Chemistry. Atomic Origins.

Transcription

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2 Chemistry Atomic Origins

3 Table of Contents: Creation of Matter The Big Bang Click on the topic to go to that section Electrons & Protons The Nucleus Formation of the Elements Isotopes Radioactive Decay Half Life 3

4 The Big Bang Return to Table of Contents 4

5 Chemistry The observable Universe is made up of amazing stuff. We more formally call this stuff matter. Humans have always been curious about the nature of matter: where did matter come from? what is it made out of? why does it behave the way it does? 5

6 Chemical Elements Scientists have discovered all of the matter in our Universe is made up of 116 different types of chemical elements. About 90 of these elements occur naturally. 6

7 The Beginning... Where did the elements that makes up the Universe come from? 7

8 The Beginning... You were correct if you said the prevailing theory is that the Universe began with the "Big Bang," which is an event thought to have occurred about 14 billion years ago. 8

9 Big Bang Theory It is believed our Universe began at a single point. This one spot was thousands of times smaller than the head of a pin. It was also hotter and more dense than any object we know of today. This heat still remains as Cosmic Background Radiation. 9

10 Big Bang Theory This Universe began expanding suddenly and rapidly from this single point. Consequently, every piece of matter, all the "stuff" in the universe came from this small, dense spot! 10

11 1 Scientists believe the Big Bang happened: A 14 million years ago B 14 trillion years ago C 14 billion years ago D within the last 3000 years Answer 11

12 Energy and Matter 14 billion years ago, in the flash of the Big Bang high energy photons (light particles) collided with each other, forming oppositely charged particles. Typically, when this happened the oppositely charged matter and antimatter annihilated each other instantly, converting back into high energy photons. Photons Photons Charged Matter Oppositely Charged Antimatter 12

13 2 Energy and Matter In the first seconds of the Universe, for reasons scientists cannot explain, it is estimated that one particle of matter for approximately every one billion particles of antimatter were not annihilated. (You could win a Nobel Prize if you figure out why!) In this environment three major particles formed: + positively charged particles Answer neutrally charged particles negatively charged particles What are these positive, negative and neutral particles called? What is the magnitude of their charge? What are their masses? 13

14 Cosmic Background Radiation "As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled and stable atoms could form, they eventually could no longer absorb the thermal radiation and the universe became transparent instead of being an opaque fog. The photons that from that time have been propagating ever since, growing fainter and less energetic." background radiation/ 14

15 3 Following the Big Bang, the universe: A expanded and then rapidly stopped expanding. B expanded and has not stopped expanding since. C rapidly expanded and then shrunk back to its original size. Answer 15

16 Formation of the Elements 3 minutes after the Big Bang, the Universe began to cool down from (1x C to 1 x 10 9 C) and protons and neutrons began to combine

17 Formation of the Elements About 300,000 years later, the universe had cooled enough for positively charged protons to attract the negatively charged electrons, and the first atoms were formed Hydrogen 1 Hydrogen 2 Deuterium Hydrogen 3 Tritium + + Helium Lithium Beryllium 9 17

18 Stellar Furnaces During the formation of the universe only atoms of the lightest elements hydrogen, helium, lithium and beryllium were formed. As the cloud of cosmic dust and gases from the Big Bang cooled, stars formed, and these then grouped together to form galaxies and stars. The high pressure and temperature within Stars caused protons and neutrons to fuse together. In smaller stars like our Sun, the temperatures are 15.5 million C at the core, hot enough to make Helium from Hydrogen only. 18

19 Larger Elements In the core of hotter, larger giant stars: hydrogens fuse to make helium heliums fuse to make atoms with 4 protons beryllium helium and beryllium fuse to make atoms with 6 protons carbon carbon and helium fuse to make atoms with 8 protons oxygen, etc., and in this manner elements with up to 12 protons formed. Red Supergiant Blue Supergiant Red Giant Sun Blue Giant 19

20 Formation of Heavier Elements Atoms of elements aluminum to iron formed in Super Giant stars The most massive elements from iron to uranium were created in star explosions called supernovae. 20

