Nuclear Decay

The Atom- Review The atom consists of two parts: 1. The nucleus which contains: protons neutrons 2. Orbiting electrons. Atom of different elements contain different numbers of protons. The mass of an atom is due to the number of protons and neutrons.

The Nucleus Super dense mass that contains protons and neutrons Has overall positive charge Protons and Neutrons together are called a nuclide But, how does the nucleus stay together? 4 forces help to explain this!

Nuclear Forces (4) in order of strength Gravity: attractive force that exists between all objects (weakest force) Weak Nuclear Force: force responsible for radioactive decay of elements (2nd weakest) Electrostatic Force: repulsive or attractive force based on charge (in nucleus repulsive as + repels +; 2nd strongest force) Strong Nuclear Force: attractive force that pull nucleus together and acts over very small distances (strongest force)

X A Z Isotope Symbol Review A = number of protons + number of neutrons Mass number = number of protons + number of neutrons X A Element symbol Z Atomic number = number of protons A = number of protons + number of neutrons Z = number of protons N = number of neutrons (A – Z) **Number of neutrons = Mass Number – Atomic Number**

Fill in the chart for each isotope U 235 92 U 238 92 A Z # of protons (Z) # of neutrons (N) 235 A Z # of protons (Z) # of neutrons (N) 238 92 92 92 92 143 146 Isotopes of any particular element contain the same number of protons, but different numbers of neutrons.

Band of Stability Strong nuclear force rules the nucleus, but acts over very short distance As nucleus gets bigger, its stability starts to change depending on number of protons and neutrons present We can predict this stability based on a chart i.e. the band of stability!

Band of Stability: Charts neutrons (n) vs protons (Z) Nuclei that fall within the band are considered stable Based on where on chart point falls, can determine if nuclei is stable or not!

Band of Stability Extras All isotopes with atomic number above 83 have an unstable nucleus Isotopes with 2, 8, 20, 28, 50, 82, or 126 protons or neutrons in their nucleus are stable (magic numbers) Isotopes with an atomic number less than 40 are most stable when the ratio of protons to neutrons is 1:1.

Band of Stability Practice Using Band of Stability (pg 662) in your book, determine if the following isotopes are stable: Uranium -215 Helium – 4 Bi - 190 Calcium – 36 Yb -120 Yb - 121

Atoms that are unstable are labelled radioactive. (golf ball example) They undergo 1 of 5 types radioactive decay to stabilize themselves. The products of this decay process are what we know as radiation or radioactivity! Different types of radiation have different properties and different strengths (penetrating power).

Radioactive Decay Radioactive decay results in the emission/absorption of either: an alpha particle (a), a negative beta particle (electron) (b-), a positive beta particle (positron) (b+), a gamma ray (g). In a nuclear reaction the MASS and ATOMIC NUMBER must be the SAME on both sides of the equations (think basic algebra)

Emission Vs. Absorption: A note If a particle is emitted: It is released (it appears on products side) Generally atom gets smaller (decrease in mass number) If a particle is absorbed: It is taken in (appears on reactants side) Generally atom gets bigger (increase in mass number)

Alpha Decay - α An alpha particle is identical to a helium nucleus. Contains two protons and two neutrons. Can be stopped by paper (weak). He 4 2

Ra Rn He Alpha Decay + Loss of 2 protons & 2 neutrons: 226 88 Rn 222 86 He 4 2 + Loss of 2 protons & 2 neutrons: Atomic # decreases by 2 Mass # decreases by 4

U + He U + Th Write the equation for the alpha decay of Uranium-234 92 + Th 230 90

X + Pb He He + Pb Po Find the missing starting material A Z 214 82 4 2 218 84

e Beta Emission - b- -1 beta particle (electron) Beta emission occurs when a neutron changes into a proton and an electron, and the electron is emitted. Can be stopped by thin metal (aluminium foil). e -1 beta particle (electron)

Beta Emission Po 218 84 At 218 85 e -1 +

X Y + e Th Y + e Th Pa + e Write your own Beta Emission What is Y? A Z -1 What is Y? Th 234 90 Y A Z + e -1 Th 234 90 Pa 91 + e -1

