Subatomic Physics Section 1 Preview Section 1 The Nucleus Section 2 Nuclear Decay Section 3 Nuclear Reactions Section 4 Particle Physics
Subatomic Physics Section 1 TEKS The student is expected to: 5A research and describe the historical development of the concepts of gravitational, electromagnetic, weak nuclear, and strong nuclear forces 5H describe evidence for and effects of the strong and weak nuclear forces in nature 8C describe the significance of mass-energy equivalence and apply it in explanations of phenomena such as nuclear stability, fission, and fusion
Subatomic Physics Section 1 What do you think? What holds a nucleus together? What particles exist within the nucleus? What force(s) exist between these particles? Are these forces attractive or repulsive?
Subatomic Physics Section 1 The Nucleus The chemical symbol for an element is written like the one shown to the left. What information is provided by this symbol? The atomic number (Z) or number of protons is 13. The mass number (A) or number of protons + neutrons is 27. The number of neutrons (N) is 14 (27 13). The element is aluminum.
Subatomic Physics Section 1 Isotopes Isotopes are atoms of the same element with different atomic masses. The number of neutrons is different. Most carbon nuclei have 6 protons and 6 neutrons and an atomic mass of 12. Called carbon-12 Others have 5 neutrons (carbon-11), 7 neutrons (carbon-13), or 8 neutrons (carbon-14).
Subatomic Physics Section 1 Isotopes Click below to watch the Visual Concept. Visual Concept
Subatomic Physics Section 1 Nuclear Mass The density of the nucleus is approximately 2.3 10 17 kg/m 3. Mass is measured in unified mass units (u). 1 u is one-twelfth the mass of one atom of carbon-12. 1 u = 1.6605 10-27 kg Protons and neutrons each have a mass of approximately 1 u.
Subatomic Physics Section 1 Nuclear Mass Find the energy equivalent of 1 u in both J and ev. (For c, use the value 2.9979 10 8 m/s.) Answers: 1.4924 10-44 J, 931.47 10 6 ev or 931.47 MeV With more significant figures, 1 u = 931.49 MeV. The mass of subatomic particles is often expressed in MeV.
Subatomic Physics Section 1 Nuclear Mass This table provides the mass and rest energy of atomic particles in kilograms, unified mass units, and MeV.
Subatomic Physics Section 1 Nuclear Stability What type of electric force would exist in the nucleus shown? Protons would repel other protons very strongly because the distance between them is small. Neutrons would produce no forces. What holds the nucleus together? A force called the strong force: a powerful attractive force between all particles in the nucleus Does not depend on charge Exists only over a very short range
Subatomic Physics Section 1 Nuclear Stability As more protons are added to the nucleus, more repulsion exists. Larger and larger nuclei require more neutrons, and more strong force, to maintain stability. Look at a periodic table to find out which elements have approximately a 1:1 ratio between neutrons and protons, and which elements have the highest ratio of neutrons to protons.
Subatomic Physics Section 1
Subatomic Physics Section 1 Nuclear Stability and Ratio of Neutrons and Protons Click below to watch the Visual Concept. Visual Concept
Subatomic Physics Section 1 Binding Energy The nucleons (protons and neutrons) have a greater mass when unbound than they do after binding to form a nucleus. Called binding energy This energy is released when the binding occurs, and must be absorbed to separate the nucleons.
Subatomic Physics Section 1 Classroom Practice Problem The mass of the individual particles in an atom is the mass of the protons, neutrons, and electrons. For the mass of the protons and electrons combined, simply multiply the atomic number times the mass of a hydrogen atom (1 electron bound to 1 proton). Find the binding energy (in u and MeV) for a helium atom with two protons and two neutrons. The atomic mass of helium-4 is 4.002602 u. Answer: 0.030378 u or 28.297 MeV
Subatomic Physics Section 1 Now what do you think? What holds a nucleus together? What particles exist within the nucleus? What forces exist between these particles? Are these forces attractive or repulsive? What happens to each of these forces when the particles are farther and farther apart? What is meant by the term binding energy?
