Chapter 25 Nuclear Chemistry

Chapter 25 Nuclear Chemistry

Isotope Examples Prob: An atom of Kr has a mass of 94 AMU. How many protons & neutrons does it have? Answer: Kr = element #36 (36 protons). Neutrons = mass - protons, = 94-36 = 58 neutrons. Prob: An atom has 20 protons and 22 neutrons. Draw the symbol for the atom. 42 20 Ca

Radioactivity Radioactivity is the process by which nuclei emit particles and rays as they break down. The name of the penetrating rays emitted by a radioactive source is called radiation. A radioactive isotope is an unstable atom which breaks down on its own, releasing energy and/or particles and often becoming a new element. (Of all known isotopes, about 400 are stable and 1200 are unstable). These isotopes break down to gain stability in nature.

Radioactivity Nuclear reactions are not affected by any chemical changes (temp, pressure, catalysts). Nuclear reactions cannot be sped up, slowed down, or turned off. Unstable radioisotopes of one element are transformed into stable (nonradioactive) isotopes of a different element.

Types of Radiation Alpha = = 2 He 4 This is a Helium nucleus, consisting of 2 protons & 2 neutrons. can penetrate 2-8cm in air or 0.05 mm into skin. They are shielded by paper or skin.

Types of Radiation Beta = - 0 = electrons = 1 e Penetrating power is medium as these can penetrate about 20 meters in air, 0.1-2 to 4 mm through the skin before being absorbed. They can be shielded by metal foil.

Types of Radiation Gamma = these rays have high energy and frequency Penetrating power is VERY HIGH, and penetrates the body easily. Shielding materials are lead or concrete, but these do not shield completely.

Nuclear Reactions 1. Alpha Decay ( ): Positive charge (+) This reduces the mass by 4, and the element number by 2. Alpha decay usually indicates a slow decay rate (long half-lives) E.g.: 232 90 Th 4 2 Eg: Smoke detectors 241 95 Am He + 228 88 Ra 212 83 Bi 4 2 He + 208 81 Tl

Nuclear Reactions 2. Beta Minus Decay ( ) negative charge (-): no change in mass, the element number increases by 1. Usually occurs in neutron-rich nuclei caused by a neutron which converts into a proton (stays in nucleus), ejecting an electron E.g.: Eg: 1 0 n 1 1 p + 0 1 e 14 6 C 14 7 N + 0 1 e Eg: 212 83 Bi 212 84 Po + 0 1 e

Nuclear Reactions 3. Gamma Decay ( ): Causes no change in element or mass. This usually follows alpha or beta decay as the nucleus rearranges to get in a less excited state. This occurs in picoseconds. (10-12 seconds) Eg: 61 28 Ni 61 28 Ni +

Nuclear Reactions 4. Combination Reactions: The resulting nucleus may also be radioactive. Eg: 238 92 U 234 90 Th + 4 2 He Ra + He Rn + He Po + He Pb Eg: 238 92 U 8 + 6-208 82 Pb One isotope can also break down by more than one method. For example, 212 Bi can undergo either alpha or beta decay. 83

Nuclear Transformations Protons and Neutrons are held together in the Nucleus of an atom by the nuclear force. Without the nuclear force, the like-charged protons would repel and fly apart. Atoms undergo radioactive decay due to the neutron-proton ratio. If the number of neutrons are much greater than the number of protons it is likely that the element will decay.

Nuclear Transformations During a nuclear reaction, the law of conservation of mass DOES NOT apply. An extremely small quantity of mass is converted into energy released in radioactive decay.

Half-Life Every radioisotope has a consistent rate of decay that is measured as a Half-Life. A half-life (t1/2) is the time required for onehalf of the nuclei of a radioisotope sample to decay. After each half-life, half of the existing radioactive atoms have decayed into atoms of a new element.

For example: Half-Life 256g of Iodine - 131 has a half-life of 8 days. Every 8 days the sample of Iodine decays by half. How much remains after 6 Half-Lives? Half Life Time passed Amount of substance Decayed (stable) 0 1 2 3 4 5 6 Amount of Original Substance (radioactive) Percentage Remaining that is radioactive

For example: Half-Life 256g of Iodine - 131 has a half-life of 8 days. Every 8 days the sample of Iodine decays by half. Half Life Time passed Amount of substance Decayed (stable) Amount of Original Substance (radioactive) Percentage Remaining that is radioactive 0 0 days 0 grams 256 g 100% 1 8 days 128 g 128 g 50% 2 16 days 192 g 64 g 25% 3 24 days 224 g 32 g 12.5% 4 32 days 240 g 16 g 6.25% 5 40 days 248 g 8 g 3.125% 6 48 days 252 g 4 g 1.5625%

Half-Life Problems 1. How much of a 150g sample of Au-198 is left after 8.10 minutes if it s half life is 2.70 minutes? 18.75 g of Au-198 2. A 50g sample of N-16 decays to 12.5g in 14.4 seconds. What is it s half life? 7.2 Seconds 3. There are 5.0g left of Tc-99 after 40.35 days. How many grams were in the original sample if its half-life is 8.07 days? 160 grams of Tc-99 remain

Carbon-14 Half-lives are used to date and age items on Earth by comparing how much radioactive particles an isotope has remaining compared to how much it began with. Carbon is the basis of all life on Earth. The half-life of Carbon-14 is 5730 years. All living organisms absorb Carbon-14. Once they die, they stop absorbing the Radioactive isotope and this is used to date up to 75,000 years ago. Approx 13 half-lives. Potassium-40 is used to date the oldest rocks of Earth, Meteorites, and the moon. K-40 has a half-life of 1.25 x 10 9 years.

Nuclear Fission With a high number of neutrons, radioisotopes split into smaller fragments, called nuclear fission. Uranium-235 and Plutonium-239 are the only fissionable atoms. Nuclear fission releases an enormous amount of energy. E.G.: 1 kg of U-235 yields a release of energy equal to 20,000 lbs of dynamite. (Atom bomb)

Nuclear Fission 1 neutron ( 1 0 n) + 235 92 U 236 92 U 91 36 K + 142 56 Ba + 3 Neutrons (3 0 n) + energy

Nuclear Fission Fission can be controlled so energy is released more slowly. Nuclear Reactors are used in this manner to create energy in the form of heat. The heat creates steam, the steam moves a turbine and the turbine creates electricity.

Nuclear Fission

Nuclear Fusion Fusion occurs when two nuclei combine to produce a nucleus of greater mass. Fusion reactions only occur at very high temperatures: 40,000,000 C. In the sun 4 Hydrogen atoms combine to form 1 Helium atom. 1 1 H + 1 1 H + 1 1 H + 1 1 H 4 2 He Each H has only 1 proton and 0 neutrons. The fusion reaction makes He which has 2 protons and 2 neutrons.