Chapter 19 Radioactivity and Nuclear Chemistry

2 GOALS Types of radioactivity Identify radioactive nuclides Nuclear equations Binding energy; per nucleon; units Kinetics of radioactive decay

3 Facts About the Atomic Nucleus Every atom of an element has the same number of protons (+ve) atomic number (Z) Atoms of the same elements can have different numbers of neutrons (no charge) Isotopes: atoms of the same element, having same atomic number, Z, but different mass number, A (diff no. of neutrons). Isotopes are identified by their mass number (A) mass number = number of protons + neutrons

4 Facts About the Atomic Nucleus mass number = number of protons + neutrons neutrons = mass number – number of protons The nucleus of an isotope is called a nuclide Each nuclide is identified by a symbol

Nuclide Symbols Boron-10 ( 10 5 B) has 5 p and 5 n Boron-10 ( 10 5 B) has 5 p and 5 n Boron-11 ( 11 5 B) has 5 p and 6 n Boron-11 ( 11 5 B) has 5 p and 6 n 10 B 11 B Oxygen-16 ( 16 8 O) has 8 p and 8 n Oxygen-16 ( 16 8 O) has 8 p and 8 n Oxygen-17 ( 17 8 O) has 8 p and 9 n Oxygen-17 ( 17 8 O) has 8 p and 9 n Oxygen-18 ( 18 8 O) has 8 p and 10 n Oxygen-18 ( 18 8 O) has 8 p and 10 n

6 The Discovery of Radioactivity Becquerel discovered that certain minerals were constantly producing penetrating energy rays he called uranic rays (1896) Marie Curie discovered 2 new elements (Po, Ra) which also emitted uranic rays. Curie changed term uranic rays to radioactivity (present in elements other than uranium). Some nuclei are unstable; they emit particles and/or electromagnetic radiation spontaneously. This is radioactivity.

7 Types of Radioactive Rays Rutherford discovered there were 3 types of radioactivity; 2 additional types were later discovered. alpha (  ) & beta (  ) decay, gamma ray (  then positron emission, and electron capture. Another type of radioactivity ( nuclear transmutation ) results from the bombardment of nuclei (heavy) by neutrons, protons or other nuclei (lighter).

8 Penetrating Ability of Radioactive Rays    0.01 mm 1 mm 100 mm Pieces of lead

ParticleSymbolNuclear Symbol protonp+p+ neutronn0n0 electrone- alpha  beta ,  - positron ,  + 9 Important Atomic Symbols

10 Nuclear Equations nuclear processes are described using nuclear equations use the symbol of the nuclide to represent the nucleus atomic numbers and mass numbers are conserved use this to predict identity of daughter nuclide if parent and emitted particle are known emitted particle: product captured particle: reactant

11 Alpha Emission an  particle contains 2 protons and 2 neutrons most ionizing, but least penetrating loss of an alpha particle means atomic number decreases by 2 mass number decreases by 4

12 Beta Emission An unstable nucleus emits an electron when an atom loses a  particle its atomic number increases by 1 mass number remains the same in beta decay, a neutron changes into a proton

If californium-251 decays by successive α, α, β emissions, what nucleus is produced? a) b) c) d) e)

14 Gamma Emission gamma (  ) rays are high energy photons of light least ionizing, but most penetrating generally occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange

15 Positron Emission The positron has a charge of +1 and negligible mass anti-electron when an atom loses a positron from the nucleus, its mass number remains the same atomic number decreases by 1 A positron appears to result from a proton changing into a neutron

16 Electron Capture occurs when an inner orbital electron is pulled into the nucleus no particle emission, but atom changes same result as positron emission proton combines with the electron to make a neutron mass number stays the same atomic number decreases by one

17 Summary of Decay Processes (Table 19.1; pg 871) DecayEmission  At #  Mass #  -2-4inc -- +10dec  -ray 00-  0inc e-captX-ray0inc

18 Write the nuclear equation for positron emission from K-40 a) Write the nuclide symbols for both the starting radionuclide and the particle b) Set up the equation (emitted particles are products; captured particles are reactants) c) Determine the mass number and atomic number of the missing nuclide (mass and atomic numbers are conserved)

19 4) Determine the element from the atomic number Write the nuclear equation for positron emission from K-40 Q. In a decay series, U-238 emits 8 alpha particles and 6 beta particles. What nuclide is formed? Mass dec by 32; charge = +6 & -16

20 Write a nuclear equation for each of the following electron capture by Be-7 positron emission from N-13 beta emission from Ne-24 alpha emission from U-238

Stability of Nuclei stable isotopes fall in a very narrow range called the island of stability. - stable isotopes fall in a very narrow range called the island of stability.

