Atomic Theory & the Atom

Transcription

1 Section III Atomic Theory & the Atom it s elemental

2 Our view of the atom has changed over time

3 the ATOM the smallest particle of an element that still retains the chemical properties of that element

4 Here is a model of an atom. - what does this model tell you about the atom? - What is something this model doesn t tell you?

5 Models of the Atom L11 ATOMIC PUDDING

6 Key question: How are the smallest bits of matter described?

7 the ATOM our current model 3 principal particles Protons positively charged Neutrons no charge Electrons negatively charged

8 the ATOM Nucleons: protons and neutrons in the nucleus Neutrons are bound to protons by the strong nuclear force one of the strongest known forces! When neutrons are found outside the nucleus they breakdown quickly into electrons and protons.

9 the ATOM electrons: (e-) are continually in motion around the nucleus of an atom. Responsible for ALL the chemical properties of an atom. The smallest of the 3 atomic particles and considered not to contribute mass to the atom.

10 atoms are neutral in charge Proton has + charge equal in magnitude to charge of electron Atoms are electrically neutral # of protons = # of neutrons

11 Subatomic particles Particle physics and nuclear physics study these particles and how they interact.

12 Elementary particles Protons and neutrons are composite particles made of even smaller particles.

13 protons and neutrons electrons

14 Dalton s atomic theory 1. All matter is composed of atoms 2. Atoms of a given element are identical 3. Atoms cannot be divided, created or destroyed 4. Atoms combine in whole number ratios and in chemical reactions, are combined, separated or rearranged

15 Current electron cloud model of an atom. How many atoms do you think there are in this gold nugget?

16 Modern Atomic Theory Evidence through discovery and experimentation shapes our model of the atom.

17 The Atomic Model Through Time But let s take a step back and look at the evidence that changed our vision of the atom over time. The Cloud Model

18 An attempt to explain our world. The Egyptians (3000 BC) formulate the theory of the Ogdoad, or the "primordial forces", from which all was formed. These were the elements of chaos, numbered in eight, that existed before the creation of the sun.

19 The Greeks (450 BC) Empedocles asserts that all things are composed of four primal elements: earth, air, fire, and water, whereby two active and opposing forces, love and hate, or affinity and antipathy, act upon these elements, combining and separating them into infinitely varied forms.

20 440 BC Gives us the term atom or Atomos meaning uncuttable

21 Natural philosophers such as Aristotle and Democritus used deductive reasoning in an attempt to explain the behavior of the world around them. Leucippus and Democritus (440 BC) propose the idea of the atom, an indivisible particle that all matter is made of. This idea is largely rejected by natural philosophers in favor of the Aristotlean view

22 John Dalton Solid Sphere Model Dalton proposed a modern atomic model based on experimentation. All matter is made of atoms. Atoms of an element are identical. Each element has different atoms. Atoms of different elements combine in constant ratios to form compounds. Atoms are rearranged in reactions. His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

23 JJ Thompson Plum Pudding Model 1897 Thompson a British scientist, zapped atoms with electricity. He observed that negatively charged particles were removed. He proposes that atoms are similar to a fluid-filled sac with negative particles floating inside. The fluid is positive and solid particles are negative. His ideas account for the discovery of the electron

24 1911 Ernest Rutherford His ideas account for the discovery of the positive nucleus of the atom

25 While protons give an atom mass, protons alone don t account for the mass of the atom. Since Rutherford s time, it was known that an atom s mass is a bit more than twice its number of protons (atomic number) and that essentially all the mass of the atom is concentrated in the nucleus. In the 1930 s it was presumed that the fundamental particles were protons and electrons, but that required that electrons were bound in the nucleus to partially cancel the charge of the protons. But it was also known that there just wasn't enough energy available to contain electrons in the nucleus.

26 Niels Bohr 1913 Solar system model Electrons orbit at different distances from the nucleus

27 more mass discovery of the neutron Until 1932, the atom was known to consist of a positively charged nucleus surrounded by enough negatively charged electrons to make the atom electrically neutral. Lord Ernest Rutherford (1917) had postulated the existence of a neutral particle This stimulated a search for the particle. However, its electrical neutrality complicated the search because almost all experimental techniques of this period measured charged particles

28 The neutron was finally discovered in 1932 when James Chadwick used scattering data to calculate the mass of this neutral particle. Chadwick proved that there was a neutral component with a mass approximately equal to that of the proton. He called it the neutron in a paper published in the February 17, 1932, issue of Nature. In 1935, Sir James Chadwick received the Nobel Prize in physics for this work.

