Unit 8.1 Nuclear Chemistry - Nuclear Reactions. Review. Radioactivity. State College Area School District Teacher: Van Der Sluys

Transcription

1 Unit 8. Nuclear Chemistry - Nuclear Reactions State College Area School District Teacher: Van Der Sluys Review Atoms consist of electrons, protons and neutrons Atoms of elements are distinguished by the number of protons in the nucleus (atomic number) Isotopes of an element have different numbers of neutrons but the same number of protons Radioactivity Reactions involving the nucleus of atoms. Stable nuclides are not radioactive. Our focus will be on the less common naturally occurring and/or man-made isotopes that are radioactive. Radioactivity results when an unstable nucleus falls apart or decays into a more stable nucleus by giving off particles or radiation.

2 Nuclear Notation An atom can be represented by an atomic symbol, where X is the symbol for the element, Y is the mass number and Z is the atomic number Y X Z Nucleons are the particles that make up the nucleus of an atom (protons and neutrons) Subatomic Particles Alpha ( 2α) particles are made up of two protons and two neutrons, which is essentially the nucleus of a helium atom ( 2He 2+ ). Beta (β ) particles are electrons ( -e) coming from the decay of an unstable nucleus. Positrons are the antiparticle of an electron and therefore have the same mass as an electron but have a positive charge ( +e). Gamma particles are high energy particles of energy ( γ) or photons. Protons ( p) have a mass of amu and a positive charge. Neutrons ( n) have a mass of amu and no charge. Nuclear Particles Atomic Symbols Name alpha particle beta particle gamma particle neutron proton positron Notation He or α e or β γ n H or p e or β + + Symbol α β - γ n p β + 2

3 What Makes a Nucleus Stable The chemist s point of view There are two important forces that occur in the nucleus of an atom. The repulsion of the positively charged protons from one another. The attractive force called the strong nuclear force which acts only at very short distances and with certain combinations of protons and neutrons ratios close to :. The neutrons act as insulators of the charge of the protons and prevent the protons from feeling each others charge. What Makes a Nucleus Stable The nuclear physicist's point of view Strong interaction From Wikipedia, the free encyclopedia The strong interaction or strong force is today understood to represent the interactions between quarks and gluons as detailed by the theory of quantum chromodynamics (QCD). The strong force is the fundamental force mediated by gluons, acting upon quarks, antiquarks, and the gluons themselves. Although the strong force only acts upon elementary particles directly, the force is observed between hadrons as the nuclear force. As has been shown by many failed free quark searches, the elementary particles affected are unobservable directly. This phenomenon is called confinement, a theory which allows only hadrons to be seen. History Note: Don t worry if you don t understand this, I barely understand this language either!!! Before the 97s, when protons and neutrons were thought to be fundamental particles, the phrase "strong force" was what is today known as the nuclear force or the residual strong force. What were being observed were the "residual" effects of the strong force, which act on hadrons, both baryons and mesons. This force was postulated to overcome the electric repulsion between protons in the nucleus, and for its strength (at short distances) it was dubbed the "strong force". After the discovery of quarks, scientists realized that the force was actually acting upon the quarks and gluons making up the protons, not the protons themselves. For some time after this realization, the older notion was referred to as the residual strong force, and the "new" strong interaction was called colour force. 3

4 Penetrating Power The penetrating abilities of the nuclear particles depends up the size of the particle, its energy and the density of the material being penetrated. For a given material, the order of penetrating ability is α < β - = β + < γ Shielding Certain materials are used as shielding because radio-particles do not penetrate them very well. These are usually very dense materials like lead or concrete. Types of Radioactive Decay Various radio-particles are given off by radioactive isotopes when the break apart to form more stable nuclei. The types of decay include: Alpha (α) emission U --> 23 9Th + 2He Beta (β ) emission 99 3 Tc --> 99 Ru + -e Gamma (γ) emission Positron (β + ) emission 6 C --> 5B + +e Electron capture

5 Nuclear Equations The basic principle in writing nuclear reactions The two sides of a nuclear reaction must have equal atomic numbers and atomic mass numbers Rn --> 28 8 Po + 2 He What is the missing particle? U --> 23 9 Th + Y Z X 37 2 Ca --> Y Z X + + e Y Z X --> 3 5 Xe + - e Rn --> Y Z X + 2 He 5

6 Natural Versus Induced Decay Natural decay results which an unstable nuclei disintegrates 99 3 Tc --> 99 Ru + -e Induced decay occurs when two particles are collided together in a nuclear reactor or particle accelerator 2 He + 7N --> 7 8O + H Radioactive Decay Sequence for U

7 Summary Nuclear Particles α or alpha particles β - or beta particles β + or positrons γ or gamma particles Nuclear decay reactions Natural Induced 7