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chemistry

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5.1 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.

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**The Development of Atomic Models**

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

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**The Development of Atomic Models**

5.1 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. Rutherford’s model fails to explain why objects change color when heated. As the temperature of this horseshoe is increased, it first appears black, then red, then yellow, and then white. The observed behavior could 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 observation.

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**The Development of Atomic Models**

5.1 The Development of Atomic Models The timeline shoes the development of atomic models from 1803 to 1911. These illustrations show how the atomic model has changed as scientists learned more about the atom’s structure.

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**The Development of Atomic Models**

5.1 The Development of Atomic Models The timeline shows the development of atomic models from 1913 to 1932. These illustrations show how the atomic model has changed as scientists learned more about the atom’s structure.

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5.1 The Bohr Model The Bohr Model What was the new proposal in the Bohr model of the atom?

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

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**Each possible electron orbit in Bohr’s model has a fixed energy.**

5.1 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.

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5.1 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. These ladder steps are somewhat like energy levels. In an ordinary ladder, the rungs are equally spaced. The energy levels in atoms are unequally spaced, like the rungs in this ladder. The higher energy levels are closer together.

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**The Quantum Mechanical Model**

5.1 The Quantum Mechanical Model The Quantum Mechanical Model What does the quantum mechanical model determine about the electrons in an atom?

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**The Quantum Mechanical Model**

5.1 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.

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**The Quantum Mechanical Model**

5.1 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.

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**The Quantum Mechanical Model**

5.1 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. The electron cloud of an atom is compared here to photographs of a spinning airplane propeller. a) The airplane propeller is somewhere in the blurry region it produces in this picture, but the picture does not tell you its exact position at any instant. b) Similarly, the electron cloud of an atom represents the locations where an electron is likely to be found.

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**The Quantum Mechanical Model**

5.1 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. The electron cloud of an atom is compared here to photographs of a spinning airplane propeller. a) The airplane propeller is somewhere in the blurry region it produces in this picture, but the picture does not tell you its exact position at any instant. b) Similarly, the electron cloud of an atom represents the locations where an electron is likely to be found.

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**Atomic Orbitals How do sublevels of principal energy levels differ?**

5.1 Atomic Orbitals Atomic Orbitals How do sublevels of principal energy levels differ?

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5.1 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.

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5.1 Atomic Orbitals Different atomic orbitals are denoted by letters. The s orbitals are spherical, and p orbitals are dumbbell-shaped. The electron clouds for the s orbital and the p orbitals are shown here.

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5.1 Atomic Orbitals Four of the five d orbitals have the same shape but different orientations in space. The d orbitals are illustrated here. Four of the five d orbitals have the same shape but different orientations in space. Interpreting Diagrams How are the orientations of the dxy and dx2 – y2 orbitals similar? How are they different?

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

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

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**Observe the characteristics of atomic orbitals.**

Animation 5 Observe the characteristics of atomic orbitals.

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5.1 Section Quiz. 5.1.

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5.1 Section Quiz. 1. Rutherford's planetary model of the atom could not explain any properties of elements. the chemical properties of elements. the distribution of mass in an atom. the distribution of positive and negative charges in an atom.

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5.1 Section Quiz. 2. Bohr's model of the atom proposed that electrons are found embedded in a sphere of positive charge. in fixed positions surrounding the nucleus. in circular orbits at fixed distances from the nucleus. orbiting the nucleus in a single fixed circular path.

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5.1 Section Quiz. 3. What is the lowest-numbered principal energy level in which p orbitals are found? 1 2 3 4

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END OF SHOW

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