1. Chapter 4- Atomic Structure

2. Studying the structure of atoms is a little like studying wind. Because you cannot see air, you must use indirect evidence to tell the direction of the wind. Atoms pose a similar problem because they are extremely small. Even with a microscope, scientists cannot see the structure of an atom.

3. If you cut a piece of aluminum foil in half, you have two smaller pieces of the same shiny, flexible substance. You could cut the pieces again and again. Can you keep dividing the aluminum into smaller pieces? Greek philosophers debated a similar question about 2500 years ago. Ancient Greek Models of Atoms

4. The philosopher Democritus believed that all matter consisted of extremely small particles that could not be divided. He called these particles atoms from the Greek word atomos, which means “uncut” or “indivisible.” Ancient Greek Models of Atoms

5. Aristotle thought that all substances were made of only four elements—earth, air, fire, and water. He did not think there was a limit to the division of matter. For many centuries, most people accepted Aristotle’s views on the structure of matter. By the 1800s, scientists had enough experimental data to support an atomic model. Ancient Greek Models of Atoms

6. What was Dalton’s theory of the structure of matter? Dalton’s Atomic Theory Dalton proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided.

7. Evidence for Atoms John Dalton studied the behavior of gases in air. Based on the way gases exert pressure, Dalton correctly concluded that a gas consists of individual particles. Dalton measured masses of elements that combine when compounds form. The ratio of the masses of the elements in each compound was always the same. In other words, compounds have a fixed composition. Dalton’s Atomic Theory

8. When magnesium burns, it combines with oxygen. In magnesium oxide, the ratio of the mass of magnesium to the mass of oxygen is always about 3 : 2. Magnesium dioxide has a fixed composition. Dalton’s Atomic Theory

9. Dalton’s Theory All elements are composed of atoms. All atoms of the same element have the same mass, and atoms of different elements have different masses. Compounds contain atoms of more than one element. In a particular compound, atoms of different elements always combine in the same way. Dalton’s Atomic Theory

10. Dalton made these wooden spheres as a model to represent the atoms of different elements. A tiny, solid sphere with a different mass represents each type of atom. Dalton’s Atomic Theory

11. A theory must explain the data from many experiments. Because Dalton’s atomic theory met that goal, the theory became widely accepted. Over time, scientists found that not all of Dalton’s ideas about atoms were completely correct. They revised the theory to take into account new discoveries. Dalton’s Atomic Theory

12. What contributions did Thomson make to the development of atomic theory? Thomson’s Model of the Atom Thomson’s experiments provided the first evidence that atoms are made of even smaller particles.

13. When some materials are rubbed, they gain the ability to attract or repel other materials. S uch materials are said to have either a positive or a negative electric charge. Objects with like charges repel, or push apart. Objects with opposite charges attract, or pull together. Thomson’s Model of the Atom

14. Amber is the hardened form of a sticky, viscous liquid that protects trees from insects and disease. If amber is rubbed with wool, it becomes charged and can attract a feather. Thomson’s Model of the Atom

15. Thomson’s Experiments In his experiments, Joseph John Thomson used a sealed tube containing a very small amount of gas. Thomson’s Model of the Atom Sealed tube filled with gas at low pressure Glowing beam Metal disk Source of electric current Metal disk Source of electric current

16. Thomson’s Experiments In his experiments, Joseph John Thomson used a sealed tube containing a very small amount of gas. Thomson’s Model of the Atom Sealed tube filled with gas at low pressure Glowing beam Metal disk Source of electric current Metal disk Source of electric current Positive plate Negative plate

17. When the current was turned on, the disks became charged, and a glowing beam appeared in the tube. Thomson hypothesized that the beam was a stream of charged particles that interacted with the air in the tube and caused the air to glow. Thomson observed that the beam was repelled by the negatively charged plate and attracted by the positively charged plate. Thomson’s Model of the Atom

18. Evidence for Subatomic Particles Thomson concluded that the particles in the beam had a negative charge because they were attracted to the positive plate. He hypothesized that the particles came from inside atoms because no matter what metal Thomson used for the disk, the particles produced were identical. the particles had about 1/2000 the mass of a hydrogen atom, the lightest atom. Thomson’s Model of the Atom

19. Thomson’s Model Thomson revised Dalton’s model to account for these subatomic particles. The atom has neither a positive nor a negative charge, but there must always be some positive charge in the atom. The atom is filled with a positively charged mass of matter that has negative charges evenly scattered throughout it. Thomson’s Model of the Atom

20. Thomson’s model is called the “plum pudding” model. Today, it might be called the “chocolate chip ice cream” model. The chips represent negatively charged particles, which are spread evenly through a mass of positively charged matter—the vanilla ice cream. Thomson’s Model of the Atom

21. What contributions did Rutherford make to the development of atomic theory? Rutherford’s Atomic Theory According to Rutherford’s model, all of an atom’s positive charge is concentrated in its nucleus.

