Nuclear Decay Radiation Nuclear breakdown Chapter 4 section 4
Introduction to Nuclear Chemistry Nuclear chemistry is the study of the structure of atomic nuclei and the changes they undergo.
The Discovery of Radioactivity (1895 – 1898): • Roentgen found that invisible rays were emitted when electrons bombarded the surface of certain materials. • Becquerel accidently discovered that phosphorescent uranium salts produced spontaneous emissions that darkened photographic plates
The Discovery of Radioactivity (1895 – 1898): • Marie Curie isolated the components (uranium atoms) emitting the rays • identified 2 new elements, polonium and radium on the basis of their radioactivity • These findings contradicted Dalton’s theory of indivisible atoms.
radioactivity • Process of emitting radiation • Radiation- rays and particles emitted by the breakdown of atomic nuclei • Chemical reactions do not change the atomic number (# of protons) in an atom. • Nuclear reactionsDO change the atom’s nucleus, changing the number of protons, which changes the chemical identity of the atom from one element to another.
radioisotope • Isotopes of elements that are radioactive • Example: carbon-14 is a radioactive isotope of carbon
Radioactive decay • Process where unstable radioactive atoms give off radiation until they form stable nonradioactive atoms, usually of a different element. • Half-life- the amount of time needed for 1/2 ½of a radioactive sample to decay and become nonradioactive
Types of radiation • Alpha- made up of 2 protons and 2 neutrons • Has a +2 charge • α • Penetrating power – low (0.05 mm body tissue) • Shielding – paper, clothing • Beta- a high-speed electron • Has a -1 charge • β • Penetrating power – moderate (4 mm body tissue) • Shielding – metal foil • Gamma- high-energy radiation, wave not particle with no charge • γ • Penetrating power – high (penetrates body easily) • Shielding – lead, concrete
Alpha decay • Composition – Alpha particles, same as helium nuclei • Changes mass number- reduces by 4 • Changes atomic number- reduces by 2 • Alpha decay of radium-226
Beta decay • Composition – Beta particles, same as an electron • Does not change mass number • Changes atomic number- adds 1 • Beta decay of carbon-14
Gamma radiation Is usually present in alpha and beta decay but since it does not affect mass number or atomic number, it is not usually shown
Review 4 2 0 -1
Nuclear stability • Nucleus held together by strong nuclear force that acts on the neutrons and protons. • The number of neutrons, in part, determines the nuclear stability. • Strong nuclear force acts on the neutrons without interference from the repulsive force of like charges.
Proton to neutron ratio • For small nuclei: <20, a ratio of 1:1 is best • For mid-size nuclei, the ratio slowly increases to a 1.5 : 1 ratio around element 83 • All elements with atomic numbers greater than 83 are radioactive, no matter how many neutrons they have
Half-life problems • The amount of time it takes for ½ ½1/2 of a radioactive sample to break down into stable, non-radioactive material • Mass must be determined in each half-life separately! • 100 g > 50 g > 25 g > 12.5 g > 6.25 g • 4 half-lives have passed