Atomic: Nuclear Reactions 2018 Unit 4: Physical Science 10.1-2, 4 Geo Ch 21.2-4

Nuclear Chemistry Nuclear Decay Nuclear Reactions Alpha Decay Beta Decay Gamma Decay Fission Fusion + -

Golden Ratio: Stable and Unstable Nuclei Smaller elements neutron to proton ratio is 1:1 to be stable isotopes Example: Helium, Nitrogen, Oxygen Heavier elements neutron to proton ratio is 3:2 to be stable isotopes Example: Iron, Nickel, Copper, Zinc Larger nuclei tend to be unstable – all nuclei containing more than 83 protons are radioactive All elements with more than 92 protons are synthetic and decay soon after they are created (UNSTABLE) Nuclei of any isotopes that differ much from these ratios are unstable, whether heavy or light

Physics - Radioactivity: The Discovery of Radioactivity Radiation When a nucleus becomes unstable, it is said to be radioactive. It decays releasing particles and energy called nuclear radiation Some radiation is more powerful than others There are four types of nuclear radiation – alpha (α) – made of 2 p+ and 2 n0 beta (β- and β+) n0 decays into p+ and e- (beta minus) p+ decays into n0 and e+ (beta plus) gamma (γ) – no mass or charge, just energy Types of Radioactive Decay (great resource comparing/contrasting)

What kind of decay will occur?

4 Types of Radiation, cont Particle Type Alpha Beta Gamma Symbol   Mass Charge Penetration Stopped by thin layer of material Stopped by thick materials Only stopped by very thick materials 2 4 He or α β γ 4 0.0005 +2 -1, +1

Prior Knowledge Isotope Notation

Alpha Decay Alpha radiation is made of alpha particles Alpha particles are made of two protons and two neutrons (a Helium atom!) They are represented by: α Properties: Large size Cannot pierce deep into matter – easily stopped by a thin layer of material When alpha decay occurs, the nucleus loses 2p+ and 2n0 Atomic number is 2 less Mass is 4 less

Beta Decay (+ or -) Beta radiation is made of beta particles Beta particles are made of a high energy electron They are represented by: β+ or β- Properties: Small and light Move fast Can only be stopped by thick materials, like stacked sheets of metal, blocks of wood or heaving clothing When Beta + decay: a proton turns into a neutron and a β+ is emitted Atomic number is 1 less Mass number is same When Beta - decay occurs, a neutron changes into a proton and a β- is emitted Atomic number is 1 more Beta plus (positron, which is a positively charged electron) and Beta minus (electron)

Gamma Decay Gamma radiation is made of gamma rays Gamma rays are a high-energy form of electromagnetic radiation They are represented by: γ Properties: No mass or charge Can pass through most types of materials – need a material thicker than blocks of concrete When gamma radiation occurs, only energy is given off Usually occurs along with another type of decay Atomic number is same Mass number is same

Half-Life Spontaneous: you cannot predict when each atom will decay Random: you cannot predict which atom will decay But, each radioisotope has a predictable half-life Half-life: amount of time it takes for half of the nuclei in a radioactive isotope to decay Original, unstable substance = parent New, (stable or unstable) substance = daughter After 1 half-life 50% of parent isotope has decayed into a daughter isotope Overall rate of decay is constant This allows you to predict when a given fraction of the sample will have decayed Reference penny lab. I cannot predict which round the penny will flip. I cannot predict which penny will flip this round. But I know that each round, about half of the pennies will have flipped.

Half-Life Values and Graph

Half-Life Values and Graph ½ or 1:1 ratio ¼ or 1:3 ratio 1/8 or 1:7 ratio

Carbon 14-Dating radiocarbon dating Carbon-14 is widely used to determine the ages of fossils All organisms take in carbon during lifetime Most of this carbon is the isotope carbon-12 1 in every million is the radioisotope carbon-14 When an organism dies, carbon is no longer being taken in The amount of carbon-14 slowly decreases the half-life of carbon-14 is short compared to ages of many fossils and geological formations Objects more than 60,000 years old cannot be dated using carbon-14 radiocarbon dating

Check for Understanding What fraction of the parent isotope is left after 9 half-lives? What is the half-life of the element below?