C + C N + e Write the equation for the beta emission of carbon-14 14 7 + e -1

X Bi + e Pb Bi + e Find the missing starting material A Z 214 83 -1 -1 Pb 214 82 Bi 83 + e -1

Electron Capture An electron is absorbed, then joined with a proton to turn into a neutron (electron Capture is the opposite of Beta Emission) The electron capture itself does not have penetrating power e -1

Electron Capture At 218 85 e -1 Po 218 84 +

X e + Y Ar e + Y Ar e + Cl Write your own Electron Capture What is Y? Z e -1 + Y Z-1 What is Y? Ar 37 18 e -1 + Y A Z-1 Ar 37 18 e -1 + Cl 17

Ni + Ni e + Co Write the equation for electron capture of nickel-59 59 28 e -1 + Co 27

X e + C N e + C Find the missing starting material A Z -1 14 6 14 7 -1 -1 + C 14 6 N 14 7 e -1 + C 6

e Positron Emission b+ +1 A positron is like an electron but it has a positive charge. During positron emission a proton changes into a neutron and the excess positive charge is emitted. Can be stopped by thin metal (aluminium foil). e +1

Positron Emission At 218 85 Po 218 84 e +1 +

X Y + e B Y + e B Be + e Write your own Positron Emission What is Y? A Z Y Z - 1 + e +1 What is Y? B 8 5 Y A Z + e +1 B 8 5 Be 4 + e +1

O + O N + e Write the equation for the positron emission of oxygen-16 7 + e +1

X Cu + e Zn Cu + e Find the missing starting material A Z 66 29 +1 66 +1 Zn 66 30 Cu 29 + e +1

Gamma Decay - γ Atoms that undergo alpha & beta decay may still have too much energy to be completely stable. These atoms will emit gamma rays to release that energy. Can only be stopped by thick concrete or lead There is no change in mass or atomic number g

Summary Reaction What happens? Mass # Atomic # -4 -2 No change +1 -1 Alpha Decay a Lose Helium Nucleus -4 -2 Beta Decay b- Lose electron from nucleus (neutron turns into proton) No change +1 Electron Capture Gain electron in nucleus (proton turns into neutron) -1 Positron Emission b+ Lose positron (proton turns into neutron) Gamma Decay g Emit high energy gamma ray

Writing Equations Practice Write the equation for the alpha decay (α) of Radon-198 Write the equation for the beta decay (β-) of Carbon-14 Write the equation for the positron emission (β+) of Oxygen-15 Write the equation if Krypton-81 were to undergo electron capture Write the equation for the gamma decay (γ) of Cobalt-60 Name the isotope that is the product of the alpha decay (α) of Polonium-194

Nuclear Stability Not all isotopes are radioactive The strong nuclear force holds all nuclei together Otherwise protons would repel each other Neutrons space out protons and make nucleus stable Not all isotopes are radioactive Only unstable nuclei decay In smaller atoms stable isotopes have equal numbers of protons and neutrons In larger atoms stable isotopes will have more neutrons than protons Too many or too few neutrons makes the nucleus unstable

Nuclear Stability Graph Dark band = stable nucleus Areas off line = radioactive

Fission Fission is when a nucleus splits This is what happens in nuclear power plants Neutrons emitted during fission reactions can cause other fission reactions This is a chain reaction In a nuclear reactor the chain reaction is controlled with control rods

Chain Reaction Each reaction allows multiple other reactions to occur Controlled vs Uncontrolled

Fusion When two or more elements fuse (combine) to form one new heavier element The energy released by the sun and all stars is due to fusion reactions in the core This process releases more energy than fission Fusion reactions are hard to contain because the reactants are a plasma and at very high temperatures, no solid material can contain a plasma

Fission Fusion Nucleus splits Nuclei combine Energy is released End product is lighter than reactants End product is heavier than reactants Nuclear Change Can’t contain reaction Reaction can be harnessed LOTS of energy released

Fission vs. Decay These are not the same! Decay is a natural process, whereas fission has to be induced (requires tremendous energy) This explains why a nuclear bomb (fission) releases so much more energy!!!!!