Nuclear Chemistry

Radioactivity One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie (1876-1934). She discovered radioactivity,the spontaneous disintegration of some elements into smaller pieces.

Isotopes Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms. There are three naturally occurring isotopes of uranium: Uranium-234 Uranium-235 Uranium-238

The Nucleus Remember that the nucleus is comprised of the two nucleons, protons and neutrons. The number of protons is the atomic number. The number of protons and neutrons together is effectively the mass of the atom.

Nuclear reactions “Normal” Chemical Reactions involve electrons, not protons and neutrons Nuclear reactions involve the nucleus, release larger amounts of energy, and produce different elements. The nucleus opens, and protons and neutrons are rearranged—more stable Nucleons: are the protons and neutrons in the nucleus. Nuclide: an atom

Binding Energy Nuclear binding energy is the energy released when a nucleus is formed. It is also the amount of energy needed to break apart a nucleus. It is a measure of nucleus stability. The higher the binding energy/nucleon, the more tightly the nucleons are held together.

Mass Defect The mass defect is caused by the conversion of mass to energy when the nucleus is formed. when protons + neutrons + electrons combine, the atom they form has less mass then the sum of the mass of the protons + neutrons + electrons. The difference is lost as energy

Mass Defect Energy Mass Speed of light Some of the mass can be converted into energy Shown by a very famous equation! E=mc2 Energy Mass Speed of light

Calculate the mass defect P 703 #4

Nuclear Stability Band of stability:The neutron-proton ratio of stable nuclei cluster around a region when plotted. The most stable nuclei have a 1:1 neutron:proton ratio. As the # protons increases, the repulsive, electrostatic force between protons increases faster than nuclear force. Beyond Atomic #83, atoms are not stable.

Neutron-Proton Ratios Any element with more than one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus. A strong nuclear force helps keep the nucleus from flying apart. Neutrons play a key role stabilizing the nucleus. Therefore, the ratio of neutrons to protons is an important factor.

Nuclear stability: magic numbers Even number nuclei tend to be stable. The nucleons that are most stable are 2,8,20,28,50,82,126 (either protons or neutrons) They have completed nuclear energy levels. Double magic He Pb

Stable Nuclei Nuclei above this belt have too many neutrons. They tend to decay by emitting beta particles.

Stable Nuclei Nuclei below the belt have too many protons. They tend to become more stable by positron emission or electron capture.

Radioactive Series Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).

Types of Radiation Beta (β) – an electron Alpha (ά) – a positively charged helium isotope Beta (β) – an electron Gamma (γ) – pure energy; called a ray rather than a particle http://library.thinkquest.org/3471/radiation_types_body.html

Other Nuclear Particles Neutron emission Positron emission – a positive electron (below band of stability) Proton – usually referred to as hydrogen-1 Electron capture (rare) proton is converted to a neutron

Penetrating Ability

Balancing Nuclear Reactions In the reactants (starting materials – on the left side of an equation) and products (final products – on the right side of an equation) Atomic numbers must balance and Mass numbers must balance Use a particle or isotope to fill in the missing protons and neutrons

Types of Radioactive Decay Alpha Decay Loss of an -particle (a helium nucleus) He 4 2 U 238 92  Th 234 90 He 4 2 +

Nuclear Reactions Alpha emission Note that mass number (A) goes down by 4 and atomic number (Z) goes down by 2. Nucleons (nuclear particles… protons and neutrons) are rearranged but conserved

Types of Radioactive Decay Beta Decay Loss of a -particle (a high energy electron)  −1 e or I 131 53 Xe 54  + e −1

Nuclear Reactions Beta emission Note that mass number (A) is unchanged and atomic number (Z) goes up by 1.

Types of Radioactive Decay Gamma Emission Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle) 

Types of Radioactive Decay Positron Emission Loss of a positron (a particle that has the same mass as but opposite charge than an electron) e 1 C 11 6  B 5 + e 1

Types of Radioactive Decay Electron Capture (K-Capture) Addition of an electron to a proton in the nucleus As a result, a proton is transformed into a neutron. p 1 + e −1  n

Neutron Emission 87 Kr → 86Kr + 1 n 36 36 0 36 36 0  a type of radioactive decay of atoms containing excess neutrons and a neutron is simply ejected from the nucleus

Other Types of Nuclear Reactions Positron (0+1b): a positive electron 207 Electron capture: the capture of an electron

Practice page 703 #11-13

Artificial Nuclear Reactions New elements or new isotopes of known elements are produced by bombarding an atom with a subatomic particle such as a proton or neutron -- or even a much heavier particle such as 4He and 11B. Reactions using neutrons are called g reactions because a g ray is usually emitted. Radioisotopes used in medicine are often made by g reactions.

