Ch18 z7e nuclear

CHAPTER 18 Nuclear Chemistry 18.I Nuclear Stability & Radioactive Decay pp I

Black dots are stable nuclides. As A (atomic mass) increases, nº/p + ratio increases.

Subatomic Particles Protons - plus charge In the nucleus Neutrons - neutral Electrons - negative charge Outside the nucleus

Radiation Radiation comes from the nucleus of an atom. Unstable nucleus emits a particle or energy  alpha  beta  gamma

Types of Radiation Alpha particle (  ) helium nucleus paper 2+ Beta particle (  -) electron 1- lead Positron (  +) positron 1+ Gamma (  ) high-energy photon 0 concrete

Radiation Protection Shielding alpha – paper, clothing beta – lab coat, gloves gamma- lead, thick concrete Limit time exposed Keep distance from source

Nuclear Decay Alpha Emission Atomic & Mass Numbers must balance!! parent nuclide daughter nuclide alpha particle

Nuclear Decay Beta Emission Positron Emission electron positron

Nuclear Decay Electron Capture (of inner orbital electrons) Gamma Emission Usually follows other types of decay. Transmutation One element becomes another. electron

Gamma radiation No change in atomic or mass number 11 B 11 B + 0  5 5 0 boron atom in a high-energy state

Table 18.2 Types of Nuclear Processes p. 845

Learning Check Write the nuclear equation for the beta emitter Cobalt-60. . . 60 Co 60 Ni + 0 e 27 28 -1

Producing Radioactive Isotopes Bombardment of atoms produces radioisotopes = 60 = 60 59 Co + 1 n 56 Mn + 4 H e 27 0 25 2 = 27 = 27 cobalt neutron manganese alpha atom radioisotope particle

Learning Check NR2 What radioactive isotope is produced in the following bombardment of boron? 10 B + 4 He ? + 1 n 5 2 0

Solution NR2 What radioactive isotope is produced in the following bombardment of boron? 10 B + 4 He 13 N + 1 n 5 2 7 0 nitrogen radioisotope

Nuclear Decay pp Why nuclides decay… need stable ratio of neutrons to protons DECAY SERIES TRANSPARENCY

18.2 Kinetics of Radioactive Decay Rate of decay is a 1st order process, which is . . . ln(N/N 0 ) = -kt (memorize -- not on AP sheet) N 0 = original number of nuclides at t = 0 N = nuclides remaining at time t Half-life (t 1/2 ) = time for nuclides to reach half their original value. t 1/2 = 0.693/k

Half-life Half-life (t 1/2 ) Time required for half the atoms of a radioactive nuclide to decay. Shorter half-life = less stable.

Examples of Half-Life Isotope Half life C-15 2.4 sec Ra-224 3.6 days Ra-223 12 days I-125 60 days C-14 5700 years U-235 710 000 000 years

Learning Check NR3 The half life of Iodine-123 is 13 hr. How much of a 64 mg sample of Iodine-123 is left after 26 hours?

Solution NR3 t 1/2 = 13 hrs 26 hours = 2 x t 1/2 Amount initial = 64mg Amount remaining = 64 mg x 1/2 x 1/2 = 16 mg

Half-life m f : final mass m i : initial mass n : # of half-lives

Half-life pp Fluorine-21 has a half-life of 5.0 seconds. If you start with 25 g of fluorine-21, how many grams would remain after 60.0 s? GIVEN: T 1/2 = 5.0 s m i = 25 g m f = ? total time = 60.0 s n = 60.0s ÷ 5.0s =12 WORK : m f = m i (1/2) n m f = (25 g)(0.5) 12 m f = 0.0061 g

Kinetics of Nuclear Decay Problems pp The rate constant for 99 43 Tc = 1.16 x 10 -1 /h What is its half life? . . . t 1/2 = 0.693/k = 0.693/(1.16 x 10 -1 /h) = 5.98 h It will take 5.98 hours for a given sample of technetium-99 to decrease to half the original number of nuclides.

Kinetics of Nuclear Decay Problems pp How long for 87.5% of a sample of cobalt-60 to decay if t 1/2 = 5.26 years? Steps. . . What % is left? . . . 12.5% How many half-lives to get to this percent? 3. So, your answer to the problem is . . . 3 x 5.26 = 15.8 years.

Actual AP question: 1989 MC #68 pp If k = 0.023 min -1 how much of X was originally present if have 40. g after 60 min.? Your answer is . . . 160. g . Solution . . . t 1/2 = 0.693/k = 0.693/0.023 min -1 = 30 min. 60 minutes is 2 half-lives so going backwards 40. g to 80. g to 160. g.

18.3 Nuclear Transformations Transmutation - change of one element into another. Particle and linear accelerators are used to synthesize new elements (currently up to element number 119). Difficult to characterize the chemical properties because with some only a few atoms are formed with very short half-lives.

A representation of a Geiger-Müller counter.

18.4 Detection & Uses of Radioactivity Half-life measurements of radioactive elements are used to determine the age of an object Decay rate indicates amount of radioactive material EX : 14 C - up to 40,000 years 238 U and 40 K - over 300,000 years

Synthetic Elements Transuranium Elements elements with atomic #s above 92 synthetically produced in nuclear reactors and accelerators most decay very rapidly

Carbon-14 Dating You will have a test question like this! pp An ancient fire in an African cave has a 14 C decay rate of 3.1 cpm (cts per minute). If fresh wood has 13.6 cpm how old is the campfire if t 1/2 = 5730 years? Steps . . . Decay rates are directly proportional to nuclides so their ratio = N/N 0 What is the numerical ratio? Your answer . . . 3.1 cpm/13.6 cpm = 0.23 Use the two previous equations to solve (next slide).

