Week 6 Sg

Chemistry 6C Section: C01

Instructor: Dr. Carl Hoeger TA: Matthew Snedaker [email protected] OH: Monday 8-10pm @ York 4020B

If you need help, then visit the CHEM Help Room in YORK 4020. Midterm 2: 5/25/07 (7-8:50pm) @ PETER 108 Quiz 3: 6/4/07 Final Exam: 6/11/07 (11:30am -2:30pm) @ a top secret location (TBA). Problem Notebook Bring your problem notebook to every discussion section and come prepared with the necessary problems completed.

This study guide will highlight some of the main concepts from lecture. Do not use it as your only study tool. The material featured is representative of the material covered up to the lecture from the previous week (the Friday before our discussion). WEEK 6- Nuclear Chemistry: the study of radioactivity, the changes that nuclei undergo, and the chemical consequences of those changes. Nuclear decay is the process of partial break up of the nucleus towards a more stable one. Parent  daughter nuclei •It is a nuclear transmutation if the daughter is a different element. A nuclear equation is used to express reactions. •Check that the nuclear equation is correct by making sure that the sum of A’s and the sum of Z’s are equal on both sides. Types of nuclear decay: •Alpha decay occurs primarily for unstable nuclei with a proton number greater than 83 (Bismuth).

•Beta decay occurs mostly with lighter elements. •Positron emission is where an anti-electron ( +10 e ≡ β + ) is ejected from the nucleus. --when an electron and a positron meet annihilation.

1 •Neutron emission is less common and it is where a neutron is ejected from the nucleus ( 0 n ). •Electron Capture is where an electron collides with the nucleus; thus, the daughter nuclei should decrease in atomic number.

Take a minute to think about each of these decays and how they would affect a parent nuclei. Do you understand how to write nuclear equations and where each of the above particles belongs? What does it mean to bombard a nucleus with a particle (e.g. alpha, beta, neutron, etc.)? What does it mean if a particle is ejected? In a nuclear equation, where would an ejected particle go (i.e. left or right side)? Make sure that you understand the language used in the homework problems and that you can interpret the language into nuclear equations. Spontaneous fission: For atoms with more protons than 92, the nucleus splits into two separate nuclei. •A variety of product nuclei may be formed. Characteristics of nuclear radiation

Induced Nuclear Reactions

If you collide a target nucleus with a projectile, then you can yield nuclei with higher atomic numbers. The projectile can be any of the particles discussed before or they can be large particles (heavy ions). The higher the atomic number of the target, the more positively charged the nucleus; thus, a greater kinetic energy is needed to penetrate the target nucleus. But if the KE is too great  fission. How do you prevent this? Predictors of Nuclear Stability •Familiarize yourself with the band of stability on page 654.

•Look at the ratio of Z to N (# protons: # neutrons). N:Z ~ 1:1 ~ stable for Z< 20.

•For Z>20, stable nuclei have ratio between 1 and 1.52. •For Z>83, there are no known stable isotopes. •Make sure you understand Slide 18 and 19 about type of decay based on the position to the band of stability. •Make sure you understand the odd/even method for nuclear stability prediction on slide 20.

•Magic numbers: 2, 8, 20, 28, 40, 50, 82, 126. When a nuclei has an N or Z with these numbers it is “magically” stable. When both N and Z are magical, the nuclei is “doubly magic”. Nuclear Kinetics •Activity: rate at which radioactive decay occur (SI unit: 1Bq = 1 decay/sec) (Traditional unit: 1 Ci = 3.7*1010 Bq).

•Radionuclides undergo 1st order kinetics; thus, t1/2 =.693/k. •The Law of Radioactive Decay states that Activity = rate of decay = kN

where N is the # of radiocuclide atoms.

•1/k is average lifetime of radionuclide. •Radioactive decay is not temperature dependent. •Don’t forget units!! − kt • N t = N 0e

use this equation to determine the amount of radioactive nuclei there was initially (N0), the amount of radioactive nuclei there is at time t (Nt), the rate constant, or the time it takes for a certain fraction to remain. --Make sure that you use the same units for both N’s. Radiation is detected by counts per second (or counts/time). This is the same as disintegrations per second (or dis/time). Nuclear Energetics ΔE = Δmc2 = Nuclear Binding Energy (BE) Can energy be created or destroyed? NO. But it can be transformed into mass and mass can be transformed into energy. •Look at Einstein’s famous equation. What is it saying? It is saying that a small amount of matter relates to a large amount of energy. •Remember that the theoretical mass is the sum of proton, neutron, and electron masses and it is greater than the actual mass because some mass is converted to energy. •Refer to Dr. Hoeger’s slides to get a better understanding of this concept. In addition, make sure you understand the difference between binding energy and binding energy per nucleon. Binding energy per nucleon is the binding energy divided by the number of protons plus neutrons. BE/nucleon = BE/(#p+#n). Concepts/terms you should know/understand: Nuclear decay, daughter nuclei, nuclear transmutation, positron emission, neutron emission, electron capture, bombardment, penetrating ability of types of radiation, induced nuclear reaction, Law of Radioactive Decay, activity, nuclear binding energy, theoretical mass, actual mass, mass defect, binding energy per nucleon.

This concludes the synopsis of the material covered in the 5/7/07 to 5/11/07 lectures.