7.1 Atomic Theory and Radioactive Decay Natural background radiation exists all around us. Natural background radiation exists all around us. Radioactivity is the release of high energy particles or waves that we call radiation Radioactivity is the release of high energy particles or waves that we call radiation –When atoms lose high energy particles and waves, new atoms can be formed. –Radiation can be good or bad:  X-rays, radiation therapy and electricity generation are beneficial.  High energy particles and waves can do damage to DNA in our cells.

The Electromagnetic Spectrum –Early discoveries of radiation relied on photographic equipment  Marie Curie and her husband Pierre named the energy radioactivity named the energy radioactivity Radium salts, after being placed on a photographic plate, leave behind the dark traces of radiation.

Isotopes are atoms of the same element, with a different number of neutrons. are atoms of the same element, with a different number of neutrons. –changing the # of neutrons changes the mass number  Remember: mass # = # protons + # neutrons –isotopes still have the same number of protons and the same element symbol Atomic Mass (the decimal #’s) Atomic mass = average of the mass numbers for all isotopes of an element. Atomic mass = average of the mass numbers for all isotopes of an element.

Representing Isotopes Isotopes are written two ways Isotopes are written two ways –with the mass number at the end Ex. Potassium – 40 –With its chemical symbol Ex. Mass number Atomic number

Radioactive Decay Can result in new atoms forming. Can result in new atoms forming. –Radioactivity results from having an unstable nucleus. –Radioactive decay = when nuclei break apart + release energy from the nucleus.  Radioactive decay continues until a stable element forms.  An element may have isotopes that are radioactive called radioisotopes –Ex. carbon-12, carbon-13 and carbon-14 (only C-14 is radioactive)

Uranium goes through many decay steps before it becomes stable:

Rutherford identified three types of radiation using an electric field. Rutherford identified three types of radiation using an electric field. –Positive alpha particles were attracted to the negative plate. –Negative beta particles were attracted to the positive plate. –Neutral gamma particles did not move towards any plate.

Alpha Radiation: is a stream of alpha particles, (shown as  ) is a stream of alpha particles, (shown as  ) –positively charged –weighs the most –are the same as a helium nucleus –Alpha particles are represented by the symbols  2 protons and 2 neutrons make a mass number of 4  it has a charge of 2+ because of the protons

Alpha particles are big and slow. A sheet of paper will stop an alpha particle. Example: the alpha decay of Radium Example: the alpha decay of Radium - 226

Beta Radiation: A Beta particle, , is a high energy electron. A Beta particle, , is a high energy electron. –negatively charged, and weigh less than alpha particles. –Beta particles are represented by the symbols  electrons are very tiny, so beta particles are assigned a mass of 0.  one electron gives a beta particle a charge of 1–.

Beta decay occurs when a neutron changes into a proton + an electron. Beta decay occurs when a neutron changes into a proton + an electron. –The proton stays in the nucleus, and the electron is released. Example: The beta decay of iodine It takes a thin sheet of aluminum foil to stop a beta particle. It takes a thin sheet of aluminum foil to stop a beta particle.

Gamma Radiation: Gamma radiation, , is a ray of high energy, short-wavelength radiation. Gamma radiation, , is a ray of high energy, short-wavelength radiation. –has no charge and no mass. –is the highest energy form of electromagnetic radiation. –It takes thick blocks of lead or concrete to stop gamma rays. –Gamma decay results from energy being released from a high-energy nucleus. Shows unstable nucleus for gamma decay

Often, other kinds of radioactive decay will also release gamma radiation. Often, other kinds of radioactive decay will also release gamma radiation. –Uranium-238 decays into an alpha particle and also releases gamma rays.

Nuclear Equations: are written like chemical equations, but represent changes in the nucleus of atoms. are written like chemical equations, but represent changes in the nucleus of atoms. –Chemical equations represent changes in the position of atoms, not changes to the atoms themselves. Remember: Remember: 1.The sum of the mass numbers on each side of the equation should equal. 2.The sum of the charges on each side of the equation should equal.

Take the Section 7.1 Quiz Summary Tables