1. Atomic Theories Chapter 17 Page 506

2. Atomic Theories First Thoughts People began wondering about matter more than 2,500 years ago. Some of the early philosophers thought that matter was composed of tiny particles. They reasoned that you could take a piece of matter, cut it in half, cut the half piece in half again, and continue to cut again and again. Eventually, you wouldn’t be able to cut any more. You would have only one particle left. They named these particles atoms, a term that means “cannot be divided.”

3. A Model of the Atom- a model is a simplified representation, a picture. During the eighteenth century, scientists in laboratories began debating the existence of atoms once more. Chemists were learning about matter and how it changes. They found that certain substances couldn’t be broken down into simpler substances. Scientists came to realize that all matter is made up of elements. An element is matter made of atoms of only one kind.

4. Classifications of Matter (p450) Matter Yes Can it be separated by physical means? Pure substance Mixture Can it be decomposed by ordinary chemical means? Is the composition uniform? ElementCompound Heterogeneous mixture Homogeneous mixture No Yes No Yes No Yes No Yes

5. Chemical Laws Law of Conservation of Mass or matter states that mass is neither created nor destroyed in ordinary chemical reactions. Law of Definite Proportions- the fact that a specific chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or the source of the sample. Law of Multiple Proportions- if two or more different compounds are composed of the same two elements, then the ration of the masses of the second element combined with a certain mass of the first element is always a ration of small whole numbers.

6. Examples Law of Definite Proportions Carbon Dioxide –Always 73% oxygen and 27% carbon by weight Law of Multiple Proportions Carbon Monoxide –6 grams of carbon, 8 grams of oxygen Carbon Dioxide –6 grams of carbon, 16 grams of oxygen

7. Dalton’s Concept (1766-1844) John Dalton, an English schoolteacher proposed the following ideas about matter: 1. Matter is made up of atoms. *2. Atoms cannot be divided into smaller pieces. *3. All the atoms of an element are exactly alike. 4. Different elements are made of different kinds of atoms. Dalton pictured an atom as a hard sphere that was the same throughout.

8. Scientific Evidence In 1870, the English scientist William Crookes did experiments with a glass tube that had almost all the air removed from it. The glass tube had two pieces of metal called electrodes sealed inside. The electrodes were connected to a battery by wires.

9. A Strange Shadow The other, called the cathode, has a negative charge. When the battery was connected, the glass tube suddenly lit up with a greenish-colored glow. One electrode, called the anode, has a positive charge.

10. A Strange Shadow A shadow of the object appeared at the opposite end of the tube—the anode. The shadow showed Crookes that something was traveling in a straight line from the cathode to the anode, similar to the beam of a flashlight.

11. Cathode Rays Crookes hypothesized that the green glow in the tube was caused by rays, or streams of particles. These rays were called cathode rays because they were produced at the cathode. Crookes’s tube is known as a cathode-ray tube, or CRT. – TV or computer monitor.

12. J.J Thompson The beam is bent in the direction of the magnet. Light cannot be bent by a magnet, so the beam couldn’t be light. In 1897, J.J. Thomson, an English physicist, tried to clear up the confusion. He placed a magnet beside the tube from Crookes’s experiments.

13. The Electron Thomson concluded that cathode rays are charged particles of matter. Based on the direction the beam bent in the magnetic field he reasoned that the beam was negative charges. He observed that these particles were attracted to the positively charged anode, so he reasoned that the particles must be negatively charged.

14. The Electron These negatively charged particles are now called electrons. Thomson also inferred that electrons are a part of every kind of atom because they are produced by every kind of cathode material. If atoms contain one or more negatively charged particles, then all matter, which is made of atoms, should be negatively charged as well. But matter is not negatively charge. Could it be that atoms also contain some positive charge?

15. Thomson’s Atomic Model Thomson pictured a sphere of positive charge. The negatively charged electrons were spread evenly among the positive charge. The atom is neutral. Plum pudding model (chocolate chip cookie)

16. Chocolate Chip Cookie Model

17. Rutherford’s Experiments Ernest Rutherford and his coworkers began an experiment to find out if Thomson’s model of the atom was correct. They wanted to see what would happen when they fired fast-moving, positively charged bits of matter, called alpha particles, at a thin film of metal such as gold.

18. Rutherford’s Gold Foil Experiment

19. A Model with a Nucleus They might have drawn diagrams like those which uses Thomson’s model and shows what Rutherford expected. Most alpha particles could move through the foil with little or no interference. Some alpha particles bounced back at them. Shoot a tissue with a 22 and the bullet bounces back

20. The Proton This figure shows how Rutherford’s new model of the atom fits the experimental data. Most alpha particles could move through the foil with little or no interference. But some bounce back.

21. Rutherford’s Model The actual results did not fit Thompson’s model, so Rutherford proposed a new one. He hypothesized that almost all the mass of the atom and all of its positive charge are crammed into an incredibly small region of space at the center of the atom called the nucleus. Most of the atom is empty space.