29 The search was over. Chadwick had found a new elementary particle, the third basic component of the nucleus. It increased the mass of elements without adding electrical charge. This changed our view of the nucleus.

30 neutrons Neutrons are particles found in the nucleus. They have no charge, but do add mass to the atom. The mass of a neutron is considered equal to the mass of a proton.

31 Atom Clash of the Titans

32 The exact path of electrons cannot be predicted. Instead they exist in a specific energy region or shell surrounding the nucleus. The region referred to as the electron cloud is an area where electrons are likely to be found at any given time.

33 Schrodinger What was the impact of Erwin Schrodinger's atomic cloud model? Erwin Schrodinger s atomic cloud model revolutionized the way scientists viewed the structure of the atom. Building on the work of Neils Bohr, Schrodinger demonstrated that it was impossible to determine the exact location of the electron at a particular point in time. Instead, Schrodinger s model showed that an electron could be found in some portion of an electron cloud at any specific point in time.

34 Schrodinger s work largely took the form of a probability equation. In essence, the equation demonstrated that while the electron was more likely to be found at a specific point at a given point in time, it was impossible to determine whether or not the electron actually was there.

35 Experimental results cannot provide any more definitive answers regarding the location of the electron at a particular point in space and time. Visible light s wavelengths are too large to view atomic structures, so light microscopes are of no use to atomic investigations. Normally, scientists examine very small objects with electron microscopes. Electron microscopes fire electrons, rather than photons of light, at the object to be seen. However, electrons used in electron microscopes cannot provide imagery of other electrons because they are the same size and they will cause the original electron s position to change.

36 More on Atoms ATOMIC NUMBER & ATOMIC MASS

37 Lesson 12 - Key Question How are the atoms of one element different from those of another element?

38 You will be able to: distinguish between atomic number, mass of an atom, and average atomic mass describe the structure of an atom and draw a simple atomic model of an atom extract information from the periodic table related to atomic structure and atomic mass

39 Atomic number Every atom has a distinct number of protons in its nucleus. remember: The number of protons determines an atom s identity boron carbon nitrogen 5 protons 6 protons 7 protons

40 Also remember: Elements are arranged in the Periodic table based on their atomic number (# of protons) the atomic number identifies the element

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42 How are atomic mass and atomic number related? Theoretically, atoms have the same number of neutrons as they do protons. Neutrons and protons are responsible for the mass of an atom. Atomic mass Atomic number So..if you know the atomic number, you can predict the approximate atomic mass.

43 Elements are often symbolized with their mass number and atomic number mass # e.g. Oxygen: 16 O 8 atomic # These values are given on the periodic table. For now, round the mass # to a whole number. These numbers tell you a lot about atoms. # of protons = # of electrons = atomic number # of neutrons = mass number atomic number Calculate # of e, n 0, p + for Ca, Ar, and Br.

44 Atomic Mass p + n 0 e Ca Ar Br P 35 N Bromine

45 Atomic mass The atomic mass (m a ) is the mass of an atom, expressed in atomic mass units (AMU). The atomic mass is the total mass of protons, neutrons and electrons in a single atom.

46 average atomic mass Why does the periodic table show atomic mass numbers that are not whole numbers?

47 Lesson 13: Subatomic Heavyweights Isotopes

48 A chemist investigating a sample of lithium found that some lithium atoms have a lower mass than other lithium atoms. The chemist drew models of the two different types of lithium atoms, as shown below. 1. What is different about the two atoms? 2. What is the atomic number of each atom? 3. What is the atomic mass of each atom?

49 Key Question How can atoms of the same element be different?

50 You will be able to: define isotope and write and interpret the symbol for a specific isotope determine the average atomic mass of an element based on the natural abundance of isotopes of that element predict the number of protons, neutrons, and electrons in the most abundant isotope of an atom, based on average atomic mass

51 isotopes Atoms of an element that have different numbers of neutrons are known as isotopes. While an atom is identified by its number of protons (atomic number), the same atom can have different numbers of neutrons

52 A helium atom has an atomic number of 2 How many protons does it have? How many electrons does it have? How many neutrons does it have?

53 Calculate average atomic mass First you need to know the % abundance of each isotope. The average atomic mass of an element is the weighted average of the masses of the isotopes in a sample of the element.