21 Periodic Table of Nucleosynthesis 21

22 "We Are Made from Star Stuff" Atoms, the building blocks of matter, formed in the intense heat and pressure of the early universe, stellar furnaces and supernovae. Everything around us was once part of a star. In this course we will explore the nature of matter and apply principles of physics to understand atomic structure, chemical properties and predict chemical behavior. Click here to watch a video on the formation of the Elements. 22

23 Atomic Structure: Electrons & Protons Return to Table of Contents 23

24 . Discovery of the Electron In the late 1800's scientists were passing electricity through glass tubes containing a very small amount of gas like oxygen. When the power was turned on, the tube emitted light and glowed. + + POWER OFF The positive electrode is called the anode and the negative called the cathode. They called the rays "cathode rays" because they appeared to be coming from the negative end of the tube. POWER ON Actual Cathode Ray Tube 24

25 . Waves vs. Particles There was much speculation about what these "cathode rays" were. When an object was placed in the path of the rays, the rays cast shadows of the objects placed in their path. Light waves casts a shadow so it could be light. Or, it could be a stream of tiny particles. 25

26 4 Scientists found that they could deflect this beam by subjecting it to an additional electrical field. Why would the beam deflect toward the positive plate? Does that indicate the rays are light rays or particles? + + Answer POWER ON 26

27 5 Scientists found that they could also deflect this beam by subjecting it to a magnetic field. Why would the beam deflect upward in the magnetic field above? Does that indicate the rays are light rays or particles? x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x + Answer POWER ON 27

28 6 Scientists determined that a very weak electrical field could deflect the beam a great deal. If the particles are really easy to deflect they either have a very small or a very large or both. + + Answer POWER ON 28

29 . Charge to Mass Ratio J.J. Thomson and team were able to determine this charge to mass ratio to be: 1.76 x10 11 Coulombs of charge/ kg of mass or C/kg Keep in mind, at this point they knew neither the charge nor the mass, just that the ratio was large indicating either a large charge or a small mass. What was very interesting was that these negatively charged particles were found in all gases they experimented on and they all had the same charge to mass ratio. 29

30 . Negatively Charged Particles Electrons Physicists proposed these negatively charged particles be called electrons. These particles have the same charge to mass ratio as the negative particles generated by static electricity, heated materials, and illuminated materials. 30

31 7 What characteristic about the cathode rays led them to believe they were negatively charged? A They were small B Their behavior in an electric field C Their behavior in a magnetic field D b and c Answer 31

32 8 Which of the following indicated the cathode rays had a large charge to mass ratio? A They were small B They were easily deflected C They were deflected towards a positive electrode D They were deflected towards a negative electrode Answer 32

33 . Millikan Oil Drop Experiment A scientist named Millikan squirted oil drops into a box and then passed high energy x rays at the box hoping to knock electrons off the air molecules and onto the oil drops. Oil drops X rays + Click here to see an animation of the experiment By measuring the energy necessary to stop the drops from descending, he was able to determine the charge per drop. The more energy needed to prevent the drop from falling, the smaller the charge of the drop. 33

34 . Millikan Oil Drop Experiment: Sample Data Here are some sample data points from Millikan's experiment. Drop Charge (Coulombs) E E E E 19 Interestingly, he found that the charges on each drop were a multiple of a number. Can you find what number they are all a multiple of? move for answer = 1.6x10 19 Coulombs He correctly interpreted this to be the charge of an electron. 34

35 9 If the charge of an electron is 1.6 x C and the charge to mass ratio is 1.76 x1011 C/kg, what is the mass of an electron? A 1.6 x kg B 2.82 x 10 8 kg Answer C 9.1 x kg D 1.1 x kg 35

36 10 Which of these could be the charge of a drop in the Millikan oil drop experiment? A B C D 0.80 x C 2.0 x C 8.0 x C 4.0 x C Answer 36