Subatomic Physics Section 2 TEKS The student is expected to: 8D give examples of applications of atomic and nuclear phenomena such as radiation therapy, diagnostic imaging, and nuclear power and examples of applications of quantum phenomena such as digital cameras
Subatomic Physics Section 2 What do you think? Often scientists use radioactive carbon dating to determine the age of fossils. What does the term radioactive mean? Are all atoms radioactive? If not, how are radioactive atoms different from those that are not radioactive? How can radioactivity be used to determine the age of a fossil?
Subatomic Physics Section 2 Nuclear Decay When nuclei are unstable, particles and photons are emitted. The process is called radioactivity. It occurs because the nucleus has too many or too few neutrons. Three types of radiation can occur: Alpha Beta Gamma
Subatomic Physics Section 2 Alpha Decay (α) An alpha particle (2 protons and 2 neutrons) is emitted from the nucleus. α particles are helium-4 nuclei. A new element is formed by alpha decay. Example of alpha decay: U!!" Th + He 238 234 4 92 90 2 The uranium atom has changed into a thorium atom by ejecting an alpha particle.
Subatomic Physics Section 2 Radioactive Decay These rules are used to determine the daughter nucleus when a parent nucleus decays. Note how these rules apply in the alpha decay of uranium-238: 238 = 234 + 4 92 = 90 + 2 238 U!!" 234 4 92 90 2 Th + He
Subatomic Physics Section 2 Beta Decay An electron or positron is emitted from the nucleus. A positron is the same as an electron but with an opposite charge. A positron is the antiparticle of an electron. Since there are no electrons or positrons in the nucleus, how can beta decay occur? A neutron is transformed into a proton and an electron, and then the electron is ejected. A proton is transformed into a positron and a neutron, and then the positron is ejected.
Subatomic Physics Section 2 Beta Decay It was discovered that, during beta decay, momentum and energy were not conserved. The ejected electron did not have as much forward momentum as the recoiling nucleus. In 1930, Wolfgang Pauli proposed the existence of a particle that was not detectable at the time. In 1956, Pauli s neutrino (ν) was detected. The neutrino and its antiparticle, the antrineutrino (ν), are emitted during beta decay. Electrons are accompanied by antineutrinos. Positrons are accompanied by neutrinos.
Subatomic Physics Section 2 Beta Decay What new element is formed by the beta decay of carbon-14? 14 ""# 14 0 6 7-1 C N + e +! The new element is nitrogen-14. The electron is shown with a mass number of zero and an atomic number of -1. The total of the mass numbers and atomic numbers are still equal.
Subatomic Physics Section 2 Gamma Decay During alpha and beta decay, the nucleons left behind are often in an excited state. When returning to ground state, the nucleus emits electromagnetic radiation in the form of a gamma ray. The nucleus remains unchanged except for its energy state.
Subatomic Physics Section 2 Types of Radioactive Decay
Subatomic Physics Section 2 Alpha, Beta, and Gamma Radiation Click below to watch the Visual Concept. Visual Concept
Subatomic Physics Section 2 Nuclear Decay Series During nuclear decay, the daughter may be unstable as well, causing further decays. What element would be formed by thorium-232 undergoing 6 alpha and 4 beta decays? Answer: lead-208
Subatomic Physics Section 2
Subatomic Physics Section 2 Classroom Practice Problems Find the missing item (X) in these reactions: Ra ""# X + e+! 228 0 88-1 Answer: Ra ""# Ac + e +! 228 228 0 88 89-1 220!!" 216 86 84 Rn Po + X Answer: Rn!!" Po + He 220 216 4 86 84 2
Subatomic Physics Section 2 Measuring Nuclear Decay The rate of decay is different for each nucleus. ΔN/Δt = -λn N is the number of nuclei, Δt is the time, and λ is the decay constant. λ differs for every element. The rate of decay is called the activity. The negative sign occurs because the number of nuclei is decreasing. SI unit: becquerel (Bq) or decays/s
Subatomic Physics Section 2 Half-Life Half life is the time required for half of the nuclei to decay. Half-lives can be very short (nanoseconds) or very long (millions of years). Half-life is inversely related to the decay constant.