54 Calculate the Atomic Mass of Neon (.77)(20) + (.12)(22) + (.11)(21) = The sample contains 100 atoms of neon = 20.35

55 Isotope notation A Z X Often, at least one isotope is unstable. It breaks down, releasing radioactivity.

56 Properties of isotopes are important There are 90 naturally occurring elements with roughly 250 stable isotopes, and over 3200 unstable or radioactive isotopes. Different isotopes of the same element often have completely different properties -- making some of them very useful

57 Isotopes are used in a wide variety of applications medical imaging in the diagnosis of a wide range of ailments cancer treatment and other therapeutic applications; As a ''fingerprint'' used in forensic analysis; Smoke detectors; Batteries that power NASA satellites; to enable new sources of energy such as nuclear fusion;

58 Calculating % abundance The percentage of each isotope of an element that occurs in nature is called the natural percent abundance of the isotope.

59 Try Problem #1: Nitrogen is made up of two isotopes, N-14 and N-15. Given nitrogen's atomic weight of , what is the percent abundance of each isotope? (14) (x) + (15) (1 - x) = Notice that the abundance of N-14 is assigned 'x' and the N- 15 is 'one minus x. The two abundances always add up to one (or, if you prefer, 100%)

60 to solve: (14) (x) + (15) (1 - x) = Solving gives: 14x x = x = = and 1 - x = So. 99.3% N-14 And 0.7% N-15

61 Problem #2: Copper is made up of two isotopes, Cu-63 ( amu) and Cu-65 ( amu). Given copper's atomic weight of , what is the percent abundance of each isotope?

62 To solve: ( ) (x) + ( ) (1 - x) = Solve for x: x = OR 69.15% (the abundance for Cu-63) = or 30.85% (abundance for Cu-65)

63 Lesson 14: Isotopia Stable and Radioactive Isotopes

64 Review: isotopes Which of the following are isotopes of copper, Cu? Explain your reasoning A. 29 Cu B. 79 Au C. 28 Cu D 29 Cu E. 29 Cu F. 29 Cu

65 lesson objective What types of isotopes do the various elements have?

66 You will be able to: interpret a graph of naturally occurring isotopes describe the general nuclear composition of a stable nucleus differentiate between a stable isotope and a radioactive isotope

67 The graph of naturally occurring isotopes gives us an idea of how many different isotopes of the elements are found in nature. The words atom, isotope, and element are interrelated. Nearly all atoms have at least one neutron for every proton in the nucleus.

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69 Many isotopes have more neutrons than protons. A handful of the isotopes on the chart are unstable. Some elements have a naturally occurring radioactive isotope. The isotopes of the elements after bismuth (atomic number 84 and up) are all radioactive.

70 How does altering the neutron number affect the properties of an atom? It changes its mass!

71 How does altering the neutron number affect the properties of an atom? It s nucleus can become unstable!

72 When atoms are not stable, they emit small particles. Small bits of the nucleus come flying out of the atom. These less stable isotopes are called radioactive isotopes. They decay over time as particles are spontaneously emitted from the nucleus in a process called radioactive decay.

73 Radioactive isotope: Any isotope that has an unstable nucleus and decays over time. The neutron-to-proton ratio is an important factor in determining the stability of the nucleus of an isotope. Atoms with small masses have a neutron-to-proton ratio of about 1:1. The most massive atoms have a neutron-to-proton ratio of about 3:2. Hank on radioactivity Some elements have isotopes that are radioactive. The nuclei of radioactive isotopes are unstable and decay over time.

74 Lesson 15: Nuclear Quest Nuclear Reactions

75 Challenge: Using the Nuclear Quest game, find the ten kinds of cards shown below. Which cards cause the nucleus of one element to change into the nucleus of a different element?

76 Key Question What are nuclear reactions?

77 You will be able to: explain the different processes involved in nuclear changes and the conditions required for those processes explain the connection between nuclear changes and changes in atomic identity

78 The goal of the game is to discover element 112 and name it. This is accomplished by moving the nucleus through the entire periodic table.

79 Nuclear Chemistry The study of changes to the nucleus. Nuclear reaction: A process that involves changes to the nucleus of an atom. Radioactive decay: A spontaneous process by which an atom emits radiation or a particle from its nucleus to become more stable. Fusion: The joining of two nuclei to form a larger nucleus accompanied by a release of energy. Fission: The splitting apart of an atomic nucleus into two smaller nuclei accompanied by a release of energy.

80 Changes in the nucleus of an atom can change the identity of an element. Nuclear changes frequently involve the transfer of large amounts of energy.

81 Nuclear Reactions There are several ways the nucleus of an atom can change: Loose particles Split into smaller nuclei Combine with another nucleus or particle

82 back to Isotopes 1. Stable: the nucleus contains enough neutrons to block the repulsive forces of the protons. This keeps the isotope from breaking down over time. 2. Unstable: the nucleus of the atom does not have right amount of neutrons to block the repulsive forces of the protons; this makes the isotope radioactive!

