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Physical Science II

by: Amaya McDermott

Physical Science II SCI 3120

Amaya McDermott
Clayton State
GPA 3.54

Harold Banke

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Harold Banke
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This 6 page Class Notes was uploaded by Amaya McDermott on Monday October 5, 2015. The Class Notes belongs to SCI 3120 at Clayton State University taught by Harold Banke in Fall. Since its upload, it has received 41 views. For similar materials see /class/219550/sci-3120-clayton-state-university in Science at Clayton State University.


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Date Created: 10/05/15
Chapter 7 The Nucleus More than 2000 years ago ancient Greek philosophers pondered the nature of matter They noted that some materials seemed to be mixtures of more elementary matter and some materials seemed to be pure substances Salt seems to disappear when mixed with water but when saltwater is heated the water seems to disappear and crystals of salt remain so saltwater is not an elementary material but is a mixture Gold on the other hand comes from a mineral mixture called an 0re When the ore is heated liquid gold comes out while the rocky material remains more or less the same Gold can be heated and solidified over and over without changing in any way Gold seems to be an elementary material Some Greek philosophers wondered how many times a piece of gold could be cut and still be gold In other words if a piece of gold is cut in two the two pieces obviously are gold but when those pieces are cut would there eventually be a piece that would be the smallest piece possible and would therefore be uncuttable Some philosophers suspected that there was in fact a smallest particle for each kind of pure matter and called it an atom not cuttable The saltwater solution could be explained by the particle theory of matter as could the evaporation of perfume The alternative theory was the continuous theory of matter The atomic theory of matter was the result of philosophical speculation and it was not until the 1840 s that chemists began to use the atomic theory to explain why materials combined in definite proportions instead of random proportions When hydrogen and oxygen are mixed for example only a certain number of grams of hydrogen will combine with any particular amount of oxygen to produce water It was not until the 1980 s that atoms could be seen with microscopes Scientist do not need to see something to believe it they need to see an idea work to explain something like how materials react with each other By 1911 the atomic theory was widely accepted and several things were know about atoms 1 They are very small 2 They are electrically neutral 3 They contain electrons The electron had been discovered by an English scientist named J J Thomson in 1897 Since Thomson knew the atom is neutral he speculated that the atom must contain positive matter of some sort as well as negative electrons and suggested that the electrons are imbedded in the positive matter in a mixture resembling a plum pudding or fruit cake as we would call it Figure 71 Rutherford model of the atom The most direct way to probe the structure of a fruitcake would be to stick a finger in it as a probe In 1911 a New Zealand scientist named Ernest Rutherford proposed an experiment to probe the structure of an atom A few years earlier it was discovered that some minerals emit a stream of very small particles called alpha particles These particles are about 8000 times more massive than electrons but still less massive than most atoms Rutherford believed that he could send these particles into atoms to test their density and internal structure The Rutherford ExperimentExplain Figure 72 Figure 73 Analogy with star entering the solar system atom is relatively more empty Nuclear structure Eventually it was discovered that the positive matter in the nucleus consists of particles called protons Protons have exactly the same electric charge as electrons but a mass that is 1836 times more than that of an electron In time it was discovered that the nucleus also contains neutral particles called neutrons These have slightly more mass than protons 1839 times that of the electron Each elementary material called an element is composed of atoms whose nuclei all contain exactly the same number of protons The number of protons is called the atomic number of the element Figure 74 The number of protons determines the number of electrons The number of electrons determines most of the properties of an element There are over 100 different atomic numbers thus over 100 elements At room temperature and normal air pressure 11 elements are gases 2 are liquids and the rest are solids Isotopes All atoms of an element have the same number of protons but not necessarily the same number of neutrons Atoms of the same element with different numbers of neutrons are called isotopes of the element Example hydrogen deuterium tritium Figure 75 A nucleus with a particular composition is called a nuclide Each nuclide is represented by a symbol AZX X atomic symbol Z atomic number A mass number Mass number is the number of nucleons in the nucleus A common nuclide of chlorine is 3517Cl This same nuclide could also be called Cl35 Radioactivity In 1896 Henri Becquerel was experimenting with the element uranium He put some uranium ore in a desk drawer along with some photographic film The film was wrapped in paper to keep it from being exposed but when Becquerel opened the paper he found that the film was exposed anyway He wondered if the uranium had anything to do with it and eventually discovered that uranium gives off some kind of invisible radiation that can penetrate paper expose film ionize gases and cause