* Nuclear Chemistry- study of nuclear reactions and their uses in chem..
* If the nucleus were the size of a ping pong ball, the e- in the 1s orbital would be 0.5km away. The mass would be 2.5 billion tons, and the energy associated would be millions of times larger than normal chemical rxns.
* Uses:
* radioactive elements are used in medicine as diagnostic tools and treatments.
* help determine the mechanisms of chemical reactions
* trace movement of atoms in biological systems
* date historical artifacts
* nuclear reactions are used to both generate electricity and to create weapons of mass destruction
Section 1: Nuclear Stability and Radioactive Decay
* Inside the nucleus, there are p+ and n0. Together, we call them nucleons.
* Atomic number - number of p+
* Mass number - number of p+ and n0
* Isotopes - atom with the same number of p+ and different number of n0.
* U → 233U 235U 238U
* Nuclei that are radioactive are called radionuclides, and atoms containing these nuclei are called radioisotopes.
* Radionuclides are unstable and spontaneously emit particles and electromagnetic radiation. This is one of the ways in which an unstable nucleus is transformed into a more stable one with less energy.
* Zone of stability – p. 874
* The emitted radiation is the carrier of the excess energy.
* Alpha particles - Helium-4 particles that are emitted.
* Types of Radioactive Decay
* 3 types: alpha ( α ), beta ( β ), and gamma ( γ )
* alpha- stream of helium-4 particles (alpha particles),
represented by 42He.
23892U → 23490Th + 42He
* beta- beta particles, high-speed e- emitted by an unstable nucleus. represented in nuclear equations by the symbol 0-1e.
23892U → 23893Np + 0-1e
* gamma- high-energy photons (electromagnetic radiation of very short wavelength). It represents energy lost when the remaining nucleons reorganize into more stable arrangements.
* 2 other types of radioactive decay that occur are positron emission and electron capture. A positron is a particle that has the same mass as an e-, but an opposite charge. It is represented as 01e.
* The emission of a positron has the effect of converting a p+ to a n0.
116C → 115B + 01e
* Electron capture- the capture by the nucleus of an inner-shell e- from the e- cloud surrounding the nucleus.
8137Rb + 0-1e → 8136Kr
* The e- is consumed rather than formed in the process.
Section 2: The Kinetics of Radioactive Decay
* Neutron-to-Proton Ratio
* Like charges repel each other, so it seems strange that p+ 's can exist in large numbers in such a confined space.
* At close distances, a strong force of attraction, called the 'strong nuclear force' exists between nucleons.
* All nuclei with 2 or more p+ contain neutrons. The more p+ packed into the nucleus, the more neutrons needed to bind the nucleus together.
* For smaller, more stable nuclei, they usually have equal numbers of p+ and n0, but nuclei with large numbers of p+ usually have a higher number of n0.
* So, the neutron-to-proton ratios of stable nuclei increase with increasing atomic number.
* All nuclei with 84 or more p+ are radioactive.
* The type of radioactive decay that a particular radionuclide undergoes depends on its neutron-to-proton ratio, compared to those of nearby nuclei within the belt of stability. 3 situations:
1. Nuclei above the belt of stability (high n0 to p+ ratios)
2. Nuclei below the belt of stability ( low n0 to p+ ratios)
3. Nuclei with atomic numbers greater than or equal to 84.
* Radioactive Series
* Some nuclei, like 238U, cannot gain stability by a single emission. So, a series of emissions occurs.
* A series of nuclear rxns that begins with an unstable nucleus and terminates with a stable one is a radioactive series of a nuclear disintegration series.
* Further Observations
* Nuclei with 2,8,20,28,50, or 82 p+, or 2,8,20,28,50,82, or 126 n0 are generally more stable. These numbers are called 'magic numbers.'
* Nuclei with even numbers of both p+ and n0 are generally more stable than those with odd numbers of nucleons.
* This relates to e- orbitals being full and, therefore, stable.
* Why are some radioisotopes not found in nature?