83 Radioactive Isotopes 1. unstable nuclei become stable by ejecting a particle (radioactive decay) 2. Nuclear decay involves the emission of energy and/or particles from the nucleus in an attempt to become more stable. 3. The energy/particles emitted from the nucleus are termed radiation and can be alpha or beta particles and gamma rays.

84 Alpha Decay Alpha decay: A nuclear reaction in which an atom emits an alpha particle. Its atomic number decreases by 2, and its mass number decreases by 4. 2 protons/2 neutrons The loss of an alpha particle from the nucleus, changes the element s identity!

85 Alpha particle: A particle composed of two protons and two neutrons, equivalent to the nucleus of a helium atom. An alpha particle carries two protons away from the nucleus of an atom

86 Beta Decay Beta decay: A nuclear reaction in which a neutron breaks up and the atom emits a beta particle (electron). The atom s atomic number increases by 1. remember: neutrons are made of electrons and protons.

87 Beta particle: An electron emitted from the nucleus of an atom during beta decay. (e-) Proton (+) stays in the nucleus This adds a proton to the nucleus, changes the element s identity!

88 Example: Plutonium-241 The radioactive isotope PU emits a beta particle Am In beta decay a neutron breaks down into a proton (stays in the nucleus) and an electron (ejected). This increases the number of protons by one and decreases the number of neutrons by one because protons and neutrons have the same mass, the mass of the element does not change, but it s identity does.

89 Isotopes disintegrate at predictable rates, so they are useful for determining age/ measuring time. (C-14 decays to N-14 for example) Half-life C-14 has a half-life of 5730 years.

90 C-14 is found in all living organisms.

91 By measuring the amount of carbon-14 in a sample compared to the amount of stable carbon, age can be determined.

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93 Gamma radiation When γ rays are emitted, the identity of the emitting atom does not change. Gamma ray: A form of high-energy electromagnetic radiation often emitted during nuclear reactions.

94 Because most elements have unstable isotopes, there is radiation present in almost every environment. Because of the high energy of gamma rays, they can be dangerous to humans.

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96 Fission and Fusion

97 Nuclear Fission: The splitting apart of a nucleus Results in 2 atoms of smaller nuclei & lots of energy. Can occur spontaneously if a nucleus is unstable or caused by an outside bombardment.

98 Fission is Exothermic The sum of the masses of the resulting nuclei is less than the original mass (about 0.1% less) The missing mass is converted to energy according to E=mc 2

99 Mass Defect = Energy! The energy released from the nuclear reaction of 1kg of uranium is equivalent to the energy released during the combustion of 4 billion kilograms of coal. This large amount of energy is due to the mass defect Every time fission occurs, there is a difference between the mass of the starting atoms and the smaller atom products. This difference in mass is converted into energy by Einstein s equation :

100 Scientific discovery Scientists discovered they could cause nuclear fission by shooting neutrons at a nucleus Hahn & Strassman (1939) Bombarded Uranium-235 samples with neutrons expecting the Uranium-235 to capture neutrons

101 Instead, the products showed different chemical properties that they could not explain Instead of heavier Uranium, it had split into smaller elements Nuclear Fission radiation

102 U.S. Electrical Power Production by Source The United States has 103 nuclear power reactors in 31 states.

103 Nuclear energy provides electricity for one of every five homes and business the second-largest source of electricity after coal. Vermont gets 76% of its electricity from nuclear reactors New Hampshire, 58%, and South Carolina, 55%. In Virginia, nuclear power provides more than 40% of the electricity used.

104 Basic process of nuclear energy

105 Nuclear Fusion Nuclear fusion is the process by which multiple nuclei join together to form a heavier nucleus. It is accompanied by the release or absorption of energy depending on the masses of the nuclei involved.

106 Hydrogen isotopes

107 Nuclear Fusion Iron and nickel nuclei have the largest binding energies per nucleon of all nuclei and therefore are the most stable.

108 The fusion of two nuclei lighter than iron or Nuclear nickel generally Fusion releases energy. energy The fusion of nuclei heavier than them absorbs energy.

109 Fusion Changes Mass to Energy energy 1kg Hydrogen.993 kg Helium But temperatures of approximately 100,000,000 C are required for nuclei to successfully fuse.

