Nuclear Chemistry

Nuclear Chemistry Mr. Montero Chemistry Dr. Michael M. Krop High School

Atomic Symbols Mass Number (A) (p+ + n0) Atomic Symbol Atomic number (Z) (number of p+)

Isotopes of Uranium Uranium - 238 Uranium - 235 Uranium - 233 n0 = 146 n0 = 143 n0 = 141 What is the difference between these three isotopes? The number of neutrons!

Radioactivity The spontaneous disintegration of an unstable atomic nucleus with accompanying emission of radiation.

Radioactivity • Only unstable nuclei decay. The majority of atoms in nature do not decay • Radioactive nuclei are called Radionuclei. Atoms containing these nuclei are called Radioisotopes • The stability of the nucleus depends on number of protons and neutrons

Radioactive Emissions α, β, γ • Alpha particles (α) are the nucleus of Helium (2 protons, 2 neutrons) +2 charge • Beta particles (β) are high-speed electrons. -1 charge. • Gamma Rays (γ)are high energy radiation, not particles. Zero Charge

Separation of α, β, γemissions MOVIE

Penetrating ability of α, β, γradiation

Alpha (α) Emissions • Helium Nucleus (2 p+ and 2 n0) • Notation: or Uranium-238 Alpha decay After alpha decay Uranium changed to Thorium

Alpha (α) Emissions • What product is formed when radium-226 undergoes alpha decay? 2 protons are lost and 2 neutrons so mass number goes down by 4 and atomic number goes down by 2

Beta (β) Emissions • High Speed electrons (1 e-) • Notation: or Iodine-131 Beta decay After beta decay Iodine changed to Xenon

Beta (β) Emissions • What product is formed when Thorium-231 undergoes beta decay? No protons are lost and 1 neutron turns into a proton. Atomic number goes up by one while mass number stays constant

Beta (β) Emissions • If there are no electrons in the nucleus, where do beta particles come from? • Why does the atomic number of an atom increases by one when there is beta decay?

Beta (β) Emissions • The reason is unknown. • Possible theories involve quarks or particles which supposedly compose protons and neutrons • However, it is known that a neutron disappears and somehow turns into a proton

Positron Emissions • Same mass as electrons but opposite charge • Notation: Carbon-11 Positron decay After positron decay carbon changed to boron

Positron Emissions • What product is formed when oxygen-15 undergoes positron emission? No neutrons are lost and 1 proton turns into a neutron. Atomic number goes down by one while mass number stays constant

Gamma Radiation (γ) • High energy photons • Very short wavelength • Always accompanies other radioactive emission • Represents the energy lost when the remaining nucleons reorganize

Why does radioactivity happen? • If protons repel each other, how can they stick together in the nucleus? • What forces are present in the nucleus? STRONG NUCLEAR FORCES

Strong nuclear forces • When nucleons get close enough a very strong force binds them together overcoming electrostatic repulsion • Neutrons are not repelled so they help to cement protons together • As soon as protons are slightly apart repulsion overcomes strong nuclear force.

Strong Nuclear Forces All nucleons attract one another by Strong Nuclear forces. Only protons repel one another

Patterns of Nuclear Stability • As Z increases the number of neutrons must increase to guarantee stability • All nuclei with 84 or more protons are radioactive and tend to undergo alpha decay

Radioactive Series of Uranium • Uranium-238 does not become stable with only one decay • This nuclei undergo a combination of alpha and beta decay and finally reaches stability as lead (Pb)

Nuclear Transmutation • Nuclear reactions which are induced (not spontaneous.) • Induced by a high speed nucleon striking the nucleus • Example: An alpha particle strikes a nitrogen nucleus to synthesize Oxygen-17

Particle Accelerators • Charged particles have to be accelerated in order to overcome the repulsive forces in the nucleus. Particle accelerator at Fermi National Lab. The circumference is 6.3 km.