zinc sulfide to glow in the dark The property of uranium which allowed it to do these things was called radioactivity This term was invented by a scientist named Marie Curie Marie Curie and her husband Pierre Curie worked in the same laboratory as Becquerel They worked on an ore called pitchblende and eventually discovered 2 new elements in the ore They named one of the elements polonium after Marie s native country Poland The second element was called radium because it was extremely radioactive Scientists discovered that the rate of radiation was unaffected by chemical reactions heating or cooling It was apparent that the radioactivity must be coming from something deep inside the atom that is not affected by external conditions Radioactivity comes from the nucleus Some nuclides are inherently unstable and have a tendency to rearrange themselves in some way This process leads to the release of energy or particles that is the basis of radioactivity There are about 7000 theoretical combinations of nucleons or 7000 possible nuclides About 2000 nuclides have actually been found or created in the lab Only about 256 nuclides are stable and not radioactive Even so most elements in nature do not have radioactive isotopes Some elements have some radioactive and some nonradioactive And some elements with large atomic numbers such as uranium have all radioactive isotopes Scientists can artificially produce radioactive isotopes of certain elements which are not usually radioactive These isotopes can be placed in a person s body where they will behave like nonradioactive atoms but their radioactivity will allow doctors to follow their paths in the body These isotopes are called tracers Figure 76 Radioactive decay Early experimenters found that a magnetic field can separate the radiation from highly radioactive elements such as radium into 3 components Figure 77 The radiation that behaves as a positive particle would behave is called alpha radiation which is composed of a stream of alpha particles We now know that these particles are composed of 2 protons and 2 neutrons just like a nuclide of Helium Sometimes alpha particles are called helium nuclei but in fact alpha particles usually do not come from a sample of helium at all The radiation that behaves as a negative particle would behave is called beta radiation which is composed of a stream of beta particles Beta particles seem to be identical to electrons but these particles come from the nucleus not the electron cloud The radiation that is unaffected by a magnetic field is obviously uncharged However this radiation does not consist of particles not even neutrons This radiation consists of highfrequency EM radiation called gamma radiation Why decays occur A nuclide that is radioactive is said to undergo a decay because it changes in some way and releases radiation in the process of change Figure 78 Gamma decay nucleus has excess potential energy Alpha decay nucleus is too large Beta decay nucleus has too many neutrons relative to protons Electron capture nucleus has too many protons relative to neutrons Positron emission nucleus has too many protons relative to neutrons Neutrons outside the nucleus are unstable They decay into a proton and an electron However a neutron is not a combination of a proton and an electron It is a unique particle and it cannot be formed by trying to combine the proton and electron A hydrogen atom would be the result of a forced combination Apparently a proton is stable even outside the nucleus When a decay occurs we can usually predict the changes that occur in the original nuclide Example 71 Example 72 Halflife Unstable nuclides are called radionuclides Each radionuclide decays at a characteristic rate The rate is indicated by a unit called halflife the time needed for one half of the atoms in a sample to decay Figure 710 The halflife is based on the structure and composition of the nucleus and is not affected by external events Some halflives are as short as a millionth of a second and others are billions of years long One of the biggest problems related to nuclear power is the problem of waste disposal since the waste is still radioactive even though it can no longer boil water Radiometric dating The slow steady decay of radionuclides forms the basis of a technique for measuring the age of materials Radiometric dating uses information about the rate of decay of radionuclides By measuring the concentration of the products of radioactive decay and comparing these products to the concentration of the original radioactive material we can calculate how long a radioactive substance has been decaying Much more will be discussed in Earth Science Radiation hazards All radiation from radioactive materials is powerful enough to ionize the matter it passes through Radiation dosage is measured in sieverts or millisieverts where l Sv is the amount of any radiation that has the same biological effects as those produced when 1 kg of body tissue absorbs l joule of Xrays or gamma rays A similar unit is called a rem or millirem This unit is somewhat outmoded but is still used All persons are eXposed to radiation Figure 71 1 The average dose is about 36 mSvy The dosage varies with location In places where there is a lot of granite the dosage is higher because granite often contains uranium At high altitudes people are more likely to be irradiated by cosmic radiation In fact astronauts may receive as much as l mSv per day Xrays are an important part of the radiation dosage of human beings The necessity of Xrays must always be balance against their risk of harm This is especially true of the concentrated form of Xrays called CT scans computed tomography which are equivalent in dosage to hundreds of ordinary chest Xrays


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