Energy and Radioactivity Time to Wrap This Energy Unit Up! We have just finished discussing energy and its relation to atomic structure as well as its relation to an atom’s ability to bond What’s interesting is much of our understanding of atomic structure came from studies of radioactive elements So, what is radioactivity? To begin answering this question, we must first revisit the structure of the nucleus of the atom The Nucleus Remember that the nucleus is comprised of the two nucleons, protons and neutrons The number of protons is the atomic number The number of protons and neutrons is the mass number This is because the number of protons and neutrons together effectively accounts for entire mass of the atom To help us understand nuclear structure, let’s use some magnets! Time for the Magnet Demonstration! Answer Question #1 on Page ______ with Your Neighbor! The Magnet Demonstration and Its Significance to the Nucleus • Any element with more than one proton (all but hydrogen) will have repulsions between the protons in the nucleus • The magnets represented the protons in the demonstration • A strong nuclear force helps keep the nucleus from flying apart • This force was represented as the tape in the magnet demonstration • The strong nuclear force It is essentially the “glue” that holds the protons and neutrons together • It is stronger that the repulsive forces that exist between two protons or other like-charges • However, it has a very short range – it can’t even reach from one end of a fair-sized atomic nucleus to the other • If the balance between the electrical force and strong nuclear force gets upset and the electrical force wins, some part of the nucleus can go shooting off! • Radioactivity! Isotopes and Radioactivity So, what helps balance the electrical force and the strong nuclear force interactions? Come to find out, neutrons play a key role stabilizing the nucleus Represented as the marble in the magnet demonstration The ratio of neutrons to protons is key to determine the stability of a nucleus Let’s Investigate! Turn to Page ______ and Answer the Questions with a Partner! Neutron-Proton Ratios As you saw in the “Belt of Stability” graph, it takes a greater number of neutrons to stabilize the nucleus as nuclei get larger This is because repulsions between protons become much greater as atomic number increases, and more neutrons are required to hold them together Neutron-Proton Ratios For smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1 Magic Numbers in the Nucleus As we just saw, some combinations of protons and neutrons appear to be more stable than others Therefore, there must be ways to predict nuclear stability One concept involves the idea of magic numbers Magic numbers are natural occurrences in isotopes and are stable Proton magic numbers - 2, 8, 20, 28, 40, 50, 82, 114 Neutron magic numbers - 2, 8, 20, 28, 40, 50, 82, 126, 184 The existence of magic numbers indicates the existence of energy levels in the nucleus, similar to those observed for electrons outside the nucleus! Referred to as the “Shell Model of the Nucleus” The magic numbers correspond to closed shells in nuclei! Predicting Nuclear Stability Another way to predict nuclear stability is based on whether the nucleus contains odd/even number of protons and neutrons Nuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons Radioactivity and Nuclear Transformations If a nucleus does not follow either one of these patterns, it is referred to as being unstable or radioactive Called radioisotopes Beyond element 82, all nuclei are radioactive because there is no amount of neutrons that is strong enough to hold the nucleus together in a stable manner So, what happens when a nucleus is unstable? These radioisotopes gain stability by undergoing radioactive decay Radioactive decay can occur by emitting mass, charge, or energy Note, nuclear reactions can not be speeded up, slowed down, or stopped – they will always occur! Discovery of Radioactivity Radioactivity was first discovered by Marie Curie and Henri Becquerel Discovery of Radioactivity 1852 - 1908 1867 - 1934 Nuclear Reactions The chemical properties of the nucleus are independent of the state of chemical combination of the atom In other words, when writing nuclear equations, it makes no difference if the atom is as an element or a compound However, mass and charges MUST BE BALANCED!!! Types of Radioactive Decay – Positron Emission This type of decay occurs when an atom doesn’t have enough neutrons to be stable A proton turns into a neutron and loses charge As a result, a positron (a particle that’s just like an electron except that it has opposite electric charge) is emitted The atomic number decreases by one, but the mass number remains the same Types of Radioactive Decay – Electron Capture This type of decay is viewed as an electron being pulled into the nucleus from the innermost electron orbit (energy level) As a result, a proton is transformed into a neutron! 