Using Neutrons • Neutrons are not charged, therefore they do not need to be accelerated. • Cobalt-60 is an important isotope used in radiation therapy for cancer. It is produced from iron-58 which is bombarded by neutrons

Transuranium Elements • Man-made elements heavier than Uranium. • Many of these only exist for brief moments and then decay rapidly • In 1940 elements 93 (Neptunium) and 94 (Plutonium) were discovered by bombarding Uranium with neutrons

Rates of Radioactive Decay • Different nuclei undergo radioactive decay at different rates • Some decay very rapidly (seconds) others decay very slowly (years) • Radioactive decay is a first-order process

First Order Process • The rate of decay is proportional to the number of radioactive nuclei (N) Rate = kN • Where k is the decay constant • The more nuclei you have the faster it decays and vice versa

Half-life • Time it takes for half of the nuclei to decay. • Half-life is always constant

First Order process and half-life MOVIE

Half-life problem • The half-life of cobalt-60 is 5.3 yr. How much of a 1000-mg sample of cobalt-60 is left after a 15.9-yr period? Every half life the initial amount is reduced by half How many half-lives is 15.9 yrs?

Half-life problem cont’ After three half-lives how much cobalt-60 will there be if we started with 1000 mg? So after 3 half-lives there will be 125 mg of 60Co The previous series is equivalent to:

Half-Life Calculations • General formula for Half-Life problems: mf= final mass mi = initial mass n = Number of Half-Lives

Number of Half-lives Answer: Another Half-Life Problem • The half-life of 131I is 0.022 yr. On July 12th there are 52.3 mg of this radioisotope. How much 131I was there on June 22nd? We must find the initial mass mi. Therefore:

Nuclear Binding Energy (Mass Defect) • Recall: The mass of the carbon-12 nucleus is exactly 12 amu. The mass of 6 protons and 6 neutrons is 12.096 amu • How can the mass of 12 subatomic particles separate be more than their mass when they combine?

Nuclear Binding Energy (Mass Defect) The mass and energy of an object are proportional. If a system loses mass it loses energy; if it gains mass it gains energy E = mc2 E = Energy m = mass c = Speed of light

Nuclear Binding Energy (Mass Defect) • In every process when energy is released, mass also changes. • In chemical reactions the change of mass is too small to be detected. • In nuclear reactions the mass change is very significant. • Because “c” is so large, even small changes of mass release huge amounts of energy

Nuclear Binding Energy (Mass Defect) • Nuclear Binding Energy: Energy required to separate a nucleus into its individual nucleons. • Mass Defect: Mass difference between a nucleus and its constituent nucleons

Finding Mass defect and binding energy for 12C The mass of one 12C atom is EXACTLY 12 amu. 12C has 6 protons and 6 neutrons. 6 x (1.0073) + 6 x (1.0087) = 12.096 u Mass Defect (Δm) = 12.096 – 12 = 0.096 u

Finding Mass defect and binding energy for 12C • To find the nuclear binding energy in Joules we must convert the mass defect to kilograms. Recall that 1 Joule = 1 kg.m2/s2 Converting to kilograms: Finding the Binding Energy “E”

Nuclear Binding Energy of Isotopes

Nuclear Fission • The splitting of a larger nucleus into smaller ones. • The splitting of the nucleus produces a mass change; therefore, energy is released. • The fission of a heavy atom like Uranium is a highly exothermic process

Nuclear Fission The splitting of Uranium produces 2 smaller nuclei and 3 neutrons. These neutrons can collide against other atoms of uranium and make them split as well.

Chain Reaction • The original fission causes 2 or 3 neutrons. These neutrons can cause 2 or 3 more fissions and produce 6 or 9 more neutrons. The process escalates quickly and violently if not controlled.

Critical Mass • A chain reaction will not occur unless a certain critical mass of fuel is present. The critical mass of 235U is about 1 kg. • If there is less than the critical mass some of the neutrons may escape and miss the nuclei of uranium and the reaction fizzles out.

Fission reactors Cadmium or boron control rods regulate the flux of neutrons

Nuclear Fusion • The joining of two lighter nuclei to form a more massive one. • When two light nuclei fuse into a heavier one a great amount of energy is released • Fusion reactions are responsible for energy produced by the Sun. • Electrostatic repulsion is the largest obstacle to overcome when trying to fuse two light atoms

Nuclear Fusion • To overcome repulsion an extreme high temperature environment is necessary. • The fusion reaction that requires the least temperature is the fusion of tritium (3H) and deuterium (2H) The temperature necessary for this reaction is 40,000,000 K

Nuclear Fusion • Such high temperatures have been achieved using atomic bombs. • Could we control fusion and harness its energy? Plasma Fusion Movie

Uses of Nuclear Energy • Nuclear Reactors (Power Plants) • Nuclear Weapons (Fission bomb, H-Bomb) • Nuclear Medicine (Drugs, Radiotherapy, Scanning) • Food irradiation

Nuclear Chemistry