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Chapter 18 The Nucleus
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Cool Nuclear Facts Nucleus has a density of 1.6x10 14 g/cm 3 Nuclear material the size of a ping pong ball would be 2.5 billion tons If the nucleus was the size of a ping pong ball the 1s orbital would be 0.3 miles away Enormous amounts of energy are within the nucleus
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Vocab Nucleons – protons and neutrons Atomic Number (Z) – Number of protons Mass Number (A) – Protons + Neutrons Isotope – Same number of protons different number of neutrons Nuclide – Specific isotope
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Notation Nuclear Symbol Hyphen Notation – Element name and mass number Carbon – 14
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Radioactive Decay Of the approximately 2000 known nuclides only 279 are stable in terms of decay In general light nuclides are stable when the n 0 p + to ratio = 1 Heavier nuclides require a slightly higher ratio No nuclide with more than 84 protons is stable
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Cont. Certain combinations of protons and neutrons are more stable too ProtonsNeutrons# of Stable isotopes Even 168 EvenOdd57 OddEven50 Odd 4
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Types of Decay Radioactive decay occurs to produce a more favorable neutron to proton ratio Almost all change the number of protons Therefore the element changes Mass is always conserved
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Alpha Emission Alpha particle (α) is a helium nucleus Usually in very heavy nuclides Mass Number decreases The n 0 to p + ratio increases
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Beta Emission Beta particle (β) is an electron emitted from the nucleus Neutron is converted into a proton and electron Mass Number stays the same n 0 to p + ratio decreases Carbon – 14 is a beta producer
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Positron Emission Positrons are particles with the same mass as an electron but with a positive charge Proton is converted into a neutron and a positron Mass Number stays the same n 0 to p + ratio increases
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Electron Capture Inner orbital electron is captured by the nucleus Combines with a proton to form an neutron Mass Number stays the same n 0 to p + ratio increases
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Gamma Emission Gamma rays ( γ ) are high energy electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground state. Usually follows other types of decay
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Decay Series Often times many decays are necessary to produce a stable isotope Original nuclide is called the “Parent” Resulting nuclides are called the “Daughters”
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Homework P. 906 #’s 10,11,13,18
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Penetrating Ability Different emissions have different penetrating abilities Alpha particles can be stopped by skin Beta can penetrate about 1cm Gamma is the highest energy –Needs a lot of lead
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Effect of Radiation Different particles ionize molecules in the body differently Alpha particles ionize molecules easily –Ingesting would be very bad Gamma not so much
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Measurement of Radiation Energy of radiation measured in rads –1 rad = 1x10 -2 J However different isotopes have different effects on humans so rems are used too The average human receives 193mrem/yr On short term exposure 500 rem causes death in half the population in 30 days
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Detection of Radioactivity Measured with a Geiger counter. Uses the fact the radioactive particles ionize atoms Chamber increases in conductivity when radiation is present Measures radioactivy in counts
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Radioactive Decay Kinetics ALL radioactive decay follows first order kinetics Just like chapter 12 There is a rate and a half life Rate changes half life remains the same
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1 st Order Kinetics ln N = -kt + ln N o N Number of nuclides or counts at time t N o Number of nuclides or counts at t=0 k is the rate constant This is the equation of a ________? –LINE –Y axis is ln N, X axis is t –The slope is the rate constant
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1 st Order Half Life The half life is the time required for a reactant to reach half of its previous concentration Meaning N = N o /2 t 1/2 =ln2/k t 1/2 =.693/k Half life is always the same for first order kinetics
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#27 page 907
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Radioactive Dating Scientists can use the half lives of nuclides to determine the age of samples Carbon -14 has a half life of 5730 years Every 5730 years half of the original sample is gone.
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Homework P. 907 #’s 23,25,28
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