* Because many undergo radioactive decay at different rates.
* Half-life- time required for half of any given quantity of a substance to react.
* Strontium-90 has a half-life of 29 years. If we start with 10.0g then 5.0g would remain after 29 years. 2.5g would remain after another 29 years, etc.
* The half-life of radioisotopes is NOT affected by temperature, pressure, or state of chemical combination.
* So, we can't do anything to stop it.
Calculations based on half-life
* ln Nt = -kt
N0
* t = time k = decay constant N0 = initial # of nuclei Nt = # of nuclei remaining
* T1/2 = 0.693
K
* If we start with 1.000g of strontium-90, 0.953g will remain after 2.00years. What is the half-life of strontium-90?
* How much strontium-90 will remain after 5.00 years?
* How much time is required for a 1.85g sample of 51Cr to decay to 0.75g if it has a half-life of 27.8 days?
Section 3: Nuclear Transformations
* A nuclear rxn that is induced by being struck by a neutron or another nucleus are nuclear transmutations.
* Ernest Rutherford was the first to discover this type or rxn.
* Using Charged Particles
* Charged particles, such as alpha particles, must be moving very fast in order to overcome the repulsion between the particle and the other atom's nucleus.
* We use 'atom smashers,' named particle accelerators.
* Using Neutrons
* Most synthetic isotopes used in medicine and research are made using neutrons as projectiles.
* Neutrons are neutral, so they do not need to be accelerated to cause nuclear rxns.
* Transmutation Elements
* Artificial transmutations have been used to produce the elements with atomic numbers above 92.
Section 4: Detection and Uses of Radioactivity
* Radioactivity is detected in many ways.
* Photographic plates and film can pick it up. It leaves black spots.
* Geiger counter- The ions and e- produced by the ionizing radiation permit conduction of an electrical current.
* Certain substances that are electronically excited by radiation can also be used to detect radiation. Different substances respond to different particles. ZnS responds to alpha particles.
* An instrument called a scintillation counter is used to detect and measure radiation, based on flashes of light produced when radiation hits a corresponding compound.
* Radiotracers
Radioisotopes can be detected easily, so they can be used to follow an element through its chemical rxn.
* ex: use C-14 in CO2 for photosynthesis
* Because a radioisotope can be used to trace the path of an element, it is called a radiotracer.
Uses
* Dating
* We can use half-life to determine the ages of different objects. C-14 is used to determine the age of organic materials. The procedure is based on the formation of C-14 by neutron capture in the upper atmosphere.
* This rxn provides a small but reasonably constant source of radioactive C-14. This C-14 has a half-life of 5730 years. Living things take in C-12 and C-14, which is identical to the atmosphere.
* When an organism dies, it not longer takes in carbon, so it can't replenish the C-14 lost from decay. So, the ratio of C-12 to C-14 decreases and can be compared to the environment.
* U-238 decays to Pb-206. Rocks can be dated by the amount of Pb-206 in them.
Section 5: Thermodynamic Stability of the Nucleus
* E = mc2 E = energy m = mass c = speed of light (3.0 x 108)
* If an object loses mass, it loses energy (exothermic) and vice versa.
* Because c is squared, even a small change in mass results in a huge change in energy.
* How much energy is lost or gained when a mole of Co-60
undergoes beta decay: 6027Co → 0-1e + 6028Ni? The mass of the
6027Co atom is 59.9338g and that of a 6028Ni atom is 59.9308g.
M = 59.9308 – 59.9338 = - 0.0030amu
E = (3.0 x 108)2(-0.0030)( 1kg )
1000g
E = -2.7 x 1011J
* Nuclear Binding Energies- energy required to separate a nucleus into its individual nucleons.
* The larger the binding energies, the more stable the nucleus toward decomposition.
* exothermic
* Heavy nuclei gain stability and therefore give off energy if they are fragmented into 2 mid-sized nuclei. This is fission, used to generate energy in nuclear power plants.