37 11 The magnitude of the charge on an electron was determined in the. A cathode ray tube, by J. J. Thomson B Millikan oil drop experiment C Dalton atomic theory D atomic theory of matter Answer 37

38 . Discovery of the Proton After the discovery of the electron, scientists believed that there must also be a positively charged particle in the atom. To look for these, they used an anode ray tube. Power Positive anode rays + By placing holes in the cathode so particles could move through it, they found that particles were indeed moving from the anode to the cathode. Since they move towards a negative plate, they must be positive. 38

39 . Discovery of the Proton The anode rays were referred to as protons, which were found to be significantly heavier than electrons. 1 proton = 1840 x mass of electron Since the heaviest anode rays in oxygen were found to be 8 x heavier than those in hydrogen, it was assumed that oxygen had 8 protons compared to hydrogen's 1. The number of protons an atom has is different for each element on the periodic table. 39

40 12 Which of the following is TRUE regarding protons? A They were originally called cathode rays B They move faster than cathode rays C They have a larger mass than electrons D They moved from the cathode to the anode Answer 40

41 13 Which of the following is NOT true regarding protons and electrons? A Both were found in all atoms B Their charges are equal in magnitude C Protons are significantly heavier than electrons D All elements have the same number of protons and electrons Answer 41

42 14 The mass of an electron was found to be 9.1 x kg. What is the mass of a proton? A 1.67x10 27 kg B 4.95x10 34 kg C 9.1x10 31 kg D 1.6x10 19 kg Answer 1 proton = 1840 x mass of electron 42

43 The Nucleus Return to Table of Contents 43

44 Models of the Atom: Plum Pudding Once it was determined that atoms are made up of negatively and positively charged particles, J.J. Thompson and team proposed that the structure of an atom resembled "plum pudding." The model featured a positive sphere of matter with negative electrons embedded in it. It was based around the idea that positive and negative charges attract and like charges repel. 44

45 Radioactivity Of course, models must be tested and the search was on to find evidence to support the "plum pudding" model. Ernest Rutherford used radioactivity used to test this theory. 45

46 Radioactivity Radioactivity is the spontaneous emission of radiation (energy) by an atom. Rutherford studied emissions from the unstable element uranium. Larger elements like uranium contain an atomic nucleus that can be either stable and does not change, or radioactive, meaning that it transforms, or decays, into another element after a certain amount of time. Decay can be as short as a fraction of a second and as long as a few million years. Radioactive Decay: Nucleus breaking into smaller nuclei and releasing energy. 46

47 Radioactivity Three types of radiation were discovered by Ernest Rutherford: α rays alpha particles (positively charged particles with a mass roughly 4x that of the proton) β rays beta particles (electrons) γ rays gamma rays (form of light with very high energy) 47

48 15 Of the three types of radioactivity characterized by Rutherford, which are particles? A α rays, β rays, and γ rays B γ rays C α rays and γ rays D α rays and β rays Answer 48

49 16 Beta particles are attracted to a charged plate, indicating they are charged. A positively, negatively B negatively, positively C neutrally, negatively D neutrally, positively Answer 49

50 17 Alpha particles are charged. A negatively B positively C neutrally D unknown Answer 50

51 Rutherford's Gold Foil Experiment Physicists Geiger and Marsden under the direction of Ernest Rutherford shot a beam of alpha particles at a thin sheet of gold foil and observed the scatter pattern of the particles. Click here to see an animation of the experiment 51

52 Discovery of the Nucleus In the Plum Pudding Model of the atom, positive and negative charges are dispersed evenly throughout the atom. If this model were correct, the high energy alpha particles would be slightly deflected by weak electric fields as they passed through the foil. Rutherford and team expected all alpha particles to pass through the atoms in the gold foil and be deflected by only a few degrees. 52

53 Discovery of the Nucleus What actually happened was very surprising. Most of the particles flew right through the foil with no deflection at all. 53

54 18 While most particles went straight through some bounced back...totally unexpected? What does this indicate about the location of protons in an atom? Answer 54

