Nuclear Chemistry Outline BackgroundIntroduction Spontaneous Nuclear Reactions Types

Nuclear Chemistry

Outline • Background/Introduction • Spontaneous Nuclear Reactions - Types of Radioactive Decay - Radioactive Half-Life • Stimulated Nuclear Reactions - Nuclear Fission - Nuclear Fusion

Nuclear Chemistry - Background • Traditional Chemistry – Reactions occur due to interactions between valence electrons (surrounding nucleus) • Late 1800 s – Early 1900 s New Developments – Discovery that Uranium emits radiation (Henry Becquerel) – Amount of radiation emitted is proportional to amount of element present (Marie Curie) – “Radioactive” substances = radiation-emitting (Curie) – Radioactivity = a inherent property of certain ATOMS, as opposed to a chemical property of compounds (Curie) • Birth of nuclear chemistry

Video: Early (Mis)uses of Radium - A radioactive element discovered in 1898 by Curie - Found to glow in the dark! - Many (at the time) thought it had health benefits Radium Toothpaste For video, see below URL: http: //www. youtube. com/watch? v=uu 96 STA 5 BDA For more complete list of “quack” cures, see below URL: http: //www. orau. org/ptp/collection/quackcures. htm

Chemical vs. Nuclear Reactions Chemical Reactions Nuclear Reactions • Loss, gain, sharing of valence electrons • Changes in nucleus • Formation of compounds (Recall the different reaction types) • Transformation of one element into a different isotope or a different element altogether • Affected by temperature, pressure, presence of other atoms • Unaffected by temperature, pressure, presence of other atoms

Atomic Structure Review Isotopes: variants of atoms of a particular chemical element, which have differing numbers of neutrons (e. g. Carbon-12, Carbon-13, Carbon-14, etc. )

Nuclear Stability • How can a stable nucleus exist? – Net positive charge in nucleus surrounded by negative charges (electrons) – Electrostatic force • Opposite charges attract • Like charges repel – Why doesn’t the nucleus break apart? • Nuclear Force (1934) – Force between protons and neutrons than binds nucleus together within atom – Very strong (must be!)

Which Elements/Isotopes Are Radioactive? • Based on the nuclear force and nuclear stability – More protons in nucleus more neutrons needed to bind nucleus together – Critical Factor = Neutral-to-Proton Ratio • Neutron-to-Proton ratios of stable nuclei increase with increasing atomic number • Unstable neutron-to-proton ratio = RADIOACTIVE – Nuclei with atomic numbers ≥ 84 = ALWAYS Radioactive • Very large nuclei! • Neutron-to-proton ratio always unstable

Spontaneous Nuclear Reactions

Radiation and Radioactive Decay • Unstable (radioactive) nuclei emit radiation (energy) in order to become more stable • Radioactive Decay – occurs when a nucleus spontaneously decomposes in this way • 3 Common Types of Nuclear Reactions – Alpha Radiation – Beta Radiation – Gamma Radiation

Alpha (α) Radiation

Alpha Radiation: Nuclear Equation • Emission of 2 protons and 2 neutrons (as an alpha particle, ) from radioactive atom’s nucleus • Atom’s atomic mass decreases by 4 units • Atom’s atomic number decreases by 2 units (a different element!)

Beta (β-) Negative Radiation

Beta negative Radiation: Nuclear Equation • Conversion of a neutron into a proton and an electron, followed by the emission of the electron (beta particle) from the nucleus • Atom’s atomic mass does NOT change • Atom’s atomic number increases by 1 units (a different element!)

Positron emission • In β+ radiation a positron is emitted. • A positron is exactly like an electron in mass and charge force except with a positive charge. • It is formed when a proton breaks into a neutron with mass and neutral charge and this positron with no mass and the positive charge. • Positron emission is most common in lighter elements with a low neutron to proton ratio

Gamma (ϒ) Radiation • • Emission of electromagnetic energy from an atom’s nucleus No particles emitted Often occurs during α and/or β decay Example: X-rays emitted during β-decay of cobalt-60 (above)

Relative Penetrating Power of Radiation Types **slide not in notes • Alpha radiation has low energy and little penetrating power compared to beta radiation • Gamma has the most energy and penetrating power

Uranium-238: Radioactive Decay Chain

Practice: WE DO 1. Write a nuclear equation for the β- decay of francium-234 2. Write a nuclear equation for the α-decay of radium-235 3. Write a nuclear equation for the ϒ-decay of uranium-239

Practice: YOU DO What isotope is present after Po-210 undergoes 2 consecutive alpha decays, followed by a beta negative decay, followed by another alpha decay.

