Unit II.  Nuclear Chemistry

Reading Assignment 1: Read & KTU Ch. 25.1-2. Answer questions 1-16

 I. Chemical reactions vs. nuclear reactions.

   A. Chemical Reactions

•  Matter undergoes a change in composition where atoms are rearranged by breaking and making chemical bond

• These reactions are affected by temperature, pressure, catalysts and concentrations of reactants.

• The type and number of atoms are conserved in a normal chemical reaction (Law of Conservation of Matter):

• If the matter is conserved, then mass is conserved which means -- Mass of Reactants = Mass of Products

   B. Nuclear reactions.

• Matter undergoes a transformation where new matter is made.

• Atoms (and mass) are not conserved.

II. Radioactivity

   A. Nucleons-  The components of the nucleus.  Protons (¹₁p⁺⁾ and Neutrons (¹₀n⁰)

1. atomic number (Z)- the number of protons

2. mass number (A)- the number of nucleons in an atom (protons + neutrons)

3. isotopes- atoms of the same elements that differ by the number of neutrons

a. nuclide- the nucleus of specific isotope of a certain element

4. radioisotope- an isotope that contains an unstable nuclide.

a. radionuclide- the unstable nucleus of a radioisotope

Video:  Radioactivity-Expect the Unexpected

Resource: The story of how radioactivity was discovered -- timeline

Resource: The theory behind radioactivity and nuclear stability

   B. Energy & mass-- Mass defect

        Question:  What comprises the helium atom?

• 2 protons (@ 1.0073 a.m.u), 2 neutrons (@1.0087 a.m.u.) and 2 electrons (@ 0.00055 a.m.u)

• The pieces of the helium atom add up to 4.0331 a.m.u. --Isotopes of Helium

• But according to the periodic table, helium has a mass of 4.0026.  Where does the other 0.0305 a.m.u. go?

1. Mass Defect- the difference in masses between the atom and the sum of the atom's components.

2. Nuclear Binding Energy- the energy associated with the mass defect defined by Einstein's equation  (E = mc²)

Example: Calculate the binding energy for the Helium atom  (42He)	Total mass doesn't include electron masses but the total mass defect is the same.
1. Calculate the mass defect
4.0026 a.m.u. - 4.0331 a.m.u. = -0.0305 a.m.u.
2. Calculate the nuclear binding energy from E = mc2 ( 1 a.m.u. = 1.6605 x 10-27 kg)
E = (-0.0305 a.m.u.* 1.6605 x 10-27 kg/ 1 a.m.u.) * (3.00 x 108 m/s)2         = -4.55 x 10-12 kg m2/s2   What is a kg m2/s2?   It is the same thing as a joule (unit for energy)   -4.55 x 10-12 J.  This doesn't seem like much energy (basically 4 trillioniths of a joule).    But remember this is for one atom.  But what about many atoms?   4.0026 grams of Helium contains approximately 6.022 x 1023 atoms (This is called Avogadros number: see mole)   So,  -4.55 x 10-12 J/atom  * 6.022 x 1023 atoms  =  -2.74 x 1012 J   (This is over 2 trillion joules for just 4 grams)   ex. 1000 ton meteorite releases 5 x 1013 J. (1 kiloton TNT = 4.2 x 1012 J, Hiroshima atomic bomb=6.3 x 1013 J)

    C. Types of Radioactive particles

All radioactive decay reactions can be thought of as an unstable parent nuclide decaying into a daughter nuclide.

1. Alpha Particles. High speed helium nuclei. written as  ⁴₂He or ⁴₂a.

Associated with heavy isotope decay (N > 83 and A >= 200)

     ex.  ²¹²₈₄Po ----> ²⁰⁸₈₂Pb  +  ⁴₂a  

Polonium was the first radioactive element found.  Discovered by Marie Curie and her husband Pierre in 1898.

²⁰⁸₈₂Pb is the most abundant isotope of lead (~52.4%)

The alpha decay of ²⁴¹Am (americium-241) to form ²³⁷Np (neptunium-237)

2. Beta Particles. High speed leptons (electrons); written as  ⁰_-1e^- or b 

Associated with neutron decay.  ¹₀n ---> ¹₁p  + ⁰_-1e^-

ex.   ⁹⁸₄₃Tc  ---> ⁹⁸₄₄Ru  +  ⁰_-1b^-

Technetium  is a radioactive element that does not occur naturally but instead was artificially prepared in 1937

 Tritium (³₁H) decaying into ³₂He

Question: What is Tritium? Click here to read about it.

Resource: Rutherford's discovery of the alpha and beta particles

Practice: Exercises in writing alpha and beta decay equations.

3. Gamma radiation.  Electromagnetic radiation with high frequency and high energy. written as g

     Usually accompanies all radioactive emissions.  Represent lost energy. 