55 The Nuclear Atom Model The only way to account for the large angles was to assume that all the positive charge was contained within a tiny volume. gold foil A small very dense nucleus must lie within a mostly empty atom. Now we know that the radius of the nucleus is 1/10,000 that of the atom. nucleus alpha particle gold atom 55

56 In Rutherford's words... Then I remember two or three days later Geiger coming to me in great excitement and saying "We have been able to get some of the alpha particles coming backward " It was quite the most incredible event that ever happened to me in my life. It was almost as incredible Rutherford as if you fired a 15 inch shell at a piece of tissue paper and it came back and hit you. 56

57 19 The gold foil experiment performed in Rutherford's lab. A confirmed the plum pudding model of the atom B led to the discovery of the atomic nucleus C was the basis for Thomson's model of the atom D utilized the deflection of beta particles by gold foil Answer 57

58 20 In the Rutherford nuclear atom model: A the heavy subatomic particles reside in the nucleus B the principal subatomic particles all have essentially the same mass C the light subatomic particles reside in the nucleus D mass is spread essentially uniformly throughout the atom Answer 58

59 . Discovery of the Neutron Since electrons were so much smaller than protons, Rutherford believed the mass of an atom would be simply related to the number of protons present. However, they found that atoms were heavier than predicted!! Example Helium (He) Helium = 2 protons, 2 electrons Expected mass = 2 x (mass of proton) Actual mass = 4 x (mass of proton) 59

60 . Discovery of the Neutron Example Helium (He) Helium = 2 protons, 2 electrons Expected mass = 2 x (mass of proton) Actual mass = 4 x (mass of proton) Where is the extra mass coming from? Rutherford guessed it came from another particle called a neutron and verified its existence. 60

61 . Subatomic Particles Neutrons have a mass that is essentially the same as a proton and no charge. The mass of a proton or neutron is described as an atomic mass unit (u). 1 u = x10 27 kg Particle Charge Mass proton +1.6 x C x10 27 kg = u neutron no charge x10 27 kg = u electron 1.6 x C 9.1 x10 31 kg = u 61

62 . Neutrons, Protons, and Atomic Masses Since electrons have a much smaller mass than a proton or neutron, the mass of an atom (in amu) is generally considered to be equal to the sum of the protons and neutrons in an atom. (# of protons) + (# of neutrons) = atomic mass (A) in amu 62

63 The Nuclear Atom Rutherford postulated a very small, dense nucleus containing protons and neutrons with the electrons around the outside of the atom. Most of the volume of the atom is empty space. Nucleus containing protons and neutrons Volume occupied by by electrons 10 o 4 A 1 5A o Click here to see Atom animation scale: o 10 A = 1 nm o A = m 63

64 21 What is the mass of an element that has 10 protons and 11 neutrons (in u)? Answer 64

65 22 How many neutrons are present in an oxygen atom with a mass of 18 u? Answer 65

66 23 How many protons are present in atom with a mass of 13 u if it has 7 neutrons? Answer 66

67 24 What is the mass of an element with 18 protons, 18 electrons, and 22 neutrons? Answer 67

68 Nomenclature The number of protons in a nucleus is called the atomic number, and it is designated by the letter Z. This number is given for each element on the periodic table, often directly above the chemical symbol. 1 H Hydrogen Atomic Number 92 Atomic Symbol and Name U Atomic Mass Uranium

69 Nomenclature Together, protons and neutrons are referred to as nucleons. The number of nucleons in a nucleus is called the mass number, and it is designated by the letter A. The neutron number, N, is given by N = A Z. 69

70 . Atomic Symbols and Atomic Masses There are two common ways to indicate the mass of a particular atom. Method 2 (Nuclear Symbol) Method 1 X A A Z X Where X is the chemical symbol, Z is the atomic number, and A is the mass number. Example: Ag 107 Ag

71 25 How many neutrons are present in a neutral atom of Sr 80? A 32 B 38 C 80 D 42 Answer 71

