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Lesson Objectives. Be able to define radiation Understand what are isotopes Understand what is radioactive decay Understand three kinds of radiation identified by Rutherford and be able to use them in nuclear equations. Open Your Textbook to p. 284!.
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Lesson Objectives Be able to define radiation Understand what are isotopes Understand what is radioactive decay Understand three kinds of radiation identified by Rutherford and be able to use them in nuclear equations (c) McGraw Hill Ryerson 2007
Open Your Textbook to p. 284! How much radiation is a “safe” amount for humans? For how long might the radioactive material stay in this person’s body? How do medical professionals shield themselves from radiation? Why might iron-59 be used in the body, rather than some other radioisotope?
7.1 - Atomic Theory and Radioactive Decay • Radioactivity is the release of high energy particles or waves of energy from a substance caused by changes in the nuclei of the atoms that make up the substance. • Exposure to radioactive materials can be beneficial / harmful. • beneficial: X-rays, radiation therapy, to generate electricity • harmful: high energy particles and waves can do damage to cells’ DNA
Wavelength …is the distance from the crest of one wave to the crest of the next. …is the length of one wave cycle. λ represents wavelength measured in some factor of metres
Frequency …refers to the number of wave cycles passing a fixed point per unit of time. frepresents frequency measured in Hertz (Hz)
The Electromagnetic Spectrum …the higher the frequency, the shorter the wavelength!
Electromagnetic Radiation also known as an electromagnetic wave consists of an electric field and a magnetic field oriented at right angles to one another and to the direction of propagation of the wave both fields travel at the speed of light (c)! electromagnetic radiation comes in discrete “packets” known as photons
The Visible Spectrum • …consists of the colours: • R – red • O – orange • Y – yellow • G – green • B – blue • I – indigo • V – violet • …can be seen when white light is passed through a prism, which bends light in different amounts, according to wavelength. • The human eye responds to wavelengths between approximately 380 nm – 750 nm.
Searching for “Invisible Rays” • Radiation is everywhere, but can be difficult to detect. • Roentgen named X-rays with an “X” 100 years ago because they were previously unknown. • Becquerel realized that uranium salts spontaneously emitted seemingly invisible energy that darkened photographic plates. • Marie and Pierre Curie isolated the radioactive particles (uranium atoms) in Becquerel’s mineral sample. They named this energy radioactivity. http://www.brainpop.com/science/energy/mariecurie/ • Devices such as the Geiger-Müller counter eventually replaced photographic plates. The counter measures radioactivity more precisely. Radium salts, after being placed on a photographic plate, leave behind the dark traces of radiation.
Isotopes and Mass Number • Isotopes are different atoms of a particular element that have the same # of protons, but different #s of neutrons. • Isotopes of a particular element… • have the same symbol • occupy the same ‘box’ on the PT • have the same # of protons - and therefore the same atomic number - as each other • have different mass numbers, since they have different #s of neutrons • In nature, most elements are a mixture of isotopes.
Isotopes and Mass Number Recall… • Mass number is a whole number and represents: the # protons + # neutrons in an atom • Mass number can be used to tell isotopes of the same element apart • Atomic mass = proportional average of the mass numbers of all isotopes of an element. • 19.9% of boron atoms have 5 neutrons, 80.1% have 6 neutrons • 19.9% have a mass number of 10, and 80.1% have a mass number of 11 • (0.199 x 10) + (0.801 x 11) = 10.801 = atomic mass of boron http://www.brainpop.com/science/matterandchemistry/isotopes/
Representing Isotopes • Isotopes are written using standard atomic notation. • Chemical symbol, atomic number, mass number • Potassium has three isotopes: • K is found in nature in a certain ratio of isotopes • 93.26% is potassium-39, 0.01% is potassium-40, and 6.73% is potassium-41 • Atomic mass = (0.9326 x 39) + (0.0001 x 40) + (0.0673 x 41) = 39.1347
Radioactive Decay • Unlike all previously discovered chemical reactions, radioactivity sometimes results in the formation of completely newatoms! • Radioactive atoms emit radiationbecause their nuclei are unstableand are likely to decay. • These nuclei gain stability by losingenergy • They lose energy by emitting radiation = undergoing radioactive decay
Radioactive Decay • Radioactive atoms release energy until they form stable, non-radioactive atoms, usually of a different element. • Radioisotopes are isotopes of an element that are capable of radioactive decay. http://www.brainpop.com/science/energy/radioactivity/ Radioisotope uranium-238 decays in several stages until it finally becomes lead-206
Three Types of Radiation • Rutherford identified 3 types of radiation using electrically charged plates. • +ve alpha particles were attracted to the -ve plate. • -ve beta particles were attracted to the +ve plate. • Electrically neutral gamma radiation passed through undeflected. Beta particles (-ve charge) Alpha particles (+ve charge)
Alpha Radiation Alpha radiation is a stream of alpha particles, . • +vely charged • more massive than beta particles or gamma radiation • the same combination of particles as the nucleus of a helium atom • represented by the symbols: • an alpha particle has 2 neutrons and 2 protons, therefore has a charge of 2+ • The release of alpha particles is called alpha decay. • Alpha particles are s l o w and fairly massive, therefore are not very penetrating. • A sheet of paper will stop an alpha particle!
Alpha Decay of the Nucleus of Radium-226 Radium-226 releases an alpha particle and becomes Radon-222. Radon has 2 less protons than radium. Note that the nuclear rxn. is balanced!
Beta Radiation Beta radiation, , is an electron. • A beta particle… • is represented by the symbol: • is assigned a mass of 0 since the mass of an e- is so small • has a charge of 1–, since there is only 1 e- • are fairly lightweight and fast-moving. • It takes a thin sheet of metal foil to stop a beta particle!
Beta Decay In beta decay, a neutron changes into a proton & an electron. • The proton stays in the nucleus (therefore the atomic number increases by 1 – it becomes an atom of another element!), and the high-energy electron is ejected. • Mass number stays the same, since the new proton replaces the original neutron.
Beta Decay • Iodine-131 releases a beta particle and becomes Xenon-131. • mass # stays the same • atomic # increases by 1 • A neutron has turned into a proton and the electron is released from the nucleus.
Gamma Radiation Gamma radiation, , is… • represented by the symbol: • high-energy, short-wavelength radiation (photons) • has no charge and no mass (release of gamma radiation does not change mass # or atomic # of a nucleus) • the highest energy form of EM radiation • the most dangerous form of EM radiation • It takes thick blocks of lead or concrete to stop gamma rays!
Gamma Decay • Gamma decay results from redistribution of energy within the nucleus • a high-energy gamma ray is released from the nucleus when an isotope goes from a high to a lower-energy state • * represents “extra energy” in the nucleus • high-energy nickel-60 undergoes gamma decay to become nickel-60 • Other kinds of radioactive decay can also release gamma rays. • Uranium-238 decays into an alpha particle and also releases gamma rays.
Nuclear Equations for Radioactive Decay A nuclear equation… • is written like a chemical equation, but represents changes in the nuclei of atoms during a nuclear rxn. • A chemical equation represents changes in the position of atoms; not changes to the atoms themselves.