Stimulated Nuclear Reactions

Nuclear Fission • Neutron fired at atom’s nucleus • Energy of neutron “bullet” causes target element to split into two (or more) elements that are lighter than the parent atom – Unpredictable composition of products – Energy released! – More neutrons released! • RESULT? SEE NEXT SLIDE

Nuclear Fission Chain Reactions Nuclear Fission Power (Controlled Nuclear Fission) Nuclear Fission (Atomic) Bomb (Uncontrolled Nuclear Fission) How a nuclear reactor works video

Nuclear Fission (Atomic) Bombs • Fuel Core (B) – Two subcritical masses of plutonium • Each contains not enough fissionable material to sustain nuclear fission – Neutron source (radioactive isotope) in separate compartment • Detonation of outer casing of TNT (A) – Two plutonium masses brought together to form supercritical mass • Mass now contains enough fissionable material to sustain nuclear fission! – Neutrons brought to speed necessary to initiate nuclear fission

Nuclear Fusion • Reactions in which two or more elements “fuse” together to form one larger element (heaver isotope formed from lighter isotopes) • Requires extremely high heat (lots of energy!) and pressure – Analogy: trying to squeeze an unopened can of Coke into a little ball without spilling any Coke • Lots of energy is released! • Example: Fusion of 2 hydrogen isotopes (deuterium and tritium) into helium

Nuclear Fusion (Hydrogen) Bombs • Central core (B) of trillions of deuterium (H-2) and tritium (H-3) isotopes - Surrounded by small atomic (fission) bombs (A) - Detonation of atomic bombs provides energy and pressure for fusion of hydrogen isotopes into helium • LOTS of energy released in this process! - Atomic bombs (nuclear fission) used to initiate nuclear fusion Fusion itself energy release Release of neutrons accompanying fusion triggers fission of bomb casing (C), which is made of Uranium!

Nuclear Fusion (Hydrogen) Bombs • Energy released = over 10 times greater than fission (atomic) bomb • Has never been used in warfare • Neutron bomb = Hydrogen bomb without uranium casing – Less explosion, due to lack of fission of casing – Large quantity of neutrons propelled outward (neutron radiation) and captured by nuclei of substances they encounter – Results: * Neutron radiation induces radioactivity in most substances it encounters * Massive radiation without massive blast – Targets: * Potential H-bomb survivors (e. g. tank drivers, stronghold inhabitants, etc. ) * Ballistic missiles (intense neutron flux damages electron components)

Nuclear Fusion in the Sun • Energy is released from these fusion reactions that we receive as LIGHT and HEAT! • NOTE: 3 hydrogen-1 atoms required (total), yet only 2 produced

Controlled Nuclear Fusion on Earth? • Advantages over nuclear fission – More energy created – Very little dangerous radiation released – Potential Source = Water (and not much of it)!! • Main Challenge – Need to find way to bring small sample of deuterium and tritium to very high temperature and pressure WITHOUT detonating atomic bombs – Subject of much current research in physics and chemistry

Half-Life (T 1/2) = The amount of time necessary for one-half of a particular radioactive sample to decay

Different Elements Different Half-Lives Data for most stable isotope of element

Half-Life Problems • Can be completed without the below formulas! – Conceptual knowledge of half-life is sufficient Half-Life Formulas Fraction remaining = (n = # of half-lives elapsed) Amount remaining = Original Amount * Fraction Remaining

Practice: WE DO The half-life of radium-226 is 1600 years. How many grams of a 0. 25 gram sample will remain after 4800 years?

Practice: YOU DO 1. How many days does it take for 16 g of palladium-103 to decay to 1. 0 g? The half-life of palladium-103 is 17 days. 2. The half-life of thorium-227 is 18. 72 days. How many days are required for 75% of a given amount to decay?