1 1 p + 0 −1 e 1 0 n Competes with positron emission This decay is a mode of decreasing atomic number while maintaining mass number Types of Radioactive Decay – Alpha Decay This type of decay involves the loss of an α-particle An α-particle is essentially a particle containing 2 protons, 2 neutrons, and a +2 charge A charged helium atom! 4 2 Why helium? Helium is held together exceptionally tight 2+ He Makes alpha particles the easiest type of “clump” to spit out Alpha particles are attracted to the negative plate when passed through an electric field In this type of decay, the mass number decreases by four and the atomic number decreases by two This type of emission is low energy, but most massive! Example - Alpha Decay of Uranium Uranium decays into thorium and an alpha particle The equation for the nuclear decay is balanced, with the sum of mass and atomic numbers being equal on both sides of the equation Sum of atomic numbers: 92 = 90 + 2 Sum of mass numbers: 238 = 234 + 4 Types of Radioactive Decay – Beta Radiation This type of decay is essentially a charge decay The emitted particle is a result from the breaking up of a neutron When a neutron decays, it produces a proton (stays in nucleus) and a beta particle (ejected electron) The beta particle is essentially a fast-moving electron The beta particle is higher energy than alpha particles It is attracted to the positive plate when passed through an electric field beta decay of uranium Types of Radioactive Decay – Gamma Emission After alpha or beta decay, a nucleus is often left in an excited state (with some extra energy) It then “calms down” by releasing this energy in the form of a high energy photon Called a -ray The equation for the nuclear decay is balanced, with the sum of mass and atomic numbers being equal on both sides of the equation This type of decay is not affected by an electric field Penetration of Various Radioactive Decays The Belt of Stability and Predicting Type of Decay The shaded region in the figure shows what nuclides would be stable, the socalled belt of stability Stable Nuclei Nuclei above this belt have too many neutrons They tend to decay by emitting beta particles This decreases the neutrons and increases the number of protons Stable Nuclei Nuclei below the belt have too many protons They tend to become more stable by positron emission or electron capture Both lower the number of protons! Positron emission is more common in lighter nuclei whereas electron capture is more common for heavier nuclei Stable Nuclei As stated earlier, there are no stable nuclei with an atomic number greater than 83 These nuclei tend to decay by alpha emission Radioactive Series Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation They undergo a series of decays until they form a stable nuclide (often a nuclide of lead) Energy Changes in Nuclear Reactions The nuclear binding energy is an energy required to break up a nucleus into its components In essence, it is a quantitative measure of the nuclear stability Remember, the nucleus contains mainly two particles – protons and neutrons Thus, the mass of the nucleus primarily comes from the masses of protons and neutrons! But, experiments have known that the sum of the masses of protons and neutrons is always greater than experimentally determined nuclear mass Wait…what?! The Nucleus and Nuclear Binding Energy To answer this question, we must look at the way nature creates the nucleus Protons and neutrons are bound together and placed in a tiny space called the nucleus In order to bind protons and neutrons together, some energy is needed, which is taken out of the masses of protons and neutrons Called the mass defect Hence, the concept of nuclear binding energy is based on Einstein’s famous equation, to which energy and mass are interconvertible: E = mc2 Einstein and Nuclear Binding Energy A more useful version of the equation is: ∆E = ∆mc2 where: ∆E = the binding energy ∆m = mass difference between the nucleus and the separate nucleons We can use this relationship to determine how much energy is produced by a decrease in mass Binding Energy We can calculate the binding energy per nucleon for all of the stable isotopes to compare their relative stabilities. We end up with the following plot.: Relative binding Energy per nucleon Fe 56 Most stable Mass number Binding Energy As the total number of nucleons increases, we reach a point where the binding energy is at a maximum Hence the reason why higher mass nucleons are less stable! This is why we can obtain energy from both fission and fusion and why alpha emission is common for heavier isotopes! Max. Binding Energy Fusion Fission Fe Getting Power from Nuclear Reactions Nuclear fusion - the direct combination of 2 radioactive atoms to produce an atom of larger masses Requires high temperature to speed up molecules Nuclei must be forced extremely close together This is how the Sun gets energy: 4 1H 1 2He 4 + 2 1e 0 4 hydrogen 1 helium + 2 positrons atoms atom Nuclear fission - splits an atom Some mass is converted into energy Nuclear reactors are designed to control the fission process via a critical chain reaction What are Chain Reactions? Critical Reaction When just enough fissions occur to keep the chain reaction going (neutrons formed = neutrons used) Creates useful nuclear power • Supercritical Reaction • When excess neutrons are produced and the rate of fission keeps increasing at an uncontrolled rate • Creates nuclear bombs Nuclear Reactions Fission and Fusion Inside a Nuclear Reactor Inside a Nuclear Reactor Inside Chernobyl The Rates of Nuclear Reactions Although we can predict the methods by which an isotope may undergo radioactive decay to become more stable, we can not predict how quickly these changes will occur This brings us to our next unit of study – chemical kinetics What is the rate at which a chemical reaction will occur?