22. Rutherford’s Hypothesis Ernest Rutherford designed an experiment to find out what happens to alpha particles when they pass through a thin sheet of gold. Alpha particles are fast- moving, positively charged particles. Based on Thomson’s model, Rutherford hypothesized that the mass and charge at any location in the gold would be too small to change the path of an alpha particle. He predicted that most particles would travel in a straight path from their source to a screen that lit up when struck. Rutherford’s Atomic Theory

23. The Gold Foil Experiment Rutherford’s Atomic Theory Beam of alpha particles Source of alpha particles Slit Deflected particle Undeflected particle Screen Gold atoms Alpha particles Nucleus

24. Discovery of the Nucleus The alpha particles whose paths were deflected must have come close to another charged object. The closer they came, the greater the deflection. However, many alpha particles passed through the gold without being deflected. These particles did not pass close to a charged object. Rutherford’s Atomic Theory

25. Thomson’s model did not explain all of the evidence from Rutherford's experiment. Rutherford proposed a new model. The positive charge of an atom is not evenly spread throughout the atom. Positive charge is concentrated in a very small, central area. The nucleus of the atom is a dense, positively charged mass located in the center of the atom. Rutherford’s Atomic Theory

26. The Houston Astrodome occupies more than nine acres and seats 60,000 people. If the stadium were a model for an atom, a marble could represent its nucleus. The total volume of an atom is about a trillion (10 12 ) times the volume of its nucleus. Rutherford’s Atomic Theory

27. Assessment Questions 1. Dalton’s theory did not include which of the following points? a. All elements are composed of atoms. b. Most of an atom’s mass is in its nucleus. c. Compounds contain atoms of more than one element. d. In a specific compound, atoms of different elements always combine in the same way.

28. Assessment Questions 1. Dalton’s theory did not include which of the following points? a. All elements are composed of atoms. b. Most of an atom’s mass is in its nucleus. c. Compounds contain atoms of more than one element. d. In a specific compound, atoms of different elements always combine in the same way. ANS:B

29. Assessment Questions 2. J. J. Thomson’s experiments provided the first evidence of a. atoms. b. a nucleus. c. subatomic particles. d. elements.

30. Assessment Questions 2. J. J. Thomson’s experiments provided the first evidence of a. atoms. b. a nucleus. c. subatomic particles. d. elements. ANS:C

31. Assessment Questions 2. J. J. Thomson’s experiments provided the first evidence of a. atoms. b. a nucleus. c. subatomic particles. d. elements.

32. Assessment Questions 1. The concept of an atom as a small particle of matter that cannot be divided was proposed by the ancient Greek philosopher, Democritus. True False ANS:T

33. 4.2 Subatomic Particles

34. Protons Based on experiments with elements other than gold, Rutherford concluded that the amount of positive charge varies among elements. A proton is a positively charged subatomic particle that is found in the nucleus of an atom. Each proton is assigned a charge of 1+. Each nucleus must contain at least one proton. Properties of Subatomic Particles

35. Electrons The particles that Thomson detected were later named electrons. An electron is a negatively charged subatomic particle that is found in the space outside the nucleus. Each electron has a charge of 1 . Properties of Subatomic Particles

36. Neutrons In 1932, the English physicist James Chadwick carried out an experiment to show that neutrons exist. Chadwick concluded that the particles he produced were neutral because a charged object did not deflect their paths. A neutron is a neutral subatomic particle that is found in the nucleus of an atom. It has a mass almost exactly equal to that of a proton. Properties of Subatomic Particles

37. What are three subatomic particles? Properties of Subatomic Particles Protons, electrons, and neutrons are subatomic particles.

38. What properties can be used to compare protons, electrons, and neutrons? Comparing Subatomic Particles Protons, electrons, and neutrons can be distinguished by mass, charge, and location in an atom.

39. Everything scientists know about subatomic particles is based on how the particles behave in experiments. Scientists still do not have an instrument that can show the inside of an atom. Comparing Subatomic Particles

40. Here are some similarities and differences between protons, electrons, and neutrons. Protons and neutrons have almost the same mass. About 2000 electrons equal the mass of one proton. An electron has a charge that is equal in size to, but the opposite of, the charge of a proton. Neutrons have no charge. Protons and neutrons are found in the nucleus. Electrons are found in the space outside the nucleus. Comparing Subatomic Particles

41. How are atoms of one element different from atoms of other elements? Atomic Number and Mass Number Atoms of different elements have different numbers of protons.

