sum of the atomic numbers must both be equal on both sides of the equation.　　Example　　Write the equation for the alpha decay of radium-221.Write the equation for the beta decay of sulfur-35. 　　Explanation　　The radium-221 atom has atomic number (A) = 88 and mass number (Z) = 221. When an alpha particle is emitted, the atomic number is reduced by 2 and the mass number is reduced by 4. The atomic number of the resulting atom is 86, so the element created as a result of this radioactive decay is radon-217.　　The sulfur-35 atom has an atomic number of 16 and a mass number of 35. When it undergoes beta decay, the atomic number is increased by 1 and the mass number remains the same. The atomic number of the atom created is 17, so the atom is chlorine-35.　　Fission and Fusion　　There are two main types of nuclear reactions: fusion and fission. In fusion reactions, two light nuclei are combined to form a heavier, more stable nucleus. In fission reactions, a heavy nucleus is split into two nuclei with smaller mass numbers. Both processes involve the exchange of huge amounts of energy: about a million times more energy than that associated with ordinary chemical reactions. In either case, if the new particles contain more stable nuclei, vast quantities of energy are released. 　　Nuclear power plants rely on fission to create vast quantities of energy. For example, U-235 nuclides can be bombarded with neutrons, and the result is lots of energy, three neutrons, and two stable nuclei (Kr-92 and Ba-141). The three neutrons formed can collide with other U-235 atoms, setting off a chain reaction and releasing tons of energy.

　　Example　　Is the following process an example of fission or fusion?　　Explanation　　This is an example of fission. Fission occurs when a large nucleus is bombarded by a small particle, such as a neutron. The result is two smaller nuclei and additional neutrons, and a chain reaction process begins.