Radioactivity Nuclear Chemistry

Discovery of Radioactivity Wilhelm Roentgen discovered x-rays in Henri Becquerel discovered that uranium salts produced similar emissions. Marie and Pierre Curie isolated the source on the emissions in Becquerel's sample—uranium. Marie Curie named the process of emitting rays— radiation.

Changes in the Nucleus A change in the nucleus of an atom is called a nuclear reaction. These reactions are associated with radioactivity – the emission of radiation in the form of rays and particles. Nuclear reactions occur because of nuclei that are unstable. Unstable isotopes are referred to as radioisotopes.

When is a nucleus stable? The strong nuclear force holds the protons and neutrons together in the nucleus of an atom. The electrostatic charge of the protons is repulsive, but the presence of the neutrons adds a stronger attractive force. To be stable, a nucleus must have from 1 to 1.5 neutrons for each proton. Atoms with more protons need the higher number of neutrons.

Transmutation The changes in the nucleus can sometimes transform an element into a different element The result = change in atomic number An element is considered radioactive because it releases Energy during transmutation

Artificial Transmutation Change in nucleus occurs when bombarded by an alpha particle or neutron Natural Transmutation Change in nucleus occurs spontaneously The ONLY reactant is the atom undergoing transmutation Ca-44 + n Ca U-238  Th 

Stability of Nuclei Most nuclei are stable ex. C-12 As atoms increase in size the ratio of neutrons to protons increases the larger the ratio the more unstable the atom ex. C-14 All nuclei with atomic numbers > 83 are unstable and radioactive

Unstable nuclei will spontaneously decay in order to form a stable nucleus

Types of Radiation NameSymbolMass (amu) ChargeDescription Alpha  42+a helium nucleus Beta  1/18401-electrons Gamma  00short wavelength electromagnetic waves

Radioactive Shielding

Alpha Particles  or He helium nucleus Heaviest Travels slowest & least amount of distance Poor penetrating power MOST DAMAGING 4242

Alpha Decay A type of radioactive decay where an atomic nucleus emits (releases) an alpha particle (2 p + and 2 n o ). The atom reduces its atomic number by 2 and its mass number by 4. This transforms the atom into a different element U Th 4 2 He

Alpha Decay U He Th Alpha Particle is one product mass and charge must be conserved

Beta Particles ß - Electrons travel near the speed of light moderate penetrating power can be stopped by aluminum e 0

Beta Decay A type of radioactive decay where a beta particle (electron or positron) is emitted. This causes a neutron to be turned into a proton.

Beta Decay

Gamma Radiation  No mass no charge similar to x-rays Fastest type of radiation Strongest penetrating power not very dangerous

Where did they come from? Uranium Decay Atoms of U-238 go through a series of decays to make Thorium Th Radon Rn Radium Ra Lead Pb Example of natural decay

 ßß      ßß ßß  

Radioactive Decay Rates Over time, radioisotopes will decay into stable atoms. The time this takes is measured in half- lives. A half-life is the time it takes one-half of radioisotope to turn into stable atoms.

Radioactive Decay Rates (cont’d) Example: 20 g of the strontium-90 will decay into 10 g over 29 years. Amount remaining = (Initial amount)(½) t/T t = total timeT = half-life

Number of half-lives Radioisotope remaining (%) Initial amount of radioisotope t 1/2 After 1 half-lifeAfter 2 half-lives After 3 half-lives Half-life of Radiation