Artificial Nuclear Reactions Example of a g reaction is production of radioactive 31P for use in studies of P uptake in the body. 3115P + 10n ---> 3215P + g

Transuranium Elements Elements beyond 92 (transuranium) made starting with an g reaction 23892U + 10n ---> 23992U + g 23992U ---> 23993Np + 0-1b 23993Np ---> 23994Pu + 0-1b

Nuclear Fission

Nuclear Fission Fission is the splitting of atoms These are usually very large, so that they are not as stable Fission chain has three general steps: 1. Initiation. Reaction of a single atom starts the chain (e.g., 235U + neutron) 2. Propagation. 236U fission releases neutrons that initiate other fissions 3. ___________ .

Stability of Nuclei Out of > 300 stable isotopes: N Even Odd Z 157 52 3115P Even Odd 50 5 21H, 63Li, 105B, 147N, 18073Ta 199F

Band of Stability and Radioactive Decay

Representation of a fission process. https://www.youtube.com/watch?v=Ezbyg2iNdQs&feature=iv&annotation_id=annotation_696116&src_vid=kHXMiYsFSrU

enrichment Natural uranium is a mixture of 0.7% U-235 and 99.3% U-238 Isotope separation is a physical process to concentrate or enrich 1 isotope relative to the other Most reactors use require fuel rods that are 3% U-235 Nuclear weapons contain 90% U-235

Nuclear Fission & POWER Currently about 103 nuclear power plants in the U.S. and about 435 worldwide. 17% of the world’s energy comes from nuclear.

Figure 19.6: Diagram of a nuclear power plant.

https://www.youtube.com/watch?v=pqn8xNWTm-c

Nuclear power plant in Florida concerns http://www.nytimes.com/2016/03/23/us/nuclear-plant-leak-threatens-drinking-water-wells-in-florida.html?_r=0

5 components of a nuclear reactor SHIELDING CONTROL RODS MODERATOR FUEL COOLANT Define Critical mass

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

Nuclear Fusion Fusion Excessive heat can not be contained Attempts at “cold” fusion have FAILED. “Hot” fusion is difficult to contain

Hydrogen bomb vs atomic bomb (thermonuclear bomb) http://www.livescience.com/53280-hydrogen-bomb-vs-atomic-bomb.html

Chernobyl http://www.world-nuclear.org/information-library/safety-and-security/safety-of-plants/chernobyl-accident.aspx

PROS AND CONS OF NUCLEAR ENERGY Pros: low cost, clean energy, provides stable load of energy (vs wind and solar), low pollution, higher energy density Cons: not renewable (U has to be mined), nuclear waste, accidents, processing, mining and enrichment

Half-Life HALF-LIFE is the time that it takes for 1/2 a sample to decompose. The rate of a nuclear transformation depends only on the “reactant” concentration.

Half-Life Decay of 20.0 mg of 15O. What remains after 3 half-lives? After 5 half-lives?

Kinetics of Radioactive Decay For each duration (half-life), one half of the substance decomposes. For example: Ra-234 has a half-life of 3.6 days If you start with 50 grams of Ra-234 After 3.6 days > 25 grams After 7.2 days > 12.5 grams After 10.8 days > 6.25 grams

Learning Check! The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 39 hours?

Effects of Radiation

Geiger Counter Used to detect radioactive substances

Radiocarbon Dating Radioactive C-14 is formed in the upper atmosphere by nuclear reactions initiated by neutrons in cosmic radiation 14N + 1on ---> 14C + 1H The C-14 is oxidized to CO2, which circulates through the biosphere. When a plant dies, the C-14 is not replenished. But the C-14 continues to decay with t1/2 = 5730 years. Activity of a sample can be used to date the sample.

Nuclear Medicine: Imaging Thyroid imaging using Tc-99m

Food Irradiation Food can be irradiated with g rays from 60Co or 137Cs. Irradiated milk has a shelf life of 3 mo. without refrigeration. USDA has approved irradiation of meats and eggs.