Chapter 18 The Nucleus: A Chemist’s View All nuclides with 84 or more protons are unstable with respect to radio active decay. Light nuclides are stable when neutron/proton = 1. For heavier elements the neutron /proton ratio required for stability is greater than 1 and increases with Z. Nuclides with even numbers of protons and neutrons are more stable. Specific numbers of protons or neutrons (magic numbers) such as 2, 8, 20, 28, 50, 82, and 126 produce stable nuclides.

Figure 18.1 The Zone of Stability

Types of Radioactive Decay A nucleus will undergo decomposition to form a different nucleus which is known as radioactive decay. Alpha production (  ): helium nucleus, Beta production (  ): (mass number remains constant). Net effect is to change a neutron to a proton. (thorium) (protactinium)

Types of Radioactive Decay G amma ray production (  ): (high energy photon) Positron production: (particle with same mass as the electron but net effect is to change a proton to a neutron. Electron capture: (inner-orbital electron is captured by the nucleus)

Decay Series Sometimes a radioactive nucleus cannot reach a stable state through a single decay process. A radioactive nucleus reaches a stable state by a series of steps.

Figure 18.2 A Decay Series

Rate of Decay Rate of decay is the negative of the change in the number of nuclides per unit time, rate = -(  N/  t)  N Rate = -  N/  t = kN (k = proportionality constant) The rate of decay is proportional to the number of nuclides. This represents a first-order process. Integrated first-order rate law is: ln(N/N o ) = -kt where, N o = original number of nuclides (at t = 0) and N = number remaining at time t.

Half-Life...the time required for the number of nuclides to reach half the original value (N 0 /2). When, t = t 1/2, N = N o /2 ln(N/N o ) = -kt  ln[(N o /2)/N o ] = -kt 1/2 (half life is constant) If the half-life of a radioactive nuclide is known, the rate constant can be calculated.

Figure 18.3 The Decay of a 10.0g Sample of Strontium-90 Over Time

Nuclear Transformation The change of one element into another.

Figure 18.5 A Schematic Diagram of a Cyclotron

Figure 18.6 A Schematic Diagram of a Linear Accelerator

Detection of Radioactivity Geiger-Muler Counter: High energy particle from radioactive decay processes produce ions when they travel through matter. The probe of the Geiger counter is filled with Ar gas which can be ionized by a rapidly moving particle. high energy Ar (g) Ar + (g) + e - particle Electric device detect the current flow and the number of events can be counted. Thus the decay rate of the radioactive sample can be determined.

Figure 18.7 A Schematic Representation of a Geiger-Müller Counter

Detection of Radioactivity Scintillation Counter: Takes the advantage of the fact that certain substances, such as zinc sulfide, gives off light when they are struck by high energy radiation. A photocell senses the flashes of light that occur as the radiation strikes and thus measures the number of decay events per unit of time.

Energy and Mass When a system gains or loses energy it also gains or loses a quantity of mass.  E =  mc 2  m = mass defect  E = change in energy If  E =  (exothermic), mass is lost from the system.

Binding Energy...is the energy required to decompose the nucleus into its components. Iron-56 is the most stable nucleus, which has a binding energy per nucleon of 8.79 MeV.

Figure 18.9 The Binding Energy Per Nucleon as a Function of Mass Number

Nuclear Fission and Fusion Fusion: Combining two light nuclei to form a heavier, more stable nucleus. Fission: Splitting a heavy nucleus into two nuclei with smaller mass numbers.

Figure Both Fission and Fusion Produce More Stable Nuclides

Figure Fission

Figure Fission Produces a Chain Reaction

Fission Processes A self-sustaining fission process is called a chain reaction.

Figure Fission Produces Two Neutrons

Key Parts of a Fission Reactor Because of tremendous energies involved, the fission process can be used as an energy source to produce electricity. Reactors were designed in which controlled fission can occur. The resulting energy is used to heat water to produce steam to run turbine generators. Reactor Core: 3% + moderator and control rods. Coolant Containment Shell

Figure A Schematic Diagram of a Nuclear Power Plant

Figure A Schematic Diagram of a Reactor Core

Breeder Reactors Fissionable fuel is produced while the reactor runs ( is split, giving neutrons for the creation of ):

Biological Effects of Radiation Somatic damage: Damage to the organism itself Genetic damage: Damage to the genetic machinery. Biological effects depend on: 1.Energy of the radiation 2.Penetration ability of the radiation 3.Ionizing ability of the radiation 4.Chemical properties of the radiation source