42. Atomic Number and Mass Number Atomic Number The atomic number of an element is the number of protons in an atom of that element. All atoms of any given element have the same atomic number. Each hydrogen atom has one proton in its nucleus. Hydrogen is assigned the atomic number 1. Each element has a unique atomic number.

43. Atomic Number and Mass Number Each element has a different atomic number. A The atomic number of sulfur (S) is 16. B The atomic number of iron (Fe) is 26. C The atomic number of silver (Ag) is 47.

44. Atomic Number and Mass Number Atoms are neutral, so each positive charge in an atom is balanced by a negative charge. That means the atomic number of an element also equals the number of electrons in an atom of that element. Hydrogen has an atomic number of 1, so a hydrogen atom has 1 electron. Sulfur has an atomic number of 16, so a sulfur atom has 16 electrons.

45. Atomic Number and Mass Number Mass Number The mass number of an atom is the sum of the protons and neutrons in the nucleus of that atom. To find the number of neutrons in an atom, you need the mass number of the atom and its atomic number. The atomic number of aluminum is 13. An atom of aluminum that has a mass number of 27 has 13 protons and 14 neutrons

46. What is the difference between two isotopes of the same element? Isotopes Isotopes of an element have the same atomic number but different mass numbers because they have different numbers of neutrons.

47. Isotopes Isotopes are atoms of the same element that have different numbers of neutrons and different mass numbers. To distinguish one isotope from another, the isotopes are referred by their mass numbers. For example, oxygen has 3 isotopes: oxygen-16, oxygen-17, and oxygen-18. All three oxygen isotopes can react with hydrogen to form water or combine with iron to form rust.

48. Isotopes With most elements, it is hard to notice any differences in the physical or chemical properties of their isotopes. Hydrogen is an exception. Hydrogen-1 has no neutrons. (Almost all hydrogen is hydrogen-1.) Hydrogen-2 has one neutron, and hydrogen-3 has two neutrons. Because a hydrogen-1 atom has only one proton, adding a neutron doubles its mass.

49. Isotopes Water that contains hydrogen-2 atoms in place of hydrogen-1 atoms is called heavy water. Hydrogen-2 atoms have twice the mass of hydrogen-1 atoms, so the properties of heavy water are different from the properties of ordinary water.

50. Assessment Questions 1. In which way do isotopes of an element differ? a. number of electrons in the atom b. number of protons in the atom c. number of neutrons in the atom d. net charge of the atom

51. Assessment Questions 1. In which way do isotopes of an element differ? a. number of electrons in the atom b. number of protons in the atom c. number of neutrons in the atom d. net charge of the atom ANS:C

52. Assessment Questions 1. Of the three subatomic particles that form the atom, the one with the smallest mass is the neutron. True False

53. Assessment Questions 1. Of the three subatomic particles that form the atom, the one with the smallest mass is the neutron. True False ANS:F, electron

54. 4.3 Compounds

55. Have you ever wondered what produces the different colors in a fireworks display? Certain compounds will produce certain colors of light when they are heated. Compounds containing the element strontium produce red light. Compounds containing barium produce green light.

56. What can happen to electrons when atoms gain or lose energy? Bohr’s Model of the Atom

57. As did Rutherford's atomic model, Bohr’s atomic model had a nucleus surrounded by a large volume of space. But Bohr’s model focused on the electrons and their arrangement. In Bohr’s model, electrons move with constant speed in fixed orbits around the nucleus, like planets around a sun. Each electron in an atom has a specific amount of energy. Bohr’s Model of the Atom

58. Energy Levels When an atom gains or loses energy, the energy of an electron can change. The possible energies that electrons in an atom can have are called energy levels. An electron cannot exist between energy levels. Bohr’s Model of the Atom

59. An electron in an atom can move from one energy level to another when the atom gains or loses energy. Bohr’s Model of the Atom Nucleus Electron Electrons gain or lose energy when they move between fixed energy levels Bohr Model

60. An analogy for energy levels of electrons is a staircase. The landing at the bottom of the staircase is the lowest level. Each step up represents a higher energy level. The step height represents an energy difference between levels. You can only move in whole numbers of stairs. Bohr’s Model of the Atom

61. An electron may move up or down two or more energy levels if it gains or loses the right amount of energy. The size of the jump between energy levels determines the amount of energy gained or lost. No two elements have the same set of energy levels. Bohr’s Model of the Atom

62. Evidence for Energy Levels Scientists can measure the energy gained when electrons absorb energy and move to a higher energy level or measure the energy released when the electron moves to a lower energy level. Light is a form of energy that can be observed. Bohr’s Model of the Atom

