Nuclear Chemistry Topic 12

Topic 12-Nuclear Chemistry Stability of the Nuclei a. Elements with atomic number 84 and above b. Proton to neutron ratio Common forms of radiation-Table O Nuclear equations a. Be able to balance Fission Fusion Artificial vs. Natural Transmutation Half-life a. Be able to solve problems Uses and Dangers of Radioisotopes a. Dating b. Chemical tracers c. Industrial applications d. Medical Applications 9. Radiation Risks

Most chemical reactions involve either the exchange or sharing of Nuclear Chemistry Most chemical reactions involve either the exchange or sharing of electrons between atoms. Nuclear chemistry is quite different in nature because it involves changes in the nucleus. When the atomic nucleus of one element is changed into the nucleus of a different element, the reaction is called a transmutation Most nuclei are stable, that is, they are found within the “belt of stability.” It is the ratio of neutrons to protons that determines the stability of a given nucleus. Elements with atomic number 84 and above, have no stable isotopes (radioisotopes).

Zone of Stability Plot of proton to neutron ratio showing the stable isotopes If the proton to neutron ratio of an atom falls outside of the zone of stability, it is an unstable isotope

Common Forms of Radiation Table O

Marie Curie November 7, 1867-July 4, 1934 First woman to win a Nobel Prize, the only woman to win in two fields and the only person to win in multiple sciences. Because of their levels of radioactivity, her papers from the 1890s are considered too dangerous to handle. Even her cookbook is highly radioactive. Her papers are kept in lead-lined boxes, and those who wish to consult them must wear protective clothing.

Notice gamma radiation has no mass and no charge. Mass number 4He 2 charge Notice gamma radiation has no mass and no charge. It is NOT a particle but instead it is a ray. The alpha particle is a helium nucleus The beta particle is an electron

Name Penetrating Power Ionizing Power Alpha Weak Strong Beta Average Gamma Because gamma has the greatest penetrating power, it is the type of radiation that is most dangerous to humans. Ionizing power is the ability of radiation to knock electrons off some atoms of the bombarded substance to produce ions.

Anything that is radioactive may give off alpha or beta particles. Anything radioactive will always give off gamma radiation. Therefore, anything radioactive is dangerous to humans.

Why does the beta particle deflect to the + side of the electric field? Beta has a – charge and is attracted to the + electric field Why does the alpha particle deflect to the – side of the electric field? Alpha has a + charge is attracted to the – electric field Why does the gamma ray pass through undeflected? Gamma has no charge and is unaffected by the field Opposite charges attract

When an unstable nucleus emits an alpha particle, the nucleus Alpha Decay When an unstable nucleus emits an alpha particle, the nucleus is called an alpha emitter. Alpha decay can be summarized as follows: 1. Atomic number decreases by two 2. Number of protons (charge) decreases by two 3. Number of neutrons decreases by two 4. Mass number decreases by four Mass # (p + n) 226Ra  222Rn + 4He 86 2 88 # protons (charge)

disintegration is said to undergo beta decay and is called a A nucleus that emits a beta particle as a result of a nuclear disintegration is said to undergo beta decay and is called a beta emitter Beta decay can be summarized as follows: 1. Atomic number increases by one 2. Number of protons increases by one 3. Number of neutrons decreases by one 4. Mass number remains the same Mass # (p + n) 214Pb  214Bi + 0e 82 83 -1 # protons (charge)

One of the things you must be able to do is balance a nuclear equation. It must be balanced in terms of mass, charge and energy. The mass numbers on both sides of the equation add up to 18 18 18 14N + 4He  17O + 1H 7 2 8 1 9 9 The atomic numbers (charges) on both sides of the equation add up to 9

Lets try some: 1. 27Al + 1n  24Na + X X = 4He 13 11 X = 14N X = 14N 2. 14C  X + 0e 6 -1 7 3. 12C + 249Cf  257Rf + 4X X= 1n 6 98 104

Fission Reactions A fission reaction begins with the capture of a neutron by the nucleus of a heavy element. The nucleus produced by the capture is unstable. It immediately splits, undergoing the process of fission. 1n + 235U  142Ba + 91Kr + 31n + energy 92 56 36 The products of fission are radioactive and must be safely stored until they become stable. This presents a huge problem since some of these products are radioactive for thousands of years. We harness the energy produced and use it for turning the turbines to generate electricity or for use in nuclear weapons