* Even greater amounts of energy are released if very light nuclei are combined or fused together to give more massive nuclei. This fusion process is the essential energy-producing process in the Sun.
Section 6: Nuclear Fission and Nuclear Fusion
* Fission and fusion are exothermic.
* Commercial nuclear power plants and most common forms of nuclear weapons depend on fission.
* U-235, U-233, and Plutonium-239 when struck by a slow-moving neutron undergo fission.
* 2 neutrons are produced by every fission of U-235. Those 2 neutron go on to produce 2 more fissions. Those go on to 4, etc. This is called a chain rxn.
* For a chain rxn to occur, the sample of fissionable material must have a minimum mass. This is the critical mass. If the mass is too low, the chain stops.
* If more than a critical mass of fissionable material is present, very few neutrons escape. The chain rxn multiplies the number of fissions which can lead to a nuclear explosion. A mass is excess of a critical mass is the supercritical mass.
* Nuclear Reactors
* Fission produces the energy in a power plant. The 'fuel' of the reactor is fissionable, such as U-235. It is used in the form of UO2 pellets. They are encased in Zr or stainless steel tubes. Rods of Cd or B control the fission by absorbing neutrons.
* A cooling liquid circulates through the reactor core to carry off heat from the fission.
* Storing the radioactive wastes is a major problem. They are currently burying them in the Yucca Mtns.
Fusion results from the fusing of 2 light nuclei.
Sun's composition: 73% H
26% He
1% other elements
* Fusion is appealing because of the abundance of light nuclei and because the products are not radioactive. Despite this, fusion is not used to generate energy.
* High energies are needed for fusion to take place, giving them the name thermonuclear reactions. Lowest temp: 40,000,000K.
Section 7: Effects of Radiation
* Everyone knows that being hit by a train is very serious. The problem is the energy transfer involved…any source of energy is potentially harmful to organisms.
* The ozone layer screens out high-energy UV radiation from the sun.
* Radioactive elements, which are sources of high-energy particles, are also potentially hazardous, although the effects are usually quite subtle.
Damages from Radiation
* 2 types of damage:
* Somatic damage – damage to the organism itself – resulting in sickness or death.
* Genetic damage – damage to the genetics, which performs malfunctions in the offspring of the organism.
* The biologic effects of radiation depend on several factors:
1. The energy of the radiation…measured in rads
2. The penetrating ability of the radiation…gamma > beta > alpha
3. Ionizing ability of radiation – extraction of e- from biomolecules to form ions is detrimental to their functions…gamma can penetrate very deeply, but cause only occasional ionization, while alpha can be stopped by the skin but cause ionization so ingestion is lethal.
4. The chemical properties of the radiation source – when ingested, its effectiveness in causing damage depends on it residence time…Kr and Sr are both beta particle producers. Kr is inert, so it passes thru the body quickly, not making beta particles. Sr, however, can be absorbed into the bones, etc, where it can cause leukemia and bone cancer.
How Gryffinium is Synthetically Produced
When Oganesson (element 118) undergoes beta emission, you would think it’d produce element 119. However, a weird behavior occurs in which there is a lot of positron emission. Exactly 100 protons are each converted into a neutron and an electron, leaving it with only 19 protons.
* P. 898 contains a chart with exposures
* Humans are continually bombarded by radiation: infrared, UV, radio waves, microwaves, x-rays, and also from soil and natural materials.
* Ionizing radiation- very harmful, causes e- to become excited, causing movement and rotation.
* When it passes through living tissues, e- are removed from H2O forming H20+ ions, which are highly reactive.
* These H2O+ react with H2O to make H3O+ and OH-. OH- is highly reactive and is a free radical, a substance with one or more unpaired e-. This molecule can attack a number of biomolecules, which can produce more free radicals.
* Radiation Doses
* Becquerel (Bq) - SI unit for the activity of a radioactive source.
* Curie (Ci) - older unit still used.
* Gray (Gy) - SI unit of absorbed dose of radiation.
* Rad - unit most often used in medicine for absorbed