Subatomic Physics Section 2 Half-Life Carbon-14 is radioactive with a half-life of 5715 years. The figure shows a decay curve for carbon-14. Does the total number of nuclei change? No How much time has passed at T 1/2? 5715 years How much time has passed at 2T 1/2? 11 430 years How many blue circles will there be at 3T 1/2? one
Subatomic Physics Section 2 Radioactive Carbon Dating All living things have about the same ratio of carbon-14 to carbon-12. Carbon-14 is radioactive, and carbon-12 is not. After death, the ratio drops because the carbon-14 decays into nitrogen-14, while the carbon-12 is stable and remains. When the ratio is half the starting ratio, 5715 years have passed since death occurred.
Subatomic Physics Section 2 Classroom Practice Problems A sample of barium-144 contains 5.0 10 9 atoms. The half-life is about 12 s. What is the decay constant of barium-144? How many atoms would remain after 12 s? How many atoms would remain after 24 s? How many atoms would remain after 36 s? Answers: 0.058 s -1 2.5 10 9 atoms, 1.2 10 9 atoms, 6.2 10 8 atoms
Subatomic Physics Section 2 Half-Life Click below to watch the Visual Concept. Visual Concept
Subatomic Physics Section 2 Now what do you think? Often scientists use radioactive carbon dating to determine the age of fossils. What does the term radioactive mean? Are all atoms radioactive? If not, how are the radioactive atoms different from those that are not radioactive? How can radioactivity be used to determine the age of a fossil?
Subatomic Physics Section 3 TEKS The student is expected to: 8C describe the significance of mass-energy equivalence and apply it in explanations of phenomena such as nuclear stability, fission, and fusion 8D give examples of applications of atomic and nuclear phenomena such as radiation therapy, diagnostic imaging, and nuclear power and examples of applications of quantum phenomena such as digital cameras
Subatomic Physics Section 3 What do you think? Nuclear power and nuclear weapons are important and frequently-discussed issues in the world today. How does a nuclear reactor produce energy? What is nuclear about it? What problems are associated with nuclear power? Do atomic bombs and hydrogen bombs differ in the way they produce energy? If so, how are they different?
Subatomic Physics Section 3 Nuclear Changes For nuclear changes to occur naturally, energy must be released. Binding energy must increase. Lighter elements must combine, and heavier elements must reduce in size. The greatest stability is for atoms with mass numbers between 50 and 60.
Subatomic Physics Section 3 Fission Fission occurs when a large nucleus absorbs a neutron and splits into two or more smaller nuclei. Example of fission: n + U!!" U!!" X + Y + neutrons 1 235 236 * 0 92 92 It only occurs for heavy atoms. The * indicates an an unstable state that lasts for about a trillionth of a second. X and Y can be different combinations of atoms that have a total atomic number of 92.
Subatomic Physics Section 3 Fission n + U!!" Ba + Kr + 3 n 1 235 140 93 1 0 92 56 36 0 A typical fission reaction is shown above. The products, Ba and Kr, have more binding energy than the uranium. As a result, energy is released. Each fission yields about 100 million times the energy released when burning a molecule of gasoline.
Subatomic Physics Section 3 Chain Reaction On the average, 2.5 neutrons are released with each fission. These neutrons are then absorbed and cause more fissions. A chain reaction occurs.
Subatomic Physics Section 3 Nuclear Fission Click below to watch the Visual Concept. Visual Concept
Subatomic Physics Section 3 Nuclear Reactors Reactors manage the fission rate by inserting control rods to absorb some of the neutrons. Nuclear power plants and navy vessels use fission reactions as an energy source. Reactors produce radioactive waste, and disposal is one difficulty. Presently 20% of the U.S. electric power is generated by nuclear reactors. Atomic bombs use uncontrolled fission.
