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Chapter 5

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Models of the Atom The scale model shown is a physical model. However, not all models are physical. In fact, several theoretical models of the atom have been developed over the last few hundred years. You will learn about the currently accepted model of how electrons behave in atoms. 5.1

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The Development of Atomic Models What was inadequate about Rutherford’s atomic model? 5.1

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The Development of Atomic Models Rutherford’s atomic model could not explain the chemical properties of elements. Rutherford’s atomic model could not explain why objects change color when heated. 5.1

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The Development of Atomic Models The timeline shoes the development of atomic models from 1803 to 1911. 5.1

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The Development of Atomic Models The timeline shows the development of atomic models from 1913 to 1932. 5.1

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The Bohr Model Bohr proposed that an electron is found only in specific circular paths, or orbits, around the nucleus. 5.1

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The Bohr Model Each possible electron orbit in Bohr’s model has a fixed energy. The fixed energies an electron can have are called energy levels. A quantum of energy is the amount of energy required to move an electron from one energy level to another energy level. 5.1

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The Bohr Model Like the rungs of the strange ladder, the energy levels in an atom are not equally spaced. The higher the energy level occupied by an electron, the less energy it takes to move from that energy level to the next higher energy level. 5.1

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The Quantum Mechanical Model The quantum mechanical model determines the allowed energies an electron can have and how likely it is to find the electron in various locations around the nucleus. 5.1

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The Quantum Mechanical Model Austrian physicist Erwin Schrödinger (1887–1961) used new theoretical calculations and results to devise and solve a mathematical equation describing the behavior of the electron in a hydrogen atom. The modern description of the electrons in atoms, the quantum mechanical model, comes from the mathematical solutions to the Schrödinger equation. 5.1

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The Quantum Mechanical Model The propeller blade has the same probability of being anywhere in the blurry region, but you cannot tell its location at any instant. The electron cloud of an atom can be compared to a spinning airplane propeller. 5.1

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The Quantum Mechanical Model In the quantum mechanical model, the probability of finding an electron within a certain volume of space surrounding the nucleus can be represented as a fuzzy cloud. The cloud is more dense where the probability of finding the electron is high. 5.1

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Atomic Orbitals An atomic orbital is often thought of as a region of space in which there is a high probability of finding an electron. Each energy sublevel corresponds to an orbital of a different shape, which describes where the electron is likely to be found. 5.1

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Atomic Orbitals Different atomic orbitals are denoted by letters. The s orbitals are spherical, and p orbitals are dumbbell-shaped. 5.1

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Atomic Orbitals Four of the five d orbitals have the same shape but different orientations in space. 5.1

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Atomic Orbitals The numbers and kinds of atomic orbitals depend on the energy sublevel. 5.1

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Atomic Orbitals The number of electrons allowed in each of the first four energy levels are shown here. 5.1

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Section 5.2 Electron Configurations

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Electron Arrangement in Atoms If this rock were to tumble over, it would end up at a lower height. It would have less energy than before, but its position would be more stable. You will learn that energy and stability play an important role in determining how electrons are configured in an atom. 5.2

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Electron Configurations The ways in which electrons are arranged in various orbitals around the nuclei of atoms are called electron configurations. Three rules—the Aufbau principle, the Pauli exclusion principle, and Hund’s rule—tell you how to find the electron configurations of atoms. 5.2

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Electron Configurations Aufbau Principle According to the aufbau principle, electrons occupy the orbitals of lowest energy first. In the aufbau diagram below, each box represents an atomic orbital. 5.2

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Electron Configurations Pauli Exclusion Principle According to the Pauli exclusion principle, an atomic orbital may describe at most two electrons. To occupy the same orbital, two electrons must have opposite spins; that is, the electron spins must be paired. 5.2

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Electron Configurations Hund’s Rule Hund’s rule states that electrons occupy orbitals of the same energy in a way that makes the number of electrons with the same spin direction as large as possible. 5.2

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Electron Configurations Orbital Filling Diagram 5.2

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for Conceptual Problem 1.1

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Exceptional Electron Configurations Some actual electron configurations differ from those assigned using the aufbau principle because half-filled sublevels are not as stable as filled sublevels, but they are more stable than other configurations. 5.2

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Exceptional Electron Configurations Exceptions to the aufbau principle are due to subtle electron-electron interactions in orbitals with very similar energies. Copper has an electron configuration that is an exception to the aufbau principle. 5.2

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Section 5.3

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Physics and the Quantum Mechanical Model Neon advertising signs are formed from glass tubes bent in various shapes. An electric current passing through the gas in each glass tube makes the gas glow with its own characteristic color. You will learn why each gas glows with a specific color of light. 5.3

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Light The electromagnetic spectrum consists of radiation over a broad band of wavelengths. 5.3

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Atomic Spectra When atoms absorb energy, electrons move into higher energy levels. These electrons then lose energy by emitting light when they return to lower energy levels. 5.3

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Atomic Spectra A prism separates light into the colors it contains. When white light passes through a prism, it produces a rainbow of colors. 5.3

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Atomic Spectra When light from a helium lamp passes through a prism, discrete lines are produced. 5.3

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Atomic Spectra The frequencies of light emitted by an element separate into discrete lines to give the atomic emission spectrum of the element. 5.3 Mercury Nitrogen

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An Explanation of Atomic Spectra In the Bohr model, the lone electron in the hydrogen atom can have only certain specific energies. When the electron has its lowest possible energy, the atom is in its ground state. Excitation of the electron by absorbing energy raises the atom from the ground state to an excited state. A quantum of energy in the form of light is emitted when the electron drops back to a lower energy level. 5.3

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An Explanation of Atomic Spectra The light emitted by an electron moving from a higher to a lower energy level has a frequency directly proportional to the energy change of the electron. 5.3

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An Explanation of Atomic Spectra The three groups of lines in the hydrogen spectrum correspond to the transition of electrons from higher energy levels to lower energy levels. 5.3

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Quantum Mechanics Today, the wavelike properties of beams of electrons are useful in magnifying objects. The electrons in an electron microscope have much smaller wavelengths than visible light. This allows a much clearer enlarged image of a very small object, such as this mite. 5.3

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Quantum Mechanics Classical mechanics adequately describes the motions of bodies much larger than atoms, while quantum mechanics describes the motions of subatomic particles and atoms as waves. 5.3

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