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111 Cold Fusion: Efforts are being made to start and sustain a fusion reaction at lower temperatures, in other words with a lower amount of input energy

112 1989 The search for an infinite source of energy. Nov. 2, 2015 As far-fetched as fusion energy has seemed to anyone who has followed its troubled history, recent developments at least mean that serious effort is being put into determining whether fusion has a shot at producing energy or not.

113 summary Alpha decay results in a decrease in the atomic number by 2. Beta decay results in an increase in the atomic number by 1. Gamma radiation usually accompanies alpha and beta decay and also fission. Gamma radiation can be quite harmful to humans.

114 summary Nuclear fission: A large nucleus splits into several small nuclei when impacted by a neutron, and energy is released in this process Nuclear fusion: Several small nuclei fuse together and release energy. More energy in fusing hydrogen that fission of uranium

115 Summary: Decay vs. Nuclear Reactions Alpha, beta, and gamma decay occur as ONE atom tries to increase it s stability by getting rid of a few neutrons, or protons & neutrons. The product is an alpha, beta, or gamma particle and ONE new atom. There is only ONE thing on the left hand side. Nuclear reactions involve more than just getting rid of a few protons or neutrons. The new atoms produced are VERY different elements than the reactant. Nuclear reactions must be started, so there are 2 things on the left hand side. Nuclear fission: makes 2 or more much smaller atoms Nuclear fusion: makes 1 much larger atom

116 Lesson 16: Old Gold Formation of Elements

117 remember: Nuclear Fusion The combining of atomic nuclei to form a larger atom is called fusion Nuclear fusion occurs in the sun where hydrogen atoms fuse to form helium

118 Fusion reactions also release very large amount of energy but require extremely high temperatures to start. Nuclear fusion also occurs in new stars and is hypothesized how all of our elements were made He + 4 He 8 Be + energy He + 8 Be C + energy

119 Remember: Nuclear Fission The splitting of a nucleus into smaller fragments is called nuclear fission. Heavy atoms (mass number>60) tend to break into smaller atoms, thereby increasing their stability. Nuclear fission releases a large amount of energy.

120 Nuclear Fission

121 What patterns do you notice in the fusion reactions? Do you think gold can be created on Earth by a fusion reaction? Explain your thinking.

122 Key Question How are new elements formed?

123 You will be able to: explain how different elements are formed through nuclear reactions write a balanced nuclear equation describe the mechanism behind a nuclear chain reaction

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125 Nuclear processes can be written as nuclear equations. During alpha decay, the nucleus of an atom emits a helium nucleus, transforming the element into an element with a smaller nucleus. 140 Ba 4 He (ά) Xe During beta decay, a neutron inside the nucleus of an atom emits an electron. 140 Ba 0 e (β) La

126 Fusion equations have two starting isotopes coming together to form a new product. 12 C + 14 N 26 Al

127 chain reaction: Neutrons can cause repeated fission by colliding with a U 235 nucleus Creates two smaller nuclides and free neutrons The free neutrons potentially collide with nearby U 235 nuclei May cause the nuclide to split as well Each split (fission) is accompanied by a large quantity of E-N-E-R-G-Y If sufficient neutrons are present, we may achieve a chain reaction

128 Fission Chain Reactions One fission reaction can lead to more fission reactions in a process called a chain reaction. Example - The fission of Uranium-235

129 A chain reaction can only occur if the starting material has enough mass to sustain a chain reaction. This amount is called the critical mass. Nuclear Fission is what occurs in Nuclear Reactors and Atomic Bombs. The Nuclear reactor is a controlled fission reaction, the bomb is not. Control chain reaction with control rods that absorb neutrons emitted after fission reaction

130 Balancing Nuclear Equations Mass numbers and Atomic numbers must add up on both sides of the reaction arrow Fm 54 Xe + 46Pd n

131 summary How are new elements formed? Radioactive decay, nuclear fusion, and nuclear fission are all nuclear processes that result in the creation of new elements. The mass of a nucleus changes when neutrons or protons are added or lost. The identity of an element changes when its nucleus gains or loses protons.

132 Summary Nuclear reactions change the identity of an element. Nuclear fusion involves the joining together of nuclei. Nuclear fusion produces bigger (heavier) elements from smaller (lighter) ones. Fission involves a nucleus breaking up into smaller nuclei.

133 summary (cont.) Radioactive decay happens in the natural world around us. Fission can be spontaneous for unstable nuclei, or it can be provoked using nuclear bombardment and other methods. Fusion of nuclei to form different isotopes happens in the stars. Nuclear fission is a process that releases enormous amounts of energy. Nuclear fission can result in a nuclear chain reaction that produces a great deal of energy.