22 What Causes Nuclei to Break Down? the particles in the nucleus are held together by a very strong attractive force found in the nucleus called the strong force acts only over very short distances the neutrons play an important role in stabilizing the nucleus, as they add to the strong force, but do not repel each other like the protons do

23 Neutron to Proton (N/Z) Ratio the ratio of neutrons : protons is an important measure of the stability of the nucleus if the N/Z ratio is too high (neutron rich) – neutrons are converted to protons via  decay if the N/Z ratio is too low (proton rich) – protons are converted to neutrons via positron emission or electron capture or via  decay – though not as efficient

24 Valley (Island) of Stability (Plot of # Neutrons vs # Protons) for Z = 1  20 (H - Ca), stable N/Z ≈ 1 for Z = 20  40, stable N/Z approaches 1.25 for Z = 40  80, stable N/Z approaches 1.5 Heavy nuclei: for Z > 83, there are no stable nuclei low N/Z

25 Determine the kind of radioactive decay that Mg-22 undergoes Mg-22 Z = 12 (protons) N = 22 – 12 = 10 (neutrons) N/Z = 10/12 = 0.83 from Z = 1  20, stable nuclei have N/Z ≈ 1 Mg-22 has low N/Z; it should convert 1 1 p into 1 0 n, therefore it will undergo positron emission or electron capture

26 Determine the kind of radioactive decay that N-18 undergoes N-18 Z = 7 (protons) N = 18 – 7 = 11 (neutrons) N/Z = 11/7 = 1.57 from Z = 1  20, stable nuclei have N/Z ≈ 1

27 Q. Which of the following will undergo beta decay? 16 O, 20 F, 13 N

28 Magic Numbers most stable when N or Z = 2 (He), 8 (O), 20 (Ca), 28 (Ni), 50 (Sn), 82 (Pb) besides the N/Z ratio, the numbers of protons and neutrons effects stability most stable nuclei have even numbers of protons and neutrons only a few have odd numbers of protons and neutrons if the total number of nucleons adds to a magic number, the nucleus is more stable (compare # electrons in noble gases)

Binding Energy, E b -. -All atoms are a little lighter than they are really supposed to be. Missing mass: ∆m = mass defect. -This missing mass is converted to energy, and released when 1 mole of atoms is formed from its subatomic particles (protons + neutrons + electrons). -Energy holds the nucleus together.

Calculating Binding Energy, E b E b is the energy required to separate the nucleus of an atom into protons, neutrons, electrons. For stability, E b > electrostatic repulsive forces between protons. In deuterium, 2 1 H 2 1 H  1 1 p nE b = 2.15  10 8 kJ/mol 2 1 H E b per mol nucleon = E b /2 nucleons = 1.08  10 8 kJ/mol nucleons = 1.08  10 8 kJ/mol nucleons Also, calc E b per nucleon (   nucleons)

Calculating Binding Energy, E b For deuterium, 2 1 H: 2 1 H  1 1 p n Actual mass of 2 1 H = g/mol (given or PT) Mass of proton = g/mol Mass of neutron = g/mol Theoretical mass = g/mol Mass defect (‘missing mass’) = – = g/mol = g/mol

Calculate Binding Energy, E b Mass defect = g/mol = (  1000) kg/mol = 2.39  kg/mol = 2.39  kg/mol From Einstein’s equation: E b = (∆m)c 2 = 2.39  kg  (3.00 × 10 8 m/s) 2 = 2.15 ×10 11 kg  m 2 /s 2 (but 1 kg  m 2 /s 2 = 1 J) = 2.15 ×10 11 kg  m 2 /s 2 (but 1 kg  m 2 /s 2 = 1 J) = 2.15  J/mol  1000 J = 2.15  10 8 kJ/mol = 2.15  J/mol  1000 J = 2.15  10 8 kJ/mol Two nucleons for deuterium, 2 1 H:  1 1 p n E b /mol nucleon = 1.08  10 8 kJ/mol nucleons

Calculating Binding Energy, E b For I-127, I: 53p + 74n (i.e. 127 nucleons) Actual mass of I = g/mol (given or PT) 53 protons = 53  g/mol = g/mol 74 neutrons = 74  g/mol = g/mol Theoretical mass defect = g/mol Mass defect = ( ) g/mol = g/mol = g/mol =  kg/mol =  kg/mol

