The Atomic Nucleus and Radioactivity Or You’re so hot you’re Glowing Standards: 6D The atomic nucleus contains protons and neutrons. It is held together by the strong nuclear force which only acts between protons and neutrons when they are next to one another and touching. Different elements differ in the number of protons they have in their nuclei while isotopes have the same number of protons but different numbers of neutrons. 6E) Atomic nuclei with over 83 protons are unstable as well as lighter elements with large numbers of neutrons. These atoms decay into different elements in a process called transmutation. During this process alpha, beta and gamma particles may be ejected. These particles differ in how they are produced, their composition, electrical charge and energy. During transmutations charge and number of nucleons is conserved.

Rutherford’s Discovery of Atomic Nucleus (1911) Alpha source produced positive particles Most particles went straight through gold foil but some came straight back (direct hits) Conclusions Nucleus has net positive charge Nucleus is small relative to entire atom Most of atom is empty space Question: Diagram the experiment that Ernst Rutherford used to discover the nucleus of an atom. What observation did he make that indicated there was a nucleus? What charge is the nucleus? What is the relative size of the nucleus to the rest of the atom? Activities: Draw Rutherford’s experiment w/ gold foil on board. Describe how most of alpha particles went straight through, some were deflected slightly and a very few came back towards the source. Baseball at 50 yard line of Rose Bowl is scale of nucleus to entire atom

Nucleons Proton Neutron Neutral or no charge Mass of 1.675 x 10 -27 kg Positive charge of 1.6 x 10 -19 C (+1 acu) Mass of 1.673 x 10 -27 kg Number of protons =atomic number =z, determines element that atom belongs to Mass number = # protons + number of neutrons Neutral or no charge Mass of 1.675 x 10 -27 kg Isotopes have same # protons but different # of neutrons Isolated neutrons decay into an electron and proton in 10 minutes (Beta decay) Question: What particles make up an atomic nucleus? What are there masses and charges? What distinguishes the atoms of different elements from one another? How about atoms of different isotopes? Activities: Discuss how Chadwick in 1932 using a similar apparatus to Rutherford’s except he alpha particles impact beryllium which then released a neutron. The neutron was then made to hit parafin wax where it hit a hydrogen atom releasing a proton which a detector could then register. Make a chart for the two nucleons including their mass, charge and forces that act on them. Show transparency of periodic chart entry. Point out that number of protons in an atom is its atomic number or Z. Also is number of electrons in neutral atom of this element.Number of neutrons + protons is the atomic weight of an atom. Atomic weight is represented on periodic chart as decimal because number reflects average number of nucleons in naturally occuring sample of element. Number of neutrons = Atomic weight – Atomic Number Final factoid that lone neutron decays in 10 minutes to proton and ejects electron (beta particle)

Strong Nuclear Force The strong nuclear force holds the atomic nucleus together (opposes electric force) Attractive force between neutron and proton only. Thus bigger atoms need more neutrons. Only acts at distances the same size as one proton or neutron Strongest of all action at a distance or field forces Question: What forces act on the particles inside the atomic nuclei? Which of these is the strongest? Why don’t we notice this force at our scale? Activities: Make chart of forces acting in nucleus of atom including the particles they are between, their strength and the distance at which they act. Go to periodic chart and show how ratio of nucleons to protons starts out at one to one for lighter elements but quickly increases as move up chart.

Radioactive Decay The breaking up of an atomic nucleus because there is not enough strong force to hold it together Occurs for light element isotopes w/ a lot of neutrons Occurs for all elements over atomic number 83 Result of radioactive decay or transmutation is a new element. Charge is conserved Number nucleons is conserved Question: What causes an atom to radioactively decay? Which elements are suseptable to decay? What is the result of a transmutation and what quantities are conserved? Activities: Discuss how if SNF cannot hold atom together it will break apart and produce a new element. Occurs for atoms with lots of neutrons that may get isolated and for all elements over atomic number 83. Write simple transmutation equation for U-238 (92) to Th-234 (90) + alpha particle. Point out how charge and nucleons are conserved. Point out top number is number of nucleons, bottom is number protons.

Radioactive Decay Products Alpha Decay Helium nucleus is ejected from nucleus 2 protons and 2 neutrons give it a mass of 4 amu and a charge of +2 Low energy, stopped by skin or paper Beta Decay Electron is ejected when neutron decays leaving proton. Charge is -1 Medium energy, stopped by foil Gamma Decay EM photon released when nucleus reorganizes, zero charge High energy (stopped by 6 inches of lead) Question: Distinguish between an alpha, beta and gamma particle in terms of how they are produced, what they are made of, their charge, and their energy. Activity: Show transparency 93 of how radioactive decay products are distinguished by their charge using a magnetic field. Show transparency 94 showing the differing energies of the radioactive decay products. Make a chart of the radioactive decay products, how they are produced, what they are made of, their electrical charge and their energy.

The Three Cookie Problem As a final cruel physics test Mr. DeVoe gives you three radioactive cookies. One each with alpha, beta and gamma radiation. Which cookie do you eat? Which cookie do you smash on your hand? Which cookie do you put in your pocket?

Sources and Uses of Radioactivity Yearly dose of radio activity includes 56% from Earth and Cosmic Rays 42% from medical and dental x-rays 2.9% weapons test fall out 0.1% Nuclear Power Plant Operation Radioactivity used to trace materials inside and outside human body Cancer cell DNA found to be more susceptible to radiation than healthy cells => radiation therapy Rate of decay unaffected by temp., pressure, physical or chemical changes. Can be used to see how old material is if know how much of radioactive element existed originally. Question: What are some sources of radiation in our environment? What are some uses of radiation that benefit us? Activities: Show the radiation exposure chart transparency and comment on how over half of our yearly exposure is natural and something our bodies have adapted to. Then point out a few ways we increase our exposure like multiple x rays, airplane flights. Point out how little nuclear power plants contribute. Discuss barium swallow, tagging liquid flow and fertilizer to see how it moves in human body and environment. Discuss discovery that Cancer DNA is more susceptible to radiation than healthy cells lead to radiation therapy for cancer patients. Loma Linda has proton accelerator which is highly effective for specifically eye and prostate therapy.

Radioactive Decay Products Composition and Process Charge and Mass Energy Alpha Particle Helium Nucleus made of 2 protons and neutrons ejected from nucleus Charge of +2 acu Mass of 4 amu Low Can be stopped by skin or paper Beta Particle High speed electron ejected from nucleus when neutron decays leaving proton Charge of -1 acu Mass very close to zero Medium Can be stopped by aluminum foil Gamma Particle High frequency photon emitted when nucleons reorganize No charge Mass = 0 High Need 6 inches of lead to stop

Radioactive Dating Half Life = time for half of radioactive element to decay into its daughter elements 1 half life, 50% parent, 50% daughter, 1:1 ratio 2 half lives, 25% parent, 75% daughter 1:3 ratio 3 half lives, 12.5% parent, 87.5% daughter 1:7 ratio Common radio element half lives Carbon 14 5780 years Uranium 238 4.6 Billion Years