72 26 Find the mass number. 23 Na 11 Answer Sodium Atom 72

73 27 How many protons does this element have? 23 Na 11 Answer Sodium Atom 73

74 28 How many electrons does this element have? 23 Na 11 Answer Sodium Atom 74

75 29 How many neutrons does this element have? 23 Na 11 Answer Sodium Atom 75

76 30 How many neutrons does this element have? 80 Br 35 Answer Bromine Atom 76

77 31 Which of the following has 45 neutrons? A B C D 80 Kr 80 Br 78 Se 103 Rh Answer 77

78 Formation of the Elements Return to Table of Contents 78

79 Atoms Recall, after the Big Bang, hydrogen, the lightest type of atom, was the first to form. Hydrogen contains one proton and one electron. + Answer Hydrogen 1 What is hydrogen's nuclear symbol? 79

80 Atoms Protons and neutrons continued to collide and were held together by the Nuclear Strong Force, creating more massive versions of Hydrogen called Deuterium and Tritium Hydrogen 1 Hydrogen 2 Deuterium Hydrogen 3 Tritium 80

81 Nuclear Fusion Reactions When protons and neutrons bind in a nuclear reaction, they lose a bit of mass, which is released as energy. The amount of energy released is called the "binding energy" and its magnitude can be found using mass energy equivalence. + E b = Δmc 2 + Energy + + Helium 4 81

82 * Binding Energy and Mass Defect To calculate the binding energy we start by converting Atomic mass units to kilograms. m = u x x kg = x kg 1u E = mc 2 Then use the energy mass equivalence to solve for binding energy. The binding energy is measured in Joules. E = mc 2 = x kg)(3 x 10 8 m/s) 2 = x J 82

83 * Binding Energy and Mass Defect For example, if we want to calculate the mass defect and binding energy of a Boron isotope B. There are 5 protons, 5 electrons and 5 neutrons. The mass of Hydrogen is equivalent to the mass of a proton n: 5 x u H: 5 x u B: u To calculate the mass defect: m = 5 x mass (neutron) + 5 x mass (proton) mass (Boron) m = (5 x u) + (5 x u) ( u) 5 83

84 * 32 Binding Energy is A the energy required to separate the nucleus into its constituent parts. B the energy required to split an atom into its constituent parts. C the energy that holds the electrons in orbit about the nucleus. Answer D the energy that pushes the protons apart. 84

85 * 33 What is the mass defect of? C: u n: u H: u 12 C 6 Answer 85

86 * 12 C 6 34 What is the binding energy (in Joules) of? C: u n: u H: u Answer 86

87 * What is the mass defect of U? U: u 1 n: u H: u Answer 87

88 * 36 What is the binding energy (in Joules) of U? U: u 1 n: u H: u Answer 88

89 Nuclear Fusion Making Helium occurs in 3 steps in the core of the star. Step 1: Two hydrogen atoms fuse H + 1 H 2 1 H + e + + v Producing a deuterium atom, a positron, and a neutrino. Positrons (e + ) are the opposite of electrons with the same mass and charge only positive. Positron emission causes a proton to become a neutron. A neutrino has no charge and does not affect the reaction. 89

90 Nuclear Fusion Making Helium occurs in 3 steps in the core of the star. Step 2: A hydrogen and a deuterium atom fuse H + 1 H 3 2 He + γ Producing a Helium 3 atom and a gamma ray. 90

91 Nuclear Fusion Making Helium occurs in 3 steps in the core of the star. Step 3: Two helium 3 atoms fuse He + He 4 2 He H + H Producing a Helium 4 atom and two hydrogen atoms. Note: Steps 1 & 2 must occur twice to produce the required helium 3 atoms. 91

92 Nuclear Fusion The net effect is to transform four protons into a helium nucleus plus two positrons, two neutrinos and two gamma rays H 2 He + 2e + + 2v + 2γ A conservation law applies to these reactions. The Law of the Conservation of Nucleon Number states that the total number of nucleons (A) remains constant for all nuclear reactions. A proton can change into a neutron (positron emission) or a neutron can change into a proton (electron emission) but the total number of nucleons stays constant. 92