Calculate Binding Energy, E b E b =  kg/mol  (3.00 × 10 8 m/s) 2 = 1.04 ×10 14 kg  m 2 /s 2 (but 1 kg  m 2 /s 2 = 1 J) = 1.04 ×10 14 kg  m 2 /s 2 (but 1 kg  m 2 /s 2 = 1 J) = 1.04 ×10 14 J/mol = 1.04 ×10 14 J/mol Also, can express E b in MeV: 1 MeV = × J E b /nucleon = ? MeV E b /mol nucleon = 1.04 ×10 14 J/ (127 nucleons) = 8.19 ×10 11 J = 8.19 ×10 11 J E b /nucleon = 8.19×10 11 J  (6.022 ×10 23 ) = 1.36 × J = 1.36 × J

35 Plot of E b vs Mass -the greater the binding energy per nucleon, the more stable the nucleus is

Nuclear Fission T he splitting of a heavy unstable nucleus of an atom into two or more fragments; Pu, U & Th! - induced reaction to produce energy! Energy released  16,800,000,000 kJ/mol (235 g Uranium)

Nuclear Fusion Free of long-lived radioactive waste. More destructive than fission bombs (WWII)! More difficult to achieve. Nuclei must travel at v. large KE’s at each other. Light nuclei fuse to generate heavier nuclei (more stable)

38 Kinetics of Radioactive Decay Rate = kN It is a first order process N = number of radioactive nuclei the shorter the half-life, the more nuclei decay every second (sample is hot!), the higher the rate

39 The half life of Pu-236 is 2.86 years. Starting with a 1.35 mg sample of Pu-236, calculate the mass that will remain after 5.00 years Concept Plan: Relationships: mass Pu-236 = 1.35 mg, t = 5.00 yr, t 1/2 = 2.86 yr mass, mg Given: Find: t 1/2 km 0, tmtmt +

40 Starting with a 1.35 mg sample of Pu-236, calculate the mass that will remain after 5.00 years units are correct, the magnitude makes sense since it is less than ½ the original mass for longer than 1 half-life Check: Solve: t 1/2 km 0, tmtmt +

41 An ancient skull gives 4.50 dis/min∙gC. If a living organism gives 15.3 dis/min∙gC, how old is the skull? 14 C-t 1/2 = 5730 yr dis = disintegrations Solve: Concept Plan: Relationships: rate t = 4.50 dis/min∙gC, rate t = 15.3 dis/min∙gC time, yr Given: Find: t 1/2 krate 0, rate t t +

42 An ancient skull gives 4.50 dis/min∙gC. If a living organism gives 15.3 dis/min∙gC, how old is the skull? 14 C-t 1/2 = 5730 yr units are correct, the magnitude makes sense since it is less than 2 half-lives Check: Solve:

43 An artifact containing carbon taken from the tomb of a king of ancient Egypt gave 8.1 dpm/gC. How old is the artifact? Carbon from a living organism gives 15.3 dis/min∙gC; 14 C-t 1/2 = 5730 yr. dis = disintegrations

44 bombardment of one nucleus with another ( 2 H, 4 He, 10 B, 12 C) causing new atoms to be made can also bombard with neutrons; protons reaction done in a particle accelerator linear cyclotron Tc-97 is made by bombarding Mo-96 with deuterium, releasing a neutron Joliot-Curies Artificial Nuclear Reactions

An example of a n,  reaction is production of radioactive 31 P for use in studies of P uptake in the body P n  P +  Reactions using neutrons are called n,  reactions because a  ray is usually emitted. Radioisotopes used in medicine are often made by n,  reactions.

Transuranium Elements Elements beyond 92 (transuranium) made starting with an n,  reaction U n  U +  U  Np  Np  Pu  Np  Pu 

47 Q. 56 Fe when bombarded with deuterium, produces 54 Mn and one other particle. Write a balanced equation for the reaction & identify the other particle He H  Mn + ?

48 Medical Uses of Radioisotopes

49 Nonmedical Uses of Radioactive Isotopes smoke detectors Am-241 smoke blocks ionized air, breaks circuit insect control sterilize males food preservation radioactive tracers follow progress of a “tagged” atom in a reaction

50 authenticating art object many older pigments and ceramics were made from minerals with small amounts of radioisotopes crime scene investigation measure thickness or condition of industrial materials corrosion track flow through process gauges in high temp processes weld defects in pipelines road thickness Nonmedical Uses of Radioactive Isotopes