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Nuclear Chemistry

Nuclear Chemistry. Particle. Where it’s found. What’s its charge?. What’s its mass?. What’s in an atom?. Proton. In nucleus. +1. ~ 1 amu. Neutron. In nucleus. 0. ~ 1 amu. Electron. Flying around nucleus. -1. ~ 1/1840 amu. Basic Terminology. Recall:

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Nuclear Chemistry

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  1. Nuclear Chemistry

  2. Particle Where it’s found What’s its charge? What’s its mass? What’s in an atom? Proton In nucleus +1 ~ 1 amu Neutron In nucleus 0 ~ 1 amu Electron Flying around nucleus -1 ~ 1/1840 amu

  3. Basic Terminology • Recall: • Z = atomic number = # of protons • N = number of neutrons • A = mass number = Z + N • Since each proton and neutron weighs roughly 1 amu, the mass of a nucleus (in amu) is roughly equal to its mass number.

  4. In the Nucleus • Most of the mass of the atom • VERY small volume • radius ~ 10-15 m • Number of protons determines identity • Number of neutrons in an element can vary • Isotopes: atoms with = protons but different numbers of neutrons • Average atomic mass = weighted average of the masses of all naturally-occurring isotopes.

  5. In the Nucleus • Electrostatic forces: protons repel each other • “Strong” nuclear forces: attract protons to neutrons • The balance between these two forces determines whether or not a given nucleus will be stable. Strong force explanation More on strong and weak forces

  6. Unstable Nuclei • wrong Z/N balance, radioactive decay • i.e. the atom will emit radiation. • Radioactive nuclei spontaneously decay . • The degree of unbalance determines the type of decay • Radioactive nuclei (parent nuclei) decay into daughter nuclei.

  7. Types of Radiation • Alpha Radiation • a positive charged particle is released • a helium nucleus • Beta Radiation • a negatively charged particle is released • an electron • Gamma Radiation • no particle is released • high energy waves are released

  8. Alpha ()Decay The nucleus is too largefor the strong force to hold it together, • an a-particle (24He nucleus) is emitted • Daughter nucleus has 2 fewer protons and 2 fewer neutrons than parent. • Alpha radiation is too weak to penetrate paper or skin • Nuclear equation: 92235U  24He +90231Th

  9. Beta ()Decay When a nucleus has too many neutrons, • a b-particle ( -10e) is emitted, a neutron in the nucleus splits into a p+ and an e- • The p+ stays in the nucleus. • The e- is ejected and called a b-particle. • Daughter nucleus has 1 more proton and 1 fewer neutron than parent. • Nuclear equation: • 614C --> 714N + -10e • Beta radiation is unable to penetrate aluminum foil or wood

  10. Gamma () Radiation • When a nucleus has too much energy, it can give off very high energy waves of light. • the nucleus is unchanged - it still has the same # of p+ and # of n0. • Nucleus goes from “excited state” to “ground state,” losing excess energy. • Gamma rays are often given off • with other types of radioactivity. • Gamma radiation can pass • through people

  11. Radiation Shielding

  12. Electron Capture When a nucleus has too few neutrons, • An electron falls into the nucleus • unites with a proton to form a neutron. • Electrons cascade in to fill in for the missing electron • Daughter nucleus has 1 more neutron and 1 fewer proton than parent. • Nuclear equation: • 69C + -10e --> 59B

  13. Chapter 18 Section2 Nuclear Change Gamma Rays Are Also Emitted in Positron Emission • Some nuclei that have too many protons can become stable by emitting positrons. • Positrons are the antiparticles of electrons. • The process is similar to electron capture in that a proton is changed into a neutron. • However, in positron emission, a proton emits a positron.

  14. When a proton changes into a neutron by emitting a positron, the mass number stays the same, but the atomic number decreases by one. • The positron is the opposite of an electron. • Unlike a beta particle, a positron seldom makes it into the surroundings. • Instead, the positron usually collides with an electron, its antiparticle.

  15. Any time a particle collides with its antiparticle, all of the masses of the two particles are converted entirely into electromagnetic energy or gamma rays. • This process is called annihilation of matter. • The gamma rays from electron-positron annihilation have a characteristic wavelength. • Therefore, these rays can be used to identify nuclei that decay by positron emission.

