Unit 13 - Nuclear Chemistry

Background

• 1896—Henri Becquerel discovered radioactivity; was studying the ability of uranium salts exposed to sunlight to fog film plates—discovered during a storm that even those not exposed to the sun caused the film to fog

The Curies•Marie and Pierre Curie—showed that this fogging was caused by rays emitted by the Uranium atoms—called it radioactivity• Nobel Prize in Physics and Chemistry• 1934—Marie died from leukemia caused by long term

exposure to radiation

• Radioactivity—spontaneous emission of energy and particles by atoms of certain elements that produces new elements

• Radiation—penetrating rays and particles to attain more stable atomic configuration

• Radioisotope—radioactive atom—unstable isotope; gains stability by undergoing change which releases large amounts of energy—nuclear reaction

• Radioisotope undergoes drastic change as it emits radiation

• Nuclei become unstable• Loses energy by emitting radiation during

radioactive decay• Eventually, unstable radioisotopes of 1 element

are transformed into stable isotopes of a different element

• This process is spontaneous

Nuclear vs. Chemical Rxn

• Not affected by • Change in temperature• Change in pressure• Presence of catalysts

• Cannot be• Sped up• Slowed down• Turned off

Three most common types

• 1899-Ernest Rutherford discovered that uranium compounds produce three different kinds of radiation

• Alpha, α• Beta, β• Gamma, γ

Alpha, α• Nucleus emits an alpha particle

• essentially a He nucleus—two protons and two neutrons—charge is 2+

• Because of mass and charge, they do not travel far and are not very penetrating• Paper can stop alpha particles

Beta, β• Fast moving electron emitted

from the decomposition of a neutron in an unstable nucleus• Neutron decomposes into a proton and an

electron

• Beta particles are light weight and fast moving• Greater penetrating power• Blocked by metal foil

Gamma, γ

• High energy electromagnetic radiation• No mass• No electrical charge

• Emission of gamma rays does not change the atomic number or mass number of a nucleus• Lead or concrete will not

completely block it

Half life

• Rate of decay—half life—time required for half of the nuclei of a radioisotope sample to decay to products• after 1 half life, ½ of the atoms remain; ½ decayed• after 2 half lives, ¾ of the atoms have decayed

• Half life can be extremely short or long• Uranium 238 has a half life of 4.5 billion years

• Used in artifact aging• Carbon 14 dating

Fission • Splitting of the nucleus into smaller

fragments—caused by neutron bombardment in a continuous chain reaction

• Releases LARGE amounts of energy

Uncontrolled chain reactions—atomic bomb

Controlled fission

• Nuclear power plants—use fission to produce useful energy, released as heat. Coolant, usually liquid Na or water, removes the heat and uses it to generate steam, which drives turbine and produces electricity• Nuclear waste—spent fuel rods—have

extremely long half lives so storage is key issue

Fusion

• Nuclei combine to produce a nucleus of greater mass

• Solar fusion—Hydrogen nuclei fuse to make He

• Fusion rxns tend to release large amounts of energy but occur only at extremely high temperatures, 40M °C

• Use of controlled fusion as an energy source on Earth is very appealing • Inexpensive• Products are not radioactive

• Problem lies in achieving the high temps needed to start the rxn• At the temps required, matter exists as

plasma, therefore difficult to contain

Radiation

• Ionizing radiation—radiation with enough energy to knock electrons off some atoms of the bombarded substance to produce ions• Cannot be detected by senses, must

use other device• Geiger counter• Scintillation counter• Film badge

Uses of radiation• Tracers—study reactions, test effects of

pesticides, etc, diagnose disease• Cancer treatment—selective destruction

of cancer cells• Pharmaceuticals• NMR (nuclear magnetic resonance)/

MRI (magnetic resonance imaging)

Effects of Radiation

• Exposure to alpha rays is not generally dangerous

• Beta rays—can cause severe burns upon prolonged exposure

• Gamma rays that penetrate skin can cause serious damage to cells and tissues• Excessive exposure can lead to severe

illness, and a greatly increased probability of developing cancer, sterility or even death

Radiation Burns