Chapter 25

Chapter 25Nuclear Chemistry11Nuclear chem the study of the structure of atomic nuclei and the changes they undergo. Nuclear Radiation2No e- / orbitals No e- sharing or transferringNo cpds formedNo bondings2In 1895, Roentgen found that invisible rays were emitted when e- bombarded the surface of certain materials. The Discovery of Radioactivitythey caused photographic plates to darken.

named these invisible high-energy emissions X rays. 33At that time, French physicist Becquerel was studying minerals that

emit light after being exposed to sunlight (phosphorescence). The Discovery of RadioactivityBuilding on Roentgens work, Becquerel wanted to determine whether phosphorescent minerals also emitted X rays. 44The Discovery of Radioactivity

Becquerel accidentally discovered that phosphorescent U salts even when not exposed to light produced spontaneous emissions that darkened photographic plates. 55Marie Curie (18671934) and her husband Pierre (18591906) took Becquerels mineral sample and isolated the components emitting the rays. The Discovery of Radioactivity

66The Discovery of RadioactivityConclusion: the darkening of the photographic plates was due to rays emitted specifically from the U atoms in the mineral sample. named the process by which materials give off such rays radioactivity; the rays and particles emitted by a radioactive source are called radiation. 77Isotopes are atoms of the same element that have different #s of n0. Types of RadiationRadioisotopesIsotopes of atoms with unstable nucleiready to emit radiations. unstable nuclei is due to big diffce in the # of p+ and n0. e.g. U has 92 p+ and over 140 n0Mg has 12 p+ and 12 n0 (stable)88Quick writeWhat kind of atoms are radioactive?99Types of Radiationradioactive decayA process that unstable nuclei emit radiation to attain more stable atomic configns.During radioactive decay, unstable atoms lose energy by emitting 1 of several types of radiation. 1010Types of Radiation3 common types of radiation alpha ()beta () gamma ()1111PropertyAlpha ()Beta ()Gamma ()compositionAlpha particlesBeta particleselectromagnetic radiations (waves)description4 (He nucleus) 0 (electron) 0 (photon)charge2+10Common sourceRa-266C-14Co-60Mass (a.m.u.)4 1 0Penetrating powerLow (0.05 mm body tissue)Moderate (4 mm body tissue)Very high shieldingPaper, clothingmetal foillead or concrete2-1018371212Quick WriteWhat are particles? How many p+, n0 and e- does each particle carry?1313

Radioactive Decay

Radioactive Decay unstable nuclei loses energy by emitting radiation: particles or energy (non-particles).

become lighter

Natural (not human initiated; cant stop or slow down)

not requiring any energy input.

Spontaneous

1414

1515Quick WriteWhat are the areas that decay and decay have in common? (4 bullet pts) 1616Radioactive Decay

1717Half-lifeHalf-lifeTime required for one half of the nuclei of a radioisotope sample to decay.

18 e.g. U Th + -particle

Each radioisotope has a characteristic t 1/218Half-lifeHalf-lifeTime required for one half of the nuclei of a radioisotope sample to decay.19 e.g. U Th + -particle

238U 234Th + 4He

Each radioisotope has a characteristic t 1/22909219IsotopeHalf lifeRadiation emittedC -145.73 x 10 3 yearsRn 2223.8 daysU-2384.46 x 109 yearsTh 2307.54 x 104 years, Half-life (t )2020

2121

2222

2323Effect of an Electric Field (1)What radiations are deflected toward the -ve plate? Why?

What radiations are deflected toward the +vely plate? Why? 2424Quick WriteWhat is radioactive decay? (5 bullet points)2525Quick WriteDescribe and explain the paths of , , and radiation under the influence of electric field?

2626Effect of an Electric Field (2)The +vely charged particles are deflected towards the -ve plate.

The vely charged particles are deflected towards the +ve plate.

the neutral radiation travels in a straight line. 2727Quick WriteWhat are particles? Do they carry any charges?2828Quick WriteWhat is radiation? What charge does it carry?2929Effect of an Electric Field (2) particles are deflected towards the +ve plate. particles undergo greater deflection because less mass.

3030Effect of an Electric Field (3) ray, (no electrical charge), are not deflected.

3131Types of Radiation- radiation 2+ due to the presence of the 2 p+.

An particle has the same composition as a He nucleus2 p+ and 2 n 3232Types of Radiation- radiation radiationa stream of particles. Ra-226, (88 p+ and 138 n0), undergoes decay by emitting an particle. 3333Types of Radiation- radiationAfter the decay, the resulting atom has an atomic # of 86, a mass # of 222. The new radiosiotope is Rn-222.

3434Types of RadiationThe particles involved are balanced.

i.e. the sum of the mass #s (superscripts) = the sum of the atomic #s (subscripts) on each side of the arrow.3535Types of RadiationBecause of their mass and charge, particles are relatively slow-moving compared with other Thus, particles are not very penetratinga single sheet of paper stops.

3636Types of Radiation Radiation A particle is a very-fast moving e- that has been emitted from a n0 of an unstable nucleus. particles are represented by the symbol . The 0 superscript indicates the insignificant mass of an e- in comparison with the mass of a nucleus.

3737

3838Types of Radiation RadiationThe 1 subscript denotes the -ve charge of the particle. radiation consists of a stream of fast-moving e-. 3939Types of RadiationAn example of the decay process is the decay of I-131 into Xe-131 by -particle emission.

4040Types of Radiation-- RadiationThe mass # of the product nucleus is the same as that of the original nucleus (both 131), but its atomic # has increased by 1 (54 instead of 53).

4141Types of Radiation-- RadiationThis change in atomic #, and thus, change in identity, occurs because the e- emitted during the decay has been removed from a n0, leaving behind a p+.