     An interesting gamma emission.  The annihilation of an electron and a positron forms gamma radiation:

⁰_-1e^-  +   ⁰₊₁e ----> 2g   ( 0.511 MeV or 8.187 x 10^-14 J)   ( 1 Megaelectron volt = 1.602189 x 10^-13 J)

4. Positron emission-  Particle that is similar to an electron but with a positive charge.  written as ⁰₊₁e or b⁺

     Positron emission essentially converts a proton into a neutron.  ¹₁p ---->  ¹₀n  +  ⁰₊₁e

ex. Carbon-11 decay.      ¹¹₆C --->  ¹¹₅B +  ⁰₊₁e.  

Resource: Positron annihilation studies at the University of Bristol, UK.

5. Electron capture- The nucleus captures an n=1 electron.  

     Electron capture produces an effect similar to positron emission, converting a proton to a neutron.¹₁p +  ⁰_-1e^- ---->  ¹₀n  

ex. ⁷₄Be  +  ⁰_-1e^-  --->  ⁷₃Li.  

Practice: Exercises in writing positron emission and electron capture equations

   D. Patterns of Stability

      1. Neutron-to-Proton ratios

       Nucleons are held together by Strong Nuclear Forces.  

       The stability of the nucleus is dependent upon the neutron-to-proton ratio. As Z increases the number of neutrons needed also increases but not in a linear relationship.

a. Belt of Stability.  A region on a neutron to proton graph that represents stable nuclides

Notes to Consider:

1. All nuclides with Z>83 are unstable

2. Z values lower than 20 have neutron/proton = 1

3. As Z increases above 20, the neutron/proton ratio increases in stable isotopes.  ex. ⁹⁰Zr = 1.25, ¹²⁰Sn = 1.4, ²⁰⁰Hg = 1.5

4. Region above the belt represents excess neutrons

5. Region below the belt represents excess protons

Predicting decay based on neutron-to-proton ratio   

Notes to consider:

1. Above the Belt: Represents high neutron/proton.  Seen as Beta Emission. Increases Z and A remains same

2. Right of the Belt  Represents Z>83.  Alpha particles emitted to reduce both A and Z

3. Below the Belt:  Represents low neutron/proton.  Positron emission or electron capture. Decreases Z and A remains same

Assignment 1: Radioactive decay equations worksheet

   E. Rates of Radioactive Decay

       1. Half-Life:  The time to decay 1/2 of a sample of radioactive nuclides into their stable daughter nuclides

a. Decay rates are measured in disintegrations per time. Also known as a sample's activity

Video: Half life (Bill Nye)

Decay rates & determining ages
R  =  k N	R is activity (disintegrations per time), k is a decay constant, and N is the number of nuclides.
	N = number of undecayed nuclei after time (T) N0 = number of original radioactive nuclei T = time which decay has occurred t1/2 = half-life of specific isotope

Applet: Identifying the rate of decay.

Practice: Looking at decay rates

Resource: List of radioactive isotopes & half-lives -- by half-lives

Resource: Law of Radioactive Decay.  Relates the amount of decayed radionuclide to undecayed radionuclide

b. Radioactive Dating 

A comparison of radioactive nuclei to the stable daughter nuclei in an artifact can predict the age.

Comparing the ratio of radioactive to stable nuclei in the same and then in the environment, scientists can infer the age by determining the number of half-life disintegrations. 

Resource: What is carbon-14 and how is it produced?

Resource: What is Carbon-dating? Resource: Radioactive Decay Calculator

Table:  Naturally occurring isotopes and half-lives.

Assignment 2 : Small-Scale lab, pg 887

b. Transmutation reactions.

Creating unstable nuclei from stable nuclei through bombardment of neutrons or other nuclei

The first transmutation was performed by Ernest Rutherford in 1919. Bombarding ¹⁴N with alpha particles

¹⁴₇N  + ⁴₂He  ---> ¹⁷₈O  +  ¹₁H

                         ** Both the oxygen-17 and hydrogen-1 are stable so there is no further decay.

c. Nuclear decay series.  Nuclides with Z much larger than 83 cannot decay to stable nuclides with one emission.  This usually requires multiples steps representing multiple decay emissions.

Applet: An applet showing decay series of some transuranium elements

 Resource: What are the transuranium elements?

Resource: A historic film showing transuranium elements. Hosted by Glenn Seaborg

reading asssignment 2: Read & KTU Ch. 25.3-25.4, answer questions 18-33

III. Nuclear Fission and Fusion

    1. Nuclear Fission. The splitting of heavy nuclei that results in the release of energy.

     -Critical mass can lead to a nuclear chain reaction

Video: Modern Marvels--The Manhattan Project

2. Nuclear Fusion.  The union of nuclides forming larger nuclide and the release of energy

Resource: Video tutorial of Fission and Fusion and the applications.

Resource: Nuclear Fusion in the Sun

Assignment 3: Standardized Test Prep pg. 905, 1-17

Resource: Common uses for radioisotopes --  The regulation & use of radioisotopes

 References:  The types of decay diagrams are adapted from Thinkquest