  16. Chapter 18 Section2 Nuclear Change Radioactive Decay, continued Stabilizing Nuclei by Converting Neutrons into Protons

  17. Radiation - Summary Problem Decay Type Symbol Resulting Daughter Nucleus is too large alpha a or 24He Atomic number -2 Mass number -4 Too many n0 in nucleus beta b- or -10e Atomic number +1 Mass number stays same Too much energy gamma emission g Same nucleus, just lower energy

  18. Radiation - Summary Problem Decay Type Symbol Resulting Daughter Too many Protons Positron emission b+or +10e Atomic number -1 Mass number stays same

  19. Stable Nuclei • For small isotopes N ≈ Z • e.g.: 16O is most stable isotope of O: • Z = N = 8 or N/Z=1 • For larger isotopes, N/Z is between 1 and 1.5 • e.g.: 208Pb is most stable isotope of Pb: • Z = 82; N = 126 or N/Z=1.5

  20. Practice Nuclear Equations

  21. Half-Life • amount of time it takes for half of a given sample to decay. • Each half-life, half of the sample decays and half remains. • Half lives vary from billionths of a second to billions of years.

  22. Half-Life: Equation Form • How do we put this into equation form? • Let t1/2 = the half-life of the isotope • Let A0 = the amount you start with • Let A(t) = the amount remaining at time t • Then t/t1/2 = the number of half-lives that have elapsed. A(t) = A0 (1/2)t/t1/2

  23. Nuclear Reactions - Fission • If nucleus too big - nuclear fission. • “fissions” (breaks up) into several smaller nuclei (and usually some extra neutrons as well). • not easily predicted. • Often initiated by absorbing a neutron. • Example: 92235U + 01n --> 56141Ba + 3692Kr + 3 01n

  24. Nuclear Energy - Fission • Energy can be harnessed. 92235U + 01n --> 56141Ba + 3692Kr + 3 01n The neutrons collide with other atoms of 235U, split, producing more neutrons . . . causing a chain reaction. • The energy given off can be harnessed to produce electricity (or, unfortunately, for more destructive purposes).

  25. Nuclear Energy - Fission

  26. Nuclear Weapons

  27. Nuclear Waste

  28. Nuclear Fusion • The opposite of nuclear fission is fusion, when smaller nuclei come together to form larger nuclei. • Example: 11H + 13H --> 24He • The fusion of hydrogen to form helium is the source of energy for the sun and many other stars.

  29. Nuclear Energy - Fusion • Release even more energy than fission. • emitted by stars (mostly hydrogen fusing to form helium). • no safe way yet to harness fusion • takes too much energy to get started

  30. Energy in Radioactive Decay • Radioactive Decay generally gives off large amounts of energy. • Where does it come from? • The answer lies in something called “mass defect.”

  31. Mass Defect • Law of Conservation of Mass • Mass cannot be created or destroyed. • Law of Conservation of Energy • Energy cannot be created or destroyed. • But, during nuclear reactions: • Mass can be converted into energy and energy can be converted into mass.

  32. Mass Defect • During nuclear reaction, some mass is either lost or gained. This change in mass is called the mass defect (Dm). • Dm = mass of products - mass of reactants • The relationship between the mass defect and the amount of energy given off or absorbed (DE) is DE = Dmc2 where c = the speed of light = 3.0 x 108 m/s.

  33. Mass Defect DE = Dmc2 • Example: If 0.01g of mass is converted into energy, 9 x 1011 J of energy is given off. That’s enough to heat 2.7 million liters of water from room temperature to boiling! • By comparison: you would need to burn 16 million grams of methane to give off the same amount of heat.

  34. Ten most common elements in the Milky Way

  35. Stellar Nucleosynthesis – the Stellar Metals H, He, Li H is exhausted, He is burned, fission creates Li, B, Be, C carbon burning white dwarfs more massive stars use nuclear fusion to create N, O, F, Ar, Na and Mg most massive stars use nuclear fusion to create Al, Si, P, S, Cl, Ar, K, Ca, Sc, Ti, V, Cr, Mn, Fe

  36. Stellar Nucleosynthesis – beyond Z=26 . . . • Most massive stars (12x our sun) • burn through Mg, Si etc, • only Fe core remains • implode from its own gravity • matter is crushed • p’s and e’s form n’s • explode during which • Fe nuclei gain n’s then decay into protons • new elements (thru 92) are formed

  37. Common Uses of Radioactivity • Food Irradiation • Archaeological Dating • Medical Detection • Medical Treatment

  38. Daily Exposure to Radiation • Cosmic rays • Radon gas • Smoke detectors

  39. Detecting Radiation • Geiger Counters • (beta) • Scintillation Counters • (alpha, beta and gamma) • Film • (beta, gamma)

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