22. The Proton In 1920 scientists identified the positive charges in the nucleus as protons. A proton is a positively charged particle present in the nucleus of all atoms.

23. The Neutron An atom’s electrons have almost no mass. According to Rutherford’s model, the only other particle in the atom was the proton. That meant that the mass of an atom should have been approximately equal to the mass of its protons.

24. The Neutron However, it wasn’t. The mass of most atoms is at least twice as great as the mass of its protons. It was proposed that another particle must be in the nucleus to account for the extra mass. The particle, which was later call the neutron (NEW trahn), would have the same mass as a proton and be electrically neutral.

25. The Neutron Proving the existence of neutrons was difficult though, because a neutron has no charge. It took another 20 years before scientists were able to show by more modern experiments that atoms contain neutrons.

26. The Neutron The model of the atom was revised again to include the newly discovered neutrons in the nucleus. The nuclear atom has a tiny nucleus tightly packed with positively charged protons and neutral neutrons. Negatively charged (electrons) particles surround this nucleus

27. Further Developments Then, electrons would travel in orbits around the nucleus. A physicist named Niels Bohr even calculated exactly what energy levels those orbits would represent for the hydrogen atom. However, scientists soon learned that electrons are in constant, unpredictable motion and can’t be described easily by an orbit.

28. Electrons as Waves Physicists began to wrestle with explaining the unpredictable nature of electrons. The unconventional solution was to understand electrons not as particles, but as waves. This unconventional solution also applied to light waves which can be thought of as particles.

29. The Electron Cloud Model The new model of the atom allows for the somewhat unpredictable wave nature of electrons by defining a region where electrons are most likely to be found. Electrons travel in a region surrounding the nucleus, which is called the electron cloud.

30. Size and Scale Drawings of the nuclear atom don’t give an accurate representation of the extreme smallness of the nucleus compared to the rest of the atom. For example, if the nucleus were the size of a table-tennis ball, the atom would have a diameter of more than 2.4 km. A football field is 100 m so 10 football fields is 1 km so 2.4 km = 24 football fields

31. Atomic Models Ancient Greek- “cannot be divided” John Dalton’s Theory- simple sphere Thompson’s Model- plum pudding Rutherford- Electron/Proton Model Bohr- Orbital Model/ Neutron Modern – electron Cloud Model –Protons, Neutrons, Electrons Current Model –Protons and neutrons made up of quarks

32. Identifying Numbers Page 512 The atoms of different elements contain different numbers of protons. The atomic number of an element is the number of protons in the nucleus of an atom of that element. Designated as Z Atoms of an element are identified by the number of protons because this number never changes without changing the identify of the element.

33. Number of Neutrons They are all carbon atoms because they all have six protons. Most atoms of carbon have six neutrons, some have 7 and some have 8. These three kinds of carbon atoms are called isotopes. Isotopes (I suh tohps) are atoms of the same element that have different numbers of neutrons.

34. Mass Number The mass number of an isotope is the number of neutrons plus protons. Also called the atomic mass. Designated A

35. Atomic Number, Atomic Mass, and # of Neutrons mass number = #protons + # neutrons # protons = atomic number Number of neutrons = mass number – atomic number Number of neutrons = A – Z

36. Find the number How many neutrons in Iodine-131? What is the atomic mass number of a copper isotope with 34 neutrons? Worksheet

37. Strong Nuclear Force Because protons are positively charged, you might expect them to repel each other just as the north ends of two magnets tend to push each other apart. It is true that they normally would do just that. However, when they are packed together in the nucleus with the neutrons, an even stronger binding force takes over. That force is called the strong nuclear force and it only works at very small distances.

38. Radioactive Decay Many atomic nuclei are stable when they have about the same number of protons and neutrons. As the number of protons increases, the ratio of neutrons to protons must increase. A nucleus can become more stable by changing the ration of protons to neutrons. In these nuclei, repulsion builds up. The nucleus must release a particle to become stable. This release of nuclear particles and energy is called radioactive decay.

39. Transmutation When the particles that are ejected from a nucleus changes the number of protons, the atomic number of the nucleus changes. When this happens, one element changes into a different element. The changing of one element into another through radioactive decay is called transmutation.

40. Radioactive Decay Transmutation is occurring in most of your homes right now. A smoke detector makes use of radioactive decay. The Nucleus 2 2 This device contains americium-241 (a muh RIH shee um), which undergoes transmutation by ejecting energy and an alpha particle.

41. Radioactive Decay In the smoke detector, the fast-moving alpha particles enable the air to conduct an electric current. As long as the electric current is flowing, the smoke detector is silent. The Nucleus 2 2

42. Radioactive Decay The Nucleus 2 2 The alarm is triggered when the flow of electric current is interrupted by smoke entering the detector.

43. Changed Identity When americium expels an alpha particle, it’s no longer americium.

44. Review Number of Protons determines the element. #Protons is Atomic Number (Z) # Protons + # Neutrons is Atomic Mass Number (A) #Neutrons = A-Z “Strong Force” keeps protons in the nucleus from flying apart.