Carbon-14 Dating You will have a test question like this! pp Ancient fire 14 C decay rate 3.1 cpm, fresh wood 13.6 cpm how old if t 1/2 = 5730 yrs ? 3.1 cpm/13.6 cpm = 0.23 = N/N 0 ln(N/N 0 ) = - k t and t 1/2 = 0.693/ k You want to solve for t (vs. t 1/2 ) so use t 1/2 to get k then plug into the 1st equation and solve for t. Your answer is . . . The campfire is 12 000 years old. ln( N/N 0 ) = ln( 0.23 ) = -(0.693/ 5730 ) t Se Ex 18.4A & 18.4B in Study Guide

Carbon-14 Dating You will have another test question like this ! pp A rock has ratio of Pb-206 to U-238 of 0.115. How old is it if t 1/2 of U-238 = 4.5 x 10 9 yrs ? Strategy: figure out N/N 0 of U-238, then use the 2 previous equations to get . . . 7.1 x 10 8 years . Calculations . . . Pb /U = 115/1000 so N 0 U238 = 1115, N = 1000 ln( 1000/1115 ) = -(0.693/ 4.5 x 10 9 ) t

Nuclear Medicine Radioisotope Tracers absorbed by specific organs and used to diagnose diseases Radiation Treatment larger doses are used to kill cancerous cells in targeted organs internal or external radiation source Radiation treatment using  -rays from cobalt-60.

Other Uses Food Irradiation  radiation is used to kill bacteria Radioactive Tracers explore chemical pathways trace water flow study plant growth, photosynthesis Consumer Products ionizing smoke detectors - 241 Am

18.5 Thermodynamic Stability of the Nucleus Mass Defect - difference from mass of an atom & the mass of its individual particles. 4.00260 amu 4.03298 amu

Nuclear Binding Energy Energy released when a nucleus is formed from nucleons. High binding energy = stable nucleus. E = mc 2 E: energy (J) m: mass defect ( kg ) c: speed of light (3.00 x 10 8 m/s)

Nuclear Binding Energy Unstable nuclides - radioactive & undergo radioactive decay. Elements with intermediate atomic masses ( e.g. , Fe) have greatest binding energy, so are the most stable .

18.6 Nuclear Fission and Nuclear Fusion Fission - splitting Fusion - Combining Both produce more stable nuclides so they are exothermic processes

A. Nuclear Fission Splitting a nucleus into two or more smaller nuclei 1 g of 235 U = 3 tons of coal

Nuclear Fission Fission large nuclei break up 235 U + 1 n 139 Ba + 94 Kr + 3 1 n + 92 0 56 36 0 Energy

Nuclear Power Fission Reactors Cooling Tower

Schematic of the reactor core.

Nuclear Power Fission Reactors

Fission chain reaction - self-propagating reaction critical mass - mass required to sustain a chain reaction

Nuclear Fusion combining of two nuclei to form one nucleus of larger mass thermonuclear reaction – requires temp of 40,000,000 K to sustain 1 g of fusion fuel = 20 tons of coal (vs. 3 in fission) occurs naturally in stars

Nuclear Fusion Fusion small nuclei combine 2 H + 3 H 4 He + 1 n + 1 1 2 0 Occurs in the sun and other stars Energy

Nuclear Power Fusion Reactors (not yet sustainable)

Nuclear Power Fusion Reactors (not yet sustainable) Tokomak Fusion Test Reactor Princeton University National Spherical Torus Experiment

Fission vs. Fusion pp 235 U is limited danger of meltdown toxic waste thermal pollution fuel is abundant no danger of meltdown no toxic waste not yet sustainable FISSION FUSION

Learning Check NR4 Indicate if each of the following are Fission (2) fusion Nucleus splits Large amounts of energy released Small nuclei form larger nuclei Hydrogen nuclei react Energy

Solution NR4 Indicate if each of the following are Fission (2) fusion 1 Nucleus splits 1 + 2 Large amounts of energy released 2 Small nuclei form larger nuclei 2 Hydrogen nuclei react

E. Nuclear Weapons Atomic Bomb chemical explosion is used to form a critical mass of 235 U or 239 Pu fission develops into an uncontrolled chain reaction Hydrogen Bomb chemical explosion  fission  fusion fusion increases the fission rate more powerful than the atomic bomb

18.7 Effects of Radiation pp Somatic - damage to the organism causing sickness or death. Genetic - damage to the genetic machinery causing birth defects.

Factors for Biological Effects of Radiation pp Energy - higher energy content (rads) causes more damage. Penetrating Ability -  >  - >  Ionizing Ability -  >  - >  (eating an  -particle producer like Pu is very deadly) Chemical Properties Kr-85 is chemically inert, passes through quickly Sr-90 collects in bone and stays a long time in the body.

Radioactive particles and rays vary greatly in penetrating power.

Diagram for the tentative plan for deep underground isolation of nuclear waste .

Ch18 z7e nuclear

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