chem_TE_ch05.fm Page 127 Saturday, April 15, 2006 10:46 AM 5.1 Models of the Atom 5.1 1 FOCUS Connecting to Your World Aeronautical engineers use Guide for Reading wind tunnels and scale models to simulate and test the forces from the Objectives moving air on each proposed design. The scale model shown is a physical Key Concepts • What was inadequate about 5.1.1 Identify the inadequacies in model. However, not all models are physical. In Rutherford’s atomic model? fact, several theoretical models of the atom • What was the new proposal in the Rutherford atomic model. have been developed over the last few the Bohr model of the atom? 5.1.2 Identify the new proposal in • hundred years. In this section, you will What does the quantum mechanical model determine the Bohr model of the atom. learn about the currently accepted model about the electrons in an atom? 5.1.3 Describe the energies and of how electrons behave in atoms. • How do sublevels of principal energy levels differ? positions of electrons accord- Vocabulary ing to the quantum mechanical energy levels model. quantum 5.1.4 Describe how the shapes of The Development of Atomic Models quantum mechanical model So far in this textbook, the model for the atom consisted of protons and atomic orbital orbitals related to different neutrons making up a nucleus surrounded by electrons. After discovering Reading Strategy sublevels differ. the atomic nucleus, Rutherford used existing ideas about the atom and Using Prior Knowledge Before proposed an atomic model in which the electrons move around the you read, jot down three things nucleus, like the planets move around the sun. Rutherford’s model you already know about atoms. Guide for Reading explained only a few simple properties of atoms. It could not explain, for As you read the section, explain example, why metals or compounds of metals give off characteristic colors how what you already knew helped you learn something new. Build Vocabulary L2 when heated in a flame, or why objects—when heated to higher and higher temperatures—first glow dull red, then yellow, then white, as shown in Word Parts The word orbital was Figure 5.1. Rutherford’s atomic model could not explain the chemical coined by scientists in 1932 to describe properties of elements. Explaining what leads to the chemical properties of the space in which an atom has a high elements requires a model that better describes the behavior of electrons probability of being found. It is derived within atoms. from the word orbit, the path around the nucleus that electrons had been postulated to follow. Reading Strategy L2 Figure 5.1 Rutherford’s Relate Text and Visuals Have model fails to explain why students follow the Time Line in Figure objects change color when heated. As the temperature of 5.2 and explain why each model was this horseshoe is increased, it inadequate or incomplete. first appears black, then red, then yellow, and then white. The observed behavior could 2 INSTRUCT be explained only if the atoms in the iron gave off light in specific amounts of energy. A better atomic model was needed to explain this Have students study the photograph observation. and read the text. Ask, What are two Section 5.1 Models of the Atom 127 types of models? (physical and theo- retical) Why are models important? (You can test a model to see if your ideas or design are correct.) Section Resources The Development of Print Technology •Guided Reading and Study Workbook, •Interactive Textbook with ChemASAP, Atomic Models Section 5.1 Animation 5, Assessment 5.1 Relate L2 •Core Teaching Resources, Section 5.1 Compare the use of atomic models to Review develop the atomic theory to the use •Transparencies, T57 of physical or computer-generated models to design, develop, and test products, such as automobiles. Electrons in Atoms 127 chem_TE_ch05_IPL.fm Page 128 Wednesday, August 4, 2004 9:34 PM Path of a moving electron Section 5.1 (continued) – – – Sphere with The Bohr Model positive charge Discuss L2 throughout – – Prepare students for the expanded view of the atom by asking, What are the three major subatomic particles that – – Nucleus comprise an atom? (electron, proton, and neutron) What are the electrical Tiny, solid – Negatively charged sphere particle (electron) Rutherford charges associated with each of these Thomson Model Dalton Model particles? (electron, 1–; proton, 1+; neu- Model 1904 Hantaro Nagaoka, a Japanese 1911 New Zealand tron, 0) Describe the structure of the 1897 J.J. Thomson, a British physicist, suggests physicist Ernest nuclear atom in terms of the locations 1803 John Dalton scientist, discovers the electron. that an atom has a Rutherford finds that an pictures atoms as The later leads to his "plum- central nucleus. atom has a small, dense, of each of the subatomic particles. tiny, indestructible