45. Review (cont) Neutrons provide the strong force. Ratio of Neutrons to Protons determines the strength of the strong force. For low Atomic # elements n/p = 1 is stable. For high Atomic # elements n/p >1.5 is stable. Nuclei emit particles to balance n/p and become stable.

46. Loss of Beta Particles Some elements undergo transmutations through a different process. Their nuclei emit an electron from the nucleus called a beta particle. A beta particle is a high-energy electron that comes from the nucleus, not from the electron cloud.

47. Neutron into a Proton Emission of Beta Particle ++ - Proton Neutron + - Beta Particle

48. Loss of Beta Particles Because a neutron has been changed into a proton, the nucleus of the element has an additional proton.

49. Loss of Beta Particles Unlike the process of alpha decay, in beta decay the atomic number of the element that results is greater by one.

50. Proton into a Neutron Electron Capture + - Neutron Proton

51. Proton into a Neutron Positron Emission + Proton Neutron + Positron

52. Types of Radioactive Decay TypeSymbolChargeAffect Alpha Particle 4 2 He2+Lowers Z and A Electron Capture e -1 Lowers Z, A same Beta Particle 0 -1 β1-Raises Z, A same Positron 0 +1 β1+Lowers Z, A-same Gamma rayγ0Energy Release

53. Particle Energy/Penetration/Damage ParticleEnergyPenetrationDamage Alpha ParticleLoses energy quickly Sheet of paper blocks Damage to cells Beta ParticleMuch faster moving Penetrate paper, stopped by aluminum foil Damage cells Gamma RaysElectromagneticStopped by dense materials, lead, thick concrete Little damage, pass right through tissues

54. Rate of Nuclear Decay The half-life of a radioactive isotope is the amount of time it takes for half of a sample of the element to decay.

55. Calculating Half-Life Decay Iodine-131 has a half-life of eight days. If you start with a sample of 4 g of iodine- 131, after eight days you would have only 2 g of iodine-131 remaining.

56. Carbon Dating Carbon-14 is used to determine the age of dead animals, plants, and humans. In a living organism, the amount of carbon-14 remains in constant balance with the levels of the isotope in the atmosphere or ocean. This balance occurs because living organisms take in and release carbon. As soon as the organism dies the amount of C-14 begins to decrease.

57. Assumptions of Carbon Dating The percentage of C-14 in a sample of carbon (containing C-12,C-13, C-14) has been the same for thousands of years. The rate of decay of C-14 has been constant for thousands of years. “All things are as they have always been!”

58. Detecting Radioactivity Alpha Particles are positively charged particles and Beta Particles are negatively charged particles. We can use the properties of charged particles to detect them.

59. Radiation Detectors Cloud Chamber – water or alcohol vapor condenses around the ion as it moves through the chamber leaving a trail like the trail of a jet plane –Alpha particle leaves a short thick trail –Beta particle leaves a long thin trail. Bubble chamber is similar device

60. Radiation Detectors (cont) Electroscope- we talked about these earlier as a way to detect electrical charge. Geiger counter- uses the fact that charged particles will allow an electrical current to flow and counts the times that a current flows in a special tube. (Page 549) Fluorescent screen- remember Rutherford’s experiment

61. Medical uses for radiation See pages 554-556 Tracers – since radioactive elements give off charged particles, these elements can be “trace” as they move through the body –Iodine 131 collects in a properly functioning thyroid gland but not as much in a gland with a tumor Destroy cancer cells- we know that radiation particles can damage cells. They damage fast growing cancer cells more than normal cells

62. Energy from the Nucleus In nuclear fission, a very heavy nucleus splits into more-stable nuclei of intermediate mass. The mass of the products is less than the original nucleus. This mass is converted into energy according to Einstein’s equation E= mc 2 Enormous amounts of energy are released. Nuclear fission can occur spontaneously or when nuclei are bombarded by particles.

64. Nuclear Fission A chain reaction is a reaction in which the material that starts the reaction is also one of the products and can start another reaction. The minimum amount of nuclide that provides the number of neutrons needed to sustain a chain reaction is called the critical mass. Nuclear reactors use controlled-fission chain reactions to produce energy and radioactive nuclides.

65. Nuclear Power Plant Model

66. Nuclear Fusion In nuclear fusion, low-mass nuclei combine to form a heavier, more stable nucleus. Nuclear fusion releases even more energy per gram of fuel than nuclear fission. If fusion reactions could be controlled, they could be used for energy generation.

67. Solar Energy and Bombs + ++ + + + + + Energy 4 1 1 H 4 2 He 2 +1 β Positrons Helium Hydrogen

68. Making Synthetic Elements Scientists now create new elements by smashing atomic particles into a target element. The absorbed particle converts the target element into another element with a higher atomic number. The new element is called a synthetic element because it is made by humans. Elements with atomic numbers 93 to 112, and 114 have been made in this way.