RadioactivityNuclear EquationsNuclear StabilityTypes of Radioactive DecayRadioactive Decay SeriesNuclear Bombardment ReactionsDetection and Biological EffectsBiological Effects and DosageRate of Radioactive DecayEnergy of Nuclear ReactionsExam Review TopicsRadioactivityIn the formation of molecules or ionic compounds, only the distribution of the electrons is affected. The nucleiof the atoms involved remain unchanged. However, in nuclear reactions, the composition of the nucleus willchange, most often changing the identity of he element itself.The two main processes we will discuss are radioactive decay, that is, the process in which a nucleusspontaneously disintegrates, giving off radiation, and nuclear bombardment reactions, where a nucleus isbombarded, or struck, by another nucleus or by a nuclear particle.Antoine Henri Becquerel is credited with the discovery radioactive phenomena. He first noticed thatphotographic plates could be exposed when placed in a box with certain uranium salts, and concluded that theminerals were radiating some type of energy.The radiation from uranium can be separated (by passing it through an electromagnetic field generated by twooppositely charged plates) into three fundamental forms. The first are alpha particles, which are repelled by thepositively charged plate indicating that they have a positive charge. We now understand that these alphaparticles consist of two protons and two neutrons (essentially the same as a helium atom that has been strippedof its electrons). The nuclear symbol for the alpha particle includes its atomic number (the number of (+)charges it carries) and its mass number (the sum of its protons and neutrons).Beta particles bend the opposite direction as alpha particles when separated, indicating they have a negativecharge. They resemble, at least in mass and charge, a very high speed electron that appears to be ejected by thenucleus of some unstable atoms. Since electrons do not reside in the nucleus, there are several theories as tohow an "electron-like" particle could originate there. One plausible theory is that a neutron is actually a protonand electron "stuck" together. If the neutron were to eject its "electron" half, it would be converted into aproton. Since elements that undergo beta decay do increase in atomic number as they radiate, this is consistentwith the theory.Since beta particles are negatively charged, they are assigned an atomic number of -1 and a mass number of 0(considering their mass to be 0.00055 amu, their mass is negligible compared to protons and neutrons). So, itssymbol isGamma rays are unaffected by electric charge; they have been demonstrated to be electromagnetic wavessimilar to x-rays, but of much shorter wavelength (and consequently, higher energy). Since gamma rays haveno charge and, as light, have no rest mass, the symbol used for gamma rays is Page 124αor24HeThe atomic numberThe mass number−10βor−10e00γNuclear EquationsEquations for nuclear reactions are similar to regular chemical reactions in that they are subject to a massbalance. In nuclear equations we use nuclide symbols to represent given isotopes of elements. The nuclidesymbol includes the atomic number and the mass number of that particular isotope. In balancing a nuclearequation, one must simply ensure that the sum of atomic numbers and mass numbers on both sides of the arroware equal.For example, the alpha decay of Uranium-238 would be represented as +In this example, Uranium is the "parent" nuclide, while thorium-234 is considered the "daughter" nuclide.Reactant and product nuclei are always represented using nuclide symbols. Other particles that areinvolved in many nuclear equations are given the following symbols in which the subscript equals thecharge and the superscript equals the mass number.proton or11H11pneutron01nelectron or−10epositron or+10egamma photonBeta emission of a nucleus involves the ejection of a beta particle from the nucleus. If you recall, this producesa daughter element with an atomic number one greater than the parent nuclide. For example, the equation forthe beta decay of Technicium-99 produces Ruthenium-99 as its daughter.Note that in all nuclear equations, the total charge is conserved. This means that the sum of the subscripts onboth sides of the arrow must tally. Likewise, the total mass is conserved requiring that the sum of thesuperscripts on both sides of the arrow must tally.Positron emission involves the ejection of a positron from an unstable nucleus. A positron is similar to a betaparticle, having the same mass but carrying a positive charge.Gamma emission is simply the emission of a gamma photon, which carries no mass and no chargeconsequently has no effect on the parent nuclide's atomic number or mass number. Gamma emission generallyaccompanies the emission of other radioactive particles, such as alpha or beta. Page 224He90234Th92238U+10β−10β00γ−10e4499Ru4399Tc+Nuclear StabilityOne might wonder how you can manage to get so many positively charged protons to coexist in the nucleus,when like charges repel. Well, the current theory involves the nuclear force which is defined as a strong forceof attraction between nucleons that acts only at very short distances (about 10-15 m). Once we go beyondnuclear distances, these nuclear forces are negligible.So, two protons that are farther apart than 10-15 m will repel one another. However, in the nucleus, whereprotons are in close proximity to one another, the nuclear force is quite effective, thereby giving a (sometimes)stable nucleus.Circumstantial evidence points to the theory that protons and neutrons exist energy levels, much like electronsin an atom existing in discrete levels. The shell model of the nucleus is a nuclear model in which protons andneutrons exist in levels, or shells, analogous to the shell structure that exists for electrons in an atom.In atoms, filled shells of electrons (such as, with the noble gases) are more stable. The total number of electronsfor these stable atoms are 2 (for He), 10 (for Ne), 18 (for Ar), etc.. Similarly, it has been noted that nuclei withcertain numbers of protons or neutrons appear to be unusually stable. These “stable” numbers (referred to asmagic numbers) have been explained using the Shell