4242

Types of Radiation Radiation

radiation: fast-moving e- formed by decomposition of a n0 in an atom.Quick write: why the mass # remain 14 while there is an additional p+?4343Types of RadiationBecause particles are both lightweight and fast moving, they have greater penetrating power than particles.A thin metal foil is required to stop particles.

4444Types of RadiationBoth the subscript and superscript are 0. rays are high-energy (short wavelength) electromagnetic radiation. They are denoted by the symbol .

4545Types of RadiationThus, the emission of rays does not change the atomic # or mass # of a nucleus. rays almost always accompany and radiation, as they account for most of the energy loss that occurs as a nucleus decays. 4646Types of Radiatione.g. rays accompany the -decay rxn of U-238. The 2 in front of the symbol indicates that 2 rays of different frequencies are emitted.Because rays have no effect on mass # or atomic #, it is customary to omit them from nuclear eqns.

4747Radioactive DecayOf all the known isotopes, only about 17% are stable and dont decay spontaneously.4848Beta DecayA radioisotope that lies above the band of stability is unstable because it has too many n relative to its # of p+.

49e.g. unstable has a n0 / p+ ratio of 1.33 : 1, whereas stable elements of similar mass, such as and , have n0 / p+ ratios 1:1

49Beta DecayThe N-14 atom now has a stable n0 / p+ ratio of 1 : 1.

50

It is not surprising then that undergoes beta decay, as this type of decay decreases the # of n0 in the nucleus.

Note that the atomic # of the product nucleus, , has increased by 1. 50Beta DecayThus, emission has the effect of increasing the stability of a n0 -rich atom by lowering its n0 / p+ ratio. The resulting atom is closer to, if not within, the band of stability.5151Alpha DecayAll nuclei with more than 83 p+ are radioactive and decay spontaneously. Both the # of n0 and the # of p+ must be reduced in order to make these radioisotopes stable. These very heavy nuclei often decay by emitting particles. 5252Alpha Decaye.g. , Po-210 spontaneously decays by emission.

53The atomic # of decreases by 2 and the mass # decreases by 4 as the nucleus decays into .

53

5454Nuclear FissionNuclear FissionNuclei of certain isotopes (e.g U-238) are bombarded with n0 split into smaller fragments of similar sizes.55Unleash enormous amt of energy e.g. 1 kg of U- 235 (explosion of 20000 tons of dynamite)55

5656Nuclear Fission

5757Nuclear Fission

5858Nuclear Fission 59In a chain rxn

some of the n0 produced react with other fissionable atoms

producing more n0 which react with still more fissionable atoms.59

6060Nuclear Fission

A Nuclear Power Plant6161

6262Process of Enrichment

6363Nuclear FissionNeutron Moderation

a process that slows down n0 so the reactor fuel (U-235 or Po-239) captures them to continue the chain rxn.

6464Nuclear FissionNeutron Absorption a process that decreases the # of slow-moving n0.65Control rodsmade of a material such a Cd, or B, are used to absorb n0. water slow down the n0 in the reactor

65

6666Nuclear FissionNuclear FissionNuclei of certain isotopes are bombarded with neutrons

67split into smaller fragments of similar sizes.

Unleash enormous amt of energy

e.g. 1 kg of U- 235 ( explosion of 20000 tons of dynamite)67Nuclear FissionUncontrolled fissionTotal energy release is instantaneous (fraction of a second); uncontrolled chain rxnatomic bomb;

68Controlled fission so energy is released more slowly (nuclear power)

68

6969Controled/uncontroled fission

7070Fission and Fusion of Atomic NucleiThe sun is not actually burning (combustion). would have burned out approximately 2000 years after it was formed.

7171Nuclear FusionNuclear Fusion

Occur when small nuclei combine to produce a nucleus of greater mass

H nuclei (p+) fuse to make He nuclei

7272Nuclear Fusion much more energy than fission rxn

occur at very high temp (40, 000, 000 C)

good energy source--- cheap fuel; readily available; controlled7373Nuclear Fusionproblems; high temp to initiate? --- needs an atomic bomb to trigger a nuclear fusion rxn74**** the energy release per g of the material is much larger in nuclear fusion or fission rxns than in chem rxns.

change in mass (calculated by E = mc2 ) is small but significant in nuclear rxns.

74Quick-writeHow do fission rxns and fusion rxns differ?

25.37575Nuclear FusionFusion occurs when nuclei combine to produce a nucleus of greater mass. In solar fusion, H nuclei (p+) fuse to make He nuclei and 2 positrons.

7676Nuclear Fusion

7777Nuclear FusionFusion rxns small nuclei combine, release much more energy than fission rxns (large nuclei split).

7878Nuclear Fusion The use of controlled fusion as an energy source on Earth is appealing. The potential fuels are inexpensive and readily available.

25.379The problems with fusion in achieving the high temperatures necessary to start the rxn and in containing the rxn once it has started.

79Applications of Nuclear PowerCancer treatment (radiation therapy)high-energy X-rays are directed at a persons body to kill cancer cellsX-rays, MRI, CT scane.g. dentists office radiation therapyNuclear medicine Uses radionuclides in the diagnosis of disease Relies on process of radioactive decay

80Wilhelm Roentgen, 1895 x-rayed his wifes hand80CST example 1A 2-cm thick piece of cardboard placed over a radiation source would be most effect in protecting against which type of radiation?AalphaBbetaCgammaDx-ray8181CST problem 2Which equation correctly represents the alpha decay of polonium-214?A214Po 2140 84858282The End8383[InternetShortcut]URL=http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/radioa7.swf

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Two Types of Nuclear Chain Reactions

Concept Simulation - reenacts controlled and uncontrolled nuclear chain reactions.

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