Subatomic Physics Section 3 Fusion Light elements can combine and release energy as well. Hydrogen atoms have less binding energy per nucleon than helium atoms. Fusion is the source of a star s energy. Hydrogen atoms fuse to form helium atoms. Much energy is released with each fusion. Hydrogen bombs use uncontrolled fusion. First tested in 1952 but never used in war
Subatomic Physics Section 3 Fusion as an Energy Source Fusion reactors are being developed. Advantages of fusion reactors: The fuel source, hydrogen from water, is cheap. The products of fusion are clean and are not radioactive. Disadvantages of fusion: It requires extremely high temperatures of roughly 10 8 K to force atoms to fuse. It is difficult to keep the hydrogen atoms contained at this temperature.
Subatomic Physics Section 3 Nuclear Fusion Click below to watch the Visual Concept. Visual Concept
Subatomic Physics Section 3 Now what do you think? Nuclear power and nuclear weapons are important and frequently-discussed issues in the world today. How does a nuclear reactor produce energy? What is nuclear about it? What problems are associated with nuclear power? Do atomic bombs and hydrogen bombs differ in the way they produce energy? If so, how are they different?
Subatomic Physics Section 4 TEKS The student is expected to: 5A research and describe the historical development of the concepts of gravitational, electromagnetic, weak nuclear, and strong nuclear forces 5H describe evidence for and effects of the strong and weak nuclear forces in nature
Subatomic Physics Section 4 What do you think? When the idea of the atom was first conceived, it was thought to be a fundamental particle, indivisible and indestructible. We now know differently. List every particle you can think of that is smaller than an atom. If you know the properties of these particles, list them as well. Which of the particles on your list are fundamental?
Subatomic Physics Section 4 Fundamental Forces There are four fundamental interactions or forces in nature: strong electromagnetic weak gravitational They exert force using the exchange of mediating particles. Photons are the mediating particle exchanged between electrons. This causes repulsion.
Subatomic Physics Section 4 Fundamental Forces Strong force Holds protons and neutrons together in the nucleus Electromagnetic force Creates forces between charged particles Holds atoms and molecules together Weak force A nuclear force that controls radioactive decay Gravitational force The weakest force Gravitons (the mediating particle) not yet discovered
Subatomic Physics Section 4 Fundamental Forces
Subatomic Physics Section 4 Classification of Particles All particles are classified as leptons, hadrons, or mediating particles. Over 300 particles are known. Leptons are thought to be fundamental. Electrons are leptons.
Subatomic Physics Section 4 Classification of Particles Hadrons are composed of smaller particles called quarks. Quarks are thought to be fundamental. Protons and neutrons are hadrons. Two types of hadrons: baryons and mesons Hadrons interact through all four of the fundamental forces, while leptons do not participate in strong force interactions.
Subatomic Physics Section 4
Subatomic Physics Section 4 Classification of Particles Protons and neutrons are baryons. What combination of up and down quarks would make a proton and a neutron? Two up quarks (+4/3) and one down quark (-1/3) gives a proton a charge of +1. One up quark (+2/3) and two down quarks (-2/3) gives a neutron a charge of zero.
Subatomic Physics Section 4 Combinations of Quarks Baryons and mesons are distinguished by their internal structure. The particles above are a proton, a neutron, a pion, and a kaon. Mesons are unstable, and are not constituents of everyday matter.
Subatomic Physics Section 4 The Standard Model of Particle Physics Click below to watch the Visual Concept. Visual Concept
Subatomic Physics Section 4 The Standard Model The Standard Model is the current model used in particle physics. How many fundamental particles are there in the standard model? Six quarks, six leptons, and an antiparticle for each (24 total)
Subatomic Physics Section 4 Evolution of the Four Forces
Subatomic Physics Section 4 Quarks and their Charges Click below to watch the Visual Concept. Visual Concept
Subatomic Physics Section 4 Now what do you think? When the idea of the atom was first conceived, it was thought to be a fundamental particle, indivisible and indestructible. We now know differently. List every particle you can think of that is smaller than an atom. If you know the properties of these particles, list them as well. Which of the particles on your list are fundamental? What are the four fundamental forces and what particle mediates each?