pudding" model. He pictures Electrons move in positively charged (Every atom, except hydrogen, consists of particles, with no electrons embedded in a sphere orbits like the rings nucleus. Electrons move of positive electrical charge. around the nucleus. a small dense nucleus composed of pro- internal structure. around Saturn. tons and neutrons that accounts for most of the mass of the atom. Hydrogen is an 1800 1805 1895 1900 1905 1910 exception because it has only one proton in its nucleus. Negatively charged elec- Figure 5.2 These illustrations The Bohr Model trons surround the nucleus and occupy show how the atomic model has changed as scientists learned Niels Bohr (1885–1962), a young Danish physicist and a student of Ruther- most of its volume. Electrons contribute more about the atom’s structure. ford, believed Rutherford’s model needed improvement. In 1913 Bohr little to the mass of an atom. ) changed Rutherford’s model to include newer discoveries about how the energy of an atom changes when it absorbs or emits light. He considered L2 the simplest atom, hydrogen, which has one electron. Bohr proposed Word Origins that an electron is found only in specific circular paths, or orbits, around The word quantity has the same the nucleus. The timeline in Figure 5.2 shows the development of atomic root as quantum and means “an models from 1800 to 1935. indefinite amount or number.” Each possible electron orbit in Bohr’s model has a fixed energy. The fixed energies an electron can have are called energy levels. The fixed energy levels of electrons are somewhat like the rungs of the ladder in Figure 5.3a. The lowest rung of the ladder corresponds to the lowest energy TEACHER Demo level. A person can climb up or down a ladder by going from rung to rung. Similarly, an electron can jump from one energy level to another. A person on a ladder cannot stand between the rungs. Similarly, the electrons in an Quantized Energy L2 atom cannot be between energy levels. To move from one rung to another, Purpose Students witness an example a person climbing a ladder must move just the right distance. To move of quantized energy. Word Origins from one energy level to another, an electron must gain or lose just the right amount of energy. In general, the higher an electron is on the energy Quantum comes from the Materials trumpet or trombone and a ladder, the farther it is from the nucleus. Latin word quantus, mean- player A quantum of energy is the amount of energy required to move an elec- ing “how much.” What tron from one energy level to another energy level. The energy of an elec- Procedure Have a student blow into a other commonly used trumpet or trombone. Challenge the English word comes from tron is said to be quantized. You have probably heard the term quantum this root? leap used to describe an abrupt change. The term originates from the ideas student to produce as many different found in the Bohr model of the atom. notes as he or she can without depressing any valves. Expected Outcomes It is not possible 128 Chapter 5 to play the entire scale without chang- ing the valve positions. In the open position, no matter how much energy is put in, the instrument will only Differentiated Instruction accept certain specific amounts of English Learners L2 energy and produce certain notes. The Have students make a poster to show the instrument accepts quantized pack- evolution of the model of the atom. Have ages of energy. them start with Dalton’s model and continue through all the modifications to the current quantum mechanical model. Have them include enough information to make the model clear and understandable. 128 Chapter 5 chem_TE_ch05.fm Page 129 Tuesday, April 18, 2006 10:09 AM An electron can gain The nucleus contains or lose energy by protons and neutrons. – changing its orbit. Use Visuals L1 Figure 5.3 Ask, What are the rungs in Electron the ladders compared to? (the energy – levels in atoms) How is the ladder in b + different from the ladder in a? (The The electron cloud is a visual model of the probable locations rungs are not evenly spaced. They get of electrons in an atom. The closer together nearer to the top.) probability of finding an electron is higher in the denser regions Nucleus of the cloud. Relate L2 When light shines on a fluorescent Bohr Electron Cloud Model Model material, its electrons absorb the 1923 French physicist Louis de Broglie energy and move to a higher energy 1913 In Niels proposes that moving 1926 Erwin Schrödinger 1932 James Chadwick, an level. Almost immediately, the mate- Bohr's model, the particles like electrons develops mathematical English physicist, confirms electron moves in have some properties of equations to describe the the existence of neutrons, rial begins to emit light as the elec- a circular orbit at waves.