63. The movement of electrons between energy levels explains the light you see when fireworks explode. Heat causes some electrons to move to higher energy levels. When those electrons move back to lower energy levels, they release energy. Some of that energy is released as visible light. Different elements emit different colors of light because no two elements have the same set of energy levels. Bohr’s Model of the Atom

64. What model do scientists use to describe how electrons behave in atoms? An electron cloud is a visual model of the most likely locations for electrons in an atom. Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus. Electron Cloud Model

65. Bohr’s model was improved as scientists made further discoveries. Bohr correctly assigned energy levels to electrons, but electrons do not move like planets in a solar system. Today, scientists use probability when trying to predict the locations and motions of electrons in atoms. An electron cloud is a visual model of the most likely locations for electrons in an atom. Electron Cloud Model

66. The electron cloud model replaced Bohr's vision of electrons moving in predictable paths. Electron Cloud Model The nucleus contains protons and neutrons The electron cloud is a visual model of the probable locations of electrons in an atom. The probability of finding an electron is higher in the denser regions of the cloud. Electron Cloud Model

67. The photographs below provide an analogy for an electron cloud. When the propeller of an airplane is at rest, you can see the location of the blades. When the propeller is moving, you see only a blur that is similar to a drawing of an electron cloud. Electron Cloud Model

68. What model do scientists use to describe how electrons behave in atoms? An orbital is a region of space around the nucleus where an electron is likely to be found. The electron cloud represents all the orbitals in an atom. An electron cloud is a good approximation of how electrons behave in their orbitals. Electron Cloud Model

69. For an analogy to the concept of an orbital, imagine a map of your school. Mark your exact location with a dot once every 10 minutes over a period of one week. The dots on your map are a model of your “orbital.” They describe your most likely locations. The places you visit the most would have the highest concentration of dots. The places you visit the least would have the lowest concentration of dots. Electron Cloud Model

70. The level in which an electron has the least energy—the lowest energy level—has only one orbital. Higher energy levels have more than one orbital. Electron Cloud Model

71. What is the most stable configuration of electrons in an atom? Electron Configurations

72. An electron configuration is the arrangement of electrons in the orbitals of an atom. When all the electrons in an atom have the lowest possible energies, the atom is said to be in its ground state. Electron Configurations

73. The most stable electron configuration is the one in which the electrons are in orbitals with the lowest possible energies. Electron Configurations

74. A lithium atom has three electrons. In the ground state, two of the lithium electrons are in the orbital of the first energy level. The third electron is in an orbital of the second energy level. If a lithium atom absorbs enough energy, one of its electrons can move to an orbital with a higher energy. This configuration is referred to as an excited state. An excited state is less stable than the ground state. Eventually, the electron that was promoted to a higher energy level loses energy, and the atom returns to the ground state. Electron Configurations

75. The ground state of a person is on the floor. A gymnast on a balance beam is like an atom in an excited state—not very stable. When she dismounts, the gymnast will return to a lower, more stable energy level. Electron Configurations

76. Assessment Questions 1. According to Bohr’s model of the atom, which of the following can happen when an atom gains energy? a. An atom returns to its ground state. b. A neutron can be changed into a proton. c. A proton can move to a higher energy level. d. An electron can move to a higher energy level.

77. Assessment Questions 1. According to Bohr’s model of the atom, which of the following can happen when an atom gains energy? a. An atom returns to its ground state. b. A neutron can be changed into a proton. c. A proton can move to a higher energy level. d. An electron can move to a higher energy level. ANS:D

78. Assessment Questions 2. How does the modern atomic theory describe the location of electrons in an atom? a. Electrons move randomly in space around the nucleus. b. Electrons can be described as a cloud based on probable locations. c. Electrons orbit the nucleus in the same way that planets orbit the sun. d. Electrons move in a spiral pattern if increasing distance from the nucleus.

79. Assessment Questions 2. How does the modern atomic theory describe the location of electrons in an atom? a. Electrons move randomly in space around the nucleus. b. Electrons can be described as a cloud based on probable locations. c. Electrons orbit the nucleus in the same way that planets orbit the sun. d. Electrons move in a spiral pattern if increasing distance from the nucleus. ANS:B

80. Assessment Questions 3. What is meant when an atom is said to be in its ground state? a. There is no net charge on the atom. b. The number of protons equals the number of neutrons. c. The atom’s electrons all have the lowest possible energies. d. It is the isotope with the least number of neutrons.

81. Assessment Questions 3. What is meant when an atom is said to be in its ground state? a. There is no net charge on the atom. b. The number of protons equals the number of neutrons. c. The atom’s electrons all have the lowest possible energies. d. It is the isotope with the least number of neutrons. ANS:C