93 37 Which of the following is true regarding a positron emission? A increases the number of protons B increases the number of electrons C increases the number of neutrons D does not affect the nucleus of the atom Answer 93

94 38 In the following fusion reaction, how many nucleons are in the unknown nucleus? C + 1 H X + γ Answer 94

95 39 Identify the unknown element in the nuclear reaction. A Boron B Carbon C Nitrogen C + 1 H X + γ Answer D Oxygen 95

96 40 In the following fusion reaction, how many nucleons are in the unknown nucleus? H + 1 H 1 X + 0 n Answer 96

97 41 Identify the unknown element in the nuclear reaction. A Hydrogen 1 B Hydrogen 2 C Helium 3 D Helium H + 1 H 1 X + 0 n Answer 97

98 Nuclear Fission While nuclear fusion reactions release energy while generating more massive elements, nuclear fission reactions also release energy. The target nucleus fissions into two nuclei of smaller masses and a number of neutrons. For example, the general equation for the fission of Uranium 235 is: U + 0 n 92 U* X + Y + neutrons + Q Note: Q represents energy released. 98

99 Nuclear Fission Here are two examples of possible fission reactions: U + 0 n 92 U* 56 Ba + 36 Kr + 30 n + Q U + 0 n 92 U* 54 Xe + 38 Sr + 20 n + Q Note that in either case the total number of nucleons is conserved

100 42 Identify the missing element in the following fission reaction U + 0 n 92 U* 56 Ba n + Q 1 A Kr B Sr Answer C U D Pu 100

101 43 Identify the missing element in the following fission reaction U + 0 n + 52 Te n A Kr B Zr C Pd Answer D Bk 101

102 44 Identify the missing element in the following fission reaction U + 0 n + 55 Cs n 1 A Rb B Np Answer C Cf D Cm 102

103 Nuclear Fission The energy release in a fission reaction is quite large. The smaller nuclei are stable with fewer neutrons, so multiple neutrons emerge from each fission. The neutrons can be used to induce fission in surrounding nuclei, causing a chain reaction. Enrico Fermi built the first self sustaining nuclear reaction in Chicago in Here's a nice simulation: fission 103

104 * Nuclear Reactions First fill in the missing component: H + 7 N 3 2 He +? Next, find the mass defect: m = u u u u = u Find the reaction energy: E = mc 2 = u x x kg 1u = x 10 8 m/s 2 E = x J 104

105 * 45 Compute the Q value of the reaction H + 1 H n + 2He? H: 2.014u H: 3.016u He: 4.003u Answer 105

106 * 46 Compute the Q value of the reaction U + 0 n Sr + 54Xe n U: u Sr: u Xe: Answer 106

107 * Nuclear Fission This is a schematic of a nuclear power plant. The fission process occurs in the Reactor Vessel (red), which heats water in a primary loop, which boils water in the secondary loop. Then, you just have a regular steam/turbine generator which generates electricity. 107

108 * Nuclear Fission The reactor is controlled by regulating how many neutrons are free to strike other Uranium atoms. Cadmium and Boron control rods are excellent neutron absorbers and are carefully adjusted to absorb the right amount of neutrons to allow a self sustained, controlled reaction. Critical Mass is the mass of the fissionable material that is required for nuclear fission to occur. Nuclear reactors are designed with layers upon layers of safety features and there is no possible way for a reactor to ever cause a nuclear explosion. Nuclear weapons are designed to explode in a massively uncontrolled chain reaction and are very, very different from a nuclear reactor. 108

109 Isotopes Return to Table of Contents 109

110 Isotopes As you have seen, atoms of the same element can have different numbers of neutrons. For example, some Carbon atoms have 6 neutrons, some carbon atoms have 8 neutrons. Atoms of the same element that have differing numbers of neutrons are called isotopes. C 12 C protons 6 neutrons electrons Note: Isotopes of an element will always have the same number of protons but differing masses due to the differing numbers of neutrons