Fusion Reactions Involve the combining of light nuclei to form heavier ones 1H + 1H  2H + 0e 1 1 1 +1 1H + 2H  3He 1 1 2 3He + 3He  4He + 21H 2 2 2 1 3He + 1H 4He + 0e 2 1 2 +1 While these reactions produce the energy from the sun, they are not yet available to produce energy here on Earth. Extremely high temperatures and pressure are needed to allow the positively charged hydrogen nuclei to fuse into helium. Remember the nuclei are positively charged and repel each other and resist being fused together

This loss of mass seems to contradict our concept that matter Fission and Fusion In both types of reactions, the total mass of the products is less than the total nuclear mass of the reactants This loss of mass seems to contradict our concept that matter (mass) can neither be created or destroyed. The loss of mass in these nuclear reactions represents a conversion of some matter into a large amount of energy. This relationship was expressed by Albert Einstein in his famous equation: E = mc2 A minute amount of matter produces an extremely large amount of energy

Particle Accelerators Accelerate the speed/velocity of particles by using electrical and magnetic fields. Neutrons cannot be accelerated by particle accelerators because they are neutral and cannot be affected by these fields

Transmutations Nuclear reactions can be either naturally occurring or artificial. Natural transmutation- Occur by radioactive decay 18F  17O + 1H 9 8 1 Notice that the element changed without us having to do anything to it Artificial transmutation- Occur when particles bombard the nucleus of the atom 9Be + 4He  12C + 1n 4 2 6 Notice that the element has to be bombarded with a particle before it will change

Half-Life Radioactive substances decay at a constant rate that is not dependent on factors such as temperature, pressure or concentration. It is also a random event. It is impossible to predict when a given unstable nucleus will decay. However, the number of unstable nuclei that will decay in a given time in a sample of the element can be predicted. The time it takes for half of the atoms in a given sample of an element to decay is called the half-life of the element.

Table N gives you the half-life of selected radioisotopes and also gives you the decay mode of these radioisotopes also

Notice that after each half-life, half of the radioactive sample remains

Most chromium atoms are stable, but Cr-51 is an unstable isotope with a half-life of 28 days. a. What fraction of a sample of Cr-51 will remain after 168 days? 28 56 112 140 168 84 1/1 1/2 1/4 1/8 1/16 1/32 1/64 If a sample of Cr-51 has an original mass of 52 g, what mass will remain after 168 days? 28 168 56 84 112 140 52 26 13 6.5 3.25 1.63 .813

How much was present originally in a sample of Cr-51 if 0.75 mg remains after 168 days? 112 140 168 28 56 84 ? 24.0 12.0 6.0 3.0 1.5 .75 48 mg

In 6. 20 hours, a 200 g sample of Ag-112 decays to 50 g In 6.20 hours, a 200 g sample of Ag-112 decays to 50 g. What is the half-life of Ag-112? 6.20 h 200 100 50 It underwent 2 half –lives (2 arrows), so we divide 6.20 by 2 6.20/2 = 3.10 hours Half-life of Ag-112 is 3.10 hours

Uses and Dangers of Radioisotopes Radioisotopes have many practical applications in industry, medicine and research. They also have potential dangers because of harm that could be done by the radiation released. 1. Dating C-14 decays to form stable C-12 is used to date living or once alive materials U-238 decays to form stable Pb-206 and is used to date geologic formations

2. Chemical Tracers Any radioisotope used to follow the path of a material in a system If P-31 is present in fertilizer administered to a plant, the uptake of the radioactive phosphorus can be traced by detectors. Scientists can then determine the proper amounts and timing of fertilizer applications.

I-131-uses in both the detection and treatment of thyroid conditions 3. Medical Applications Certain radioisotopes that are quickly eliminated from the body and have short half-lives are important as tracers in medical diagnosis I-131-uses in both the detection and treatment of thyroid conditions Co-60-aimed at cancerous tumors and kills cells of tumor Some radioisotopes are used to irradiate food and kill bacteria. The food lasts longer without spoiling and causes fewer bacterial Infections in those who consume it Tc-99-absorbed by cancerous cells and is easily detected by a scan.

Radiation Risks 1. Kill cancerous cells but have the potential to damage normal cells 2. High doses of radiation can cause serious illness and death Radiation can cause mutations in sex cells that could potentially be passed from generation to generation Nuclear power plants have waste products with long half-lives which make them difficult to store and dispose of