Chapter 10 Nuclear Changes

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Presentation transcript:

Chapter 10 Nuclear Changes

What is Radioactivity? Certain isotopes of many elements undergo a process called radioactive decay During radioactive decay, the unstable nuclei of these isotopes emit particles, or release energy, to become stable isotopes The released energy and matter is referred to as nuclear radiation After radioactive decay, the element changes into a different isotope of the same element or into an entirely different element

What is Radioactivity? Recall that isotopes of an element are atoms that have the same number of protons but different numbers of neutrons in their nuclei Different elements are distinguished by having different numbers of protons

What is Radioactivity? Four types of nuclear radiation Alpha particles Beta particles Gamma rays Neutrons

What is Radioactivity?

What is Radioactivity?

What is Radioactivity? Alpha particles Made of 2 protons and two neutrons Same as the helium nuclei Positively charged More massive than any other type of nuclear radiation Do not travel far through materials Barely pass through a sheet of paper Can ionize other atoms thus losing energy

What is Radioactivity? Beta particles Often fast-moving electrons but may also be positively charged particles called positrons Positrons have the same mass as electrons Neutrons, which are not charged, decay to form a proton and an electron The electron, which has a very small mass, is then ejected at a high speed from the nucleus as a beta particle Pass through a piece of paper , but most are stopped by 3 mm of aluminum or 10 mm of wood Not as massive as alpha particles Can ionize other atoms thus losing energy

What is Radioactivity? Gamma rays Not made up of matter and do not have an electric charge Form of electromagnetic energy Consist of energy packets called photons Like light and X-rays Have more energy than light or X-rays Can ionize and cause damage in matter Can penetrate up to 60 cm of aluminum or 7 cm of lead Greater damage to health

What is Radioactivity? Neutron radiation No charge Therefore, do not ionize matter Therefore, they are able to travel farther through matter than alpha and beta particles A block of lead about 15 cm thick is required to stop most fast neutrons

What is Radioactivity? Anytime that an unstable nucleus emits alpha or beta particles, the number of protons and neutrons change Example: radium-226 (an isotope of radium with the mass number 226)changess to radon-222 by emitting an alpha particle

What is Radioactivity? Gamma decay changes the energy of the nucleus No change in the atomic number of the element

What is Radioactivity? Beta Decay: Loses electron Atomic number changes (increase of one) Mass number before and after the decay does not change The atomic number of the product nucleus increases by one (neutron changes into a proton) Example: carbon-14 changes into nitrogen-14

What is Radioactivity? Alpha Decay: Loses 2 protons and 2 neutrons (helium nuclei) Both atomic mass and number change’

What is Radioactivity?

What is Radioactivity? What do the colors mean? Zone 1 counties have a predicted average indoor radon screening level greater than 4 pCi/L (picocuries per liter) (red zones)Highest Potential Zone 2 counties have a predicted average indoor radon screening level between 2 and 4 pCi/L (orange zones)Moderate Potential Zone 3 counties have a predicted average indoor radon screening level less than 2 pCi/L (yellow zones)Low Potential

PHILADELPHIA COUNTY

MONTGOMERY COUNTY

BUCKS COUNTY

RADON

RADON

RADON

What is Radioactivity? Radioactive Decay Rates Half-Life: time in which half of a radioactive substance decays Last from a nanosecond to billions of years Used to diagnose medical problems Detector follows the element as it moves through the patient’s body Predict the age of fossils/rocks

What is Radioactivity?

What is Radioactivity?

What is Radioactivity? Radium-226 has a half-life of 1,599 years. How long will 7/8 of a sample of radium-226 take o decay? Fraction remaining 1/8 Amount of sample remaining after 3 half-lives ½ x ½ x ½ = 1/8 3 half-lives X 1,599 years/half-life = 4,797 years

What is Radioactivity? Radioactive decay is exponential decay Example: Carbon-14

What is Radioactivity?

Nuclear Change

Nuclear Change The stability of a nucleus depends on the nuclear forces that hold the nucleus together. These forces act between the protons and the neutrons Like charges repel, so how can so many positively charged protons fit into an atomic nucleus without flying apart? Strong nuclear forces cause neutrons and protons to attract one another—this attraction is much stronger than the electric repulsion between protons

Nuclear Change Nuclei with more than 83 protons are always unstable These nuclei will always decay and, in the process, release large amounts of energy and nuclear radiation

Nuclear Change Nuclear fission: process of splitting heavier nuclei into lighter nuclei Neutrons and energy are released Some mass will be changed into energy The equivalence of mass and energy is explained by the Theory of Relativity (Einstein) Energy= mass X (speed of light)2 = mc2 Under ordinary conditions this change does not happen spontaneously

Nuclear Change Neutrons released by fission can start a chain reaction (one reaction triggers another and so on) Continuous series of nuclear fission reactions

Nuclear Change Critical mass: the minimum mass of a fissionable isotope that provides the number of neutrons needed to sustain a chain reaction In nature the concentration is low Chain reactions can be controlled to be used to generate electricity Control rods regulate fission by slowing the chain reaction

Nuclear Change

Nuclear Change Nuclear Fusion: energy can be obtained when very light nuclei are combined to form heavier nuclei Example: Sun: energy is formed primarily when hydrogen nuclei combine releasing energy Large amount of energy is needed to start a fusion reaction Reason is that all nuclei are positively charged and repel one another with electric force Energy needed to overcome this force (Stars: the extreme temperature overcomes this force)

Nuclear Change

Nuclear Radiation Today We are continually exposed to radiation from natural sources (sun, soil, rocks, plants)(80%) Human-made sources (computer monitors, smoke detectors, X-rays, etc.)(20%) Measured in units called rems (1rem = 1,000 millirems) Dental X-ray= 1 millirem Safe limit for workers is about 5,000 rems per year

Nuclear Radiation Today Exposure varies from location to location, activities (jobs/recreational), habits (smoking), home to home (smoke detectors: emit alpha particles)