Practice Example Problem: 228 Ac has a half life of 6.0 hours. How much of a 5.0 mg sample would remain after one day (24 hours)? The first step is to determine the number of half lives that have elapsed. number of half lives = total time / half-life = 24 hours/6.0 hours = 4.0 half lives For each half life, the total amount of the isotope is reduced by half. Amount remaining = Original amount x half-life % = 5.0 mg x 6.25% = 0.31 mg

Practice Americium-242 has a half-life of 6.0 hours. If you started with 24 g and you now have 1.5 g, how much time has passed? 24 g  12 g  6.0 g  3.0 g  1.5 g 1st ½ 2nd ½ 3rd ½ 4th ½ 4.0 ½ lives x 6.0 hrs = 24 hrs The half-life of cobalt-60 is 5.0 years. If you have 10. grams of Co-60, how much do you have after 15 years? 15 years ÷ 5.0 years = 3.0 half lives 10. g  5.0 g  2.5 g  1.25 g 1st ½ 2nd ½ 3rd ½

KEY TERMS: Ch 10.4 Nuclear Reactions Nuclear Fission – the process of splitting a nucleus into two nuclei with smaller masses Chain reaction – ongoing series of fission reactions Critical mass – the amount of fissionable material required so that each fission reaction produce approximately one more fission reaction Nuclear Fusion – two nuclei with low masses are combined to form one nucleus of larger mass Fission and Fusion Video (~4 minutes) KEY TERMS: Ch 10.4 Nuclear Reactions

Nuclear Fission Fission means “to divide” The process of splitting a nucleus into two nuclei with smaller masses This occurs when a neutron hits the nucleus of an atom Only large nuclei can undergo fission reactions Two uses from book: generate electricity, nuclear weapons The total mass of the products is slightly less than the mass of the original nuclear and the neutrons that break free The missing mass is converted into large amounts of energy

Nuclear Fission Lighter element Neutrons + Energy Neutron Uranium - 235 Lighter element

Nuclear Fission Chain reactions are continuing series of reactions in which each produces a product that can react again If chain reaction occurs too quickly, explosions occur, releasing a lot of energy all at once energy energy energy energy energy energy

Fission of Uranium-238

Nuclear Fusion Fusion = combine Nuclear Fusion is when two or more nuclei combine to form a larger nucleus (requires very high temps to overcome the repulsive forces) The sun uses this process to produce energy It fuses hydrogen into helium The problem lies in the energy involved to start fusion Most nuclei that can undergo fusion are fairly unreactive Energy given off (output) does not outweigh the energy needed (input) Hydrogen Bomb Just like in fission, a small amount of mass is converted into a large amount of energy

Fusion of Hydrogen into helium

Nuclear Fusion positron energy neutrino positron energy neutrino

Energy to Mass Conversions Think back to the law of conservation of energy We need to include mass in this law when talking about fission and fusions reactions This relationship is shown by Einstein’s theory of relativity This states that energy and mass are equivalent and can be converted using the equation E=mc2

Extra Practice What type of nuclear reaction shown below? Write an equation for the reaction. You only need to include the symbol and mass number. What type of nuclear reaction is shown below?

Extra Practice Gold-198 has a half-life of 2.7 days. How much of a 96 g sample of gold-198 will be left after 8.1 days? What is the half-life of a 100.0 g sample of nitrogen-16 that decays to 12.5 g of nitrogen-16 in 20.0 s? (Hint: 12.5 is 12.5% of 100.0, so figure out what fraction that is!) The half-life of Zn-71 is 2.4 minutes. If one had 100.0 g at the beginning, how many grams would be left after 7.2 minutes has elapsed?

Review: Types of Radioactive Decay and Nuclear Equations Videos: Types of Radioactive Decay and their effects on the Nucleus Balancing Nuclear Equations and Predicting Products of Nuclear Reactions