111 Isotopes Write the complete symbol for each of these isotopes. Neon protons 10 neutrons 10 electrons Neon protons 11 neutrons 10 electrons Neon protons 12 neutrons 10 electrons Ne Ne Ne 111

112 47 Which pair of atoms constitutes a pair of isotopes of the same element? A 14 6 X 14 7 X B C X X X X Answer D X 19 9 X 112

113 48 Which of the following is TRUE of isotopes of an element? A They have the same number of protons B The have the same number of neutrons C They have the same mass D They have the same atomic number E A and D Answer 113

114 49 An atom that is an isotope of potassium (K) must... A Have 20 protons B Have 19 neutrons C Have 19 protons D A mass of 39 Answer 114

115 * 50 Which species is an isotope of 39 Cl? A 40 Ar + B 34 S 2 C 36 Cl D 39 Ar Answer 115

116 Isotopes and Atomic Masses Not all isotopes are found in the same abundances in nature. Neon protons 10 neutrons 10 electrons Neon protons 11 neutrons 10 electrons Neon protons 12 neutrons 10 electrons 90.48% 0.27% 9.25% So in a 10,000 atom sample of neon, you would on average find (atoms of each isotope of neon) 116

117 Atomic Masses and Mass Number The atomic mass indicates the average atomic mass of all the isotopes of a given element. This is the number reported on the periodic table. The mass number indicates the exact relative mass of a particular isotope of that element. These numbers are NOT reported on the periodic table. 10 Ne Atomic mass (an average no single neon atom has this mass)

118 Calculating Atomic Masses To determine the atomic mass of an element, one must know the masses of the isotopes and how commonly they are found in nature. Then a weighted average is calculated as shown below. Example: As we have seen, a sample of neon will consist of three stable isotopes Ne 20, Ne 21, and Ne 22. If the relative abundance of these are 90.48%, 0.27%, and 9.25% respectively, what is the atomic mass of neon? How to calculate average atomic mass: 1. Multiply each isotope by its % abundance expressed as a decimal 2. Add the products together 20(.9048) + 21(0.0027) + 22(0.0925) = amu 118

119 Example: Calculate Atomic Mass Carbon consists of two isotopes that are stable (C 12 and C 13). Assuming that 98.89% of all carbon in a sample are C 12 atoms, what is the atomic mass of carbon? First, = 1.10% C 14 move then... for answer 12(.9889) + 13(.011) = amu 119

120 * 51 Calculate the atomic mass of oxygen if it's abundance in nature is: 99.76% oxygen 16, 0.04% oxygen 17, and 0.20% oxygen 18. Answer (liquid oxygen) 120

121 52 Calculate the atomic mass of copper. Copper has 2 isotopes. 69.1% has a mass of 62.9 amu, the rest has a mass of amu. Answer 121

122 53 Sulfur has two stable isotopes: S 32 and S 34. Using the average atomic mass on the periodic table, which of the following best approximates the natural relative abundances of these isotopes of sulfur? A 50% S 32 and 50% S 34 B 25% S 32 and 75% S 34 C 75% S 32 and 25% S 34 D 95% S 32 and 5% S 34 Answer 122

123 Application of Isotopes Elephants are hunted for the ivory in their tusks. Game wardens use isotopes to track where elephants are going so they can help protect them. If an elephant eats plants from a wet climate, the ratio of N 15 to N 14 in the hair will be lower than is typically found in nature. If they graze plants grown in a dry climate, they will have a higher ratio of N 15 to N 14 than normal. Answer Where would you look for an elephant that had a hair sample with a ratio of N 15/N 14 where the normal ratio is N 15/ N 14? 123

124 Radioactive Decay Return to Table of Contents 124

125 Nuclear Stability Curve There are around 260 stable nuclear isotopes. The curve on the right plots N (neutron number) vs. Z (proton number). The most stable nuclei are shown in red, with the least stable shown in blue. More neutrons are required in stable higher mass nuclei the short range nuclear force's ability to counteract the repulsive Coulomb force is reduced as the nucleus grows larger. 125

126 Radioactivity Non stable nuclei become stable nuclei by emitting radiation. This is called radioactivity and was first observed and studied by Henri Becquerel, Marie Curie and Pierre Curie. Recall there are three types: Alpha particles helium nuclei. Beta particles a neutron is converted into a proton and emits an electron and an anti neutrino. When a proton is converted into a neutron, it emits a positron (postively charged electron) and a neutrino. The beta particles are these electrons and positrons emitted from the nucleus. Gamma rays high energy (high frequencey) electromagnetic radiation released when an excited nucleus moves to a lower energy level and releases the excess energy in the form of a photon. 126

127 Radioactivity Stopping Power Alpha particles are stopped by a sheet of paper. Beta particles are stopped by a thin sheet of aluminum. Gamma rays are the most penetrating and are stopped by several meters of lead. 127

128 Decay Nomenclature Alpha Decay is when a nucleus emits a Helium nucleus (2 protons, 2 neutrons, 0 electrons, with a charge of +2e). It is represented as shown below: A A 4 4 Z Z 2 2 X + Y + He Beta Decay is when a neutron converts into a proton and emits an electron and an anti neutrino (to conserve momentum) OR a proton converts into a neutron and emits a positron and a neutrino. A Z A Z X + X + 4 Z Z 1 Y + e + v Z + e + + v Gamma Radiation is the emission of a photon when an excited nucleus decays to a lower energy level. A Z A X* Z X + γ 128

129 Alpha Decay An example of a nucleus that undergoes alpha decay is the following isotope of polonium. We can find out what it decays into by balancing out the atomic (Z) and mass numbers (A) Po ? 4 2 Pb + He Another example is Radium Ra? Rn + He 2 129

130 Beta Decay Here are two examples of Beta Decay. Electron & Anti neutrino 11 4 Be 11? 5 0 B + 1e Positron & Neutrino Na 22 10? 0 B + 1e 130

131 54 Which type of radiation is the hardest to shield a person from? A Alpha particles. B Beta particles. C Gamma rays. Answer D X rays. 131

132 55 Which type of radiation is stopped by the shirt you wear? A Alpha particles. B Beta particles. C Gamma rays. Answer D X rays. 132

133 56 What is the missing component? B 6 C +? Answer 133

134 57 What is the missing component? Po 2He +? 4 Answer 134

135 58 What is the missing component? U 90Th +? Answer 135

136 Nuclear Half life Return to Table of Contents 136

137 Nuclear Half life A macroscopic sample of any radioactive substance consists of a great number of nuclei. These nuclei do not decay at one time. The decay is random and the decay of one nucleus has nothing to do with the decay of any other nuclei. The number of decays during a specific time period is proportional to the number of nuclei as well as the time period. Mathematically, it is defined as an exponential decay. After each specific time period, half of the nuclei decay. This specific time period is called the isotope's half life. The isotopes of a specific element have very different half lives; ranging from μseconds to never decaying at all. 137

138 Nuclear Half life The half life of an isotope is defined as the amount of time it takes for half of the original amount of the isotope to decay. For example, find how much of a starting sample of 200 g of an isotope, whose half life is 2 years, is left after 6 years: After 2 years (one half life), 100 g are left. After 4 years (two half lives), 50 g are left. After 6 years (three half lives), 25 g are left. 138

139 Nuclear Half life Another way of solving this problem is to recognize that a time interval of 6 years will include 3 half life periods of 2 years. n = number of half lives = 3 x = original sample size y = sample size after 3 half lives The 2 in the denominator represents the sample size being cut in half after each half life. y = x = 200g = 25g 2 n

140 59 The half life of an isotope is 5.0 seconds. What is the mass of the isotope after 30.0 seconds from a starting sample of 8.0 g? Answer 140

141 60 The half life of an isotope is 3 hours. How long (in hours) will it take for a sample of g to be reduced to g? Answer 141