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5.1 Revising the Atomic Model >

Chapter 5 In

5.1 Revising the Atomic Model

5.2 Arrangement in Atoms

5.3 Atomic Emission Spectra and the Quantum Mechanical Model

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5.1 Revising the Atomic Model > Levels in Atoms

The

In 1913, (1885–1962), a young Danish physicist and a student of Rutherford, developed a new atomic model.

• He changed Rutherford’s model to incorporate newer discoveries about how the energy of an changes when the atom absorbs or emits light.

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The Bohr Model

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

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5.1 Revising the Atomic Model > Energy Levels in Atoms

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

• The fixed an electron can have are called energy levels. • A quantum of energy is the amount of energy required to move an electron from one to another energy level.

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The Bohr Model The rungs on this ladder are somewhat like the energy levels in Bohr’s model of the atom.

• A person on a ladder cannot stand between the rungs. Similarly, the electrons in an atom cannot exist between energy levels.

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5.1 Revising the Atomic Model > Energy Levels in Atoms

The Bohr Model The rungs on this ladder are somewhat like the energy levels in Bohr’s model of the atom.

• The energy levels in atoms are unequally spaced, like the rungs in this unusual ladder. The higher energy levels are closer together.

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How does the Bohr model improve upon the ?

The Rutherford model could not explain why elements that have been heated to higher and higher temperatures give off different colors of light. The Bohr model explains how the energy levels of electrons in an atom change when the atom emits light.

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5.1 Revising the Atomic Model > 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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• Austrian physicist Erwin Schrödinger (1887– 1961) used new theoretical calculations and experimental results to devise and solve a mathematical equation describing the behavior of the electron in a atom.

• The modern description of the electrons in atoms, the quantum mechanical model, came from the mathematical solutions to the Schrödinger equation.

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5.1 Revising the Atomic Model > The Quantum Mechanical Model

• Like the Bohr model, the quantum mechanical model of the atom restricts the energy of electrons to certain values. • Unlike the Bohr model, however, the quantum mechanical model does not specify an exact path the electron takes around the nucleus.

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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 of an atom.

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5.1 Revising the Atomic Model > The Quantum Mechanical Model

Probability describes how likely it is to find an electron in a particular location around the nucleus of an atom.

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

Electron cloud

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5.1 Revising the Atomic Model >

How are the quantum mechanical model and the Bohr model alike? How are they different?

Like the Bohr model, the quantum mechanical model restricts the energy of electrons to certain values. Unlike the Bohr model, the quantum mechanical model does not specify an exact path the electron takes around the nucleus.

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Atomic Orbitals

How do sublevels of principal energy levels differ?

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5.1 Revising the Atomic Model > Atomic Orbitals

• For each energy level, the Schrödinger equation also to a mathematical expression, called an . • An atomic orbital is represented pictorially as a region of space in which there is a high probability of finding an electron.

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• The energy levels of electrons in the quantum mechanical model are labeled by principal quantum numbers (n). • These numbers are assigned the values n = 1, 2, 3, 4, and so forth. • For each principal energy level greater than 1, there are several orbitals with different shapes and at different energy levels.

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5.1 Revising the Atomic Model > Atomic Orbitals

• The energy levels of electrons in the quantum mechanical model are labeled by principal quantum numbers (n). • These numbers are assigned the values n = 1, 2, 3, 4, and so forth. • For each principal energy level greater than 1, there are several orbitals with different shapes and at different energy levels. • These energy levels within a principal energy level constitute energy sublevels.

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Each energy sublevel corresponds to one or more orbitals of different shapes. The orbitals describe where an electron is likely to be found.

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5.1 Revising the Atomic Model > Atomic Orbitals

For a given principal energy level greater than 1, there is one s orbital, 3 p orbitals, and 5 d orbitals.

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The numbers and types of atomic orbitals depend on the principal energy level.

Summary of Principal Energy Levels and Sublevels Maximum Principal Number of Type of sublevel number of energy level sublevels electrons n = 1 1 1s (1 orbital) 2

n = 2 2 2s (1 orbital), 2p (3 orbitals) 8 3s (1 orbital), 3p (3 orbitals), n = 3 3 18 3d (5 orbitals) 4s (1 orbital), 4p (3 orbitals), n = 4 4 32 4d (5 orbitals), 4f (7 orbitals)

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5.1 Revising the Atomic Model > & YOU

Why do scientists no longer use physical models to describe the of electrons?

• Previous models of the atom were physical models based on the motion of large objects. • Theoretical calculations and experimental results showed that these models did not always correctly describe electron motion. • Schrödinger devised a mathematical equation describing the behavior of the electron in a . The quantum mechanical model came from the solutions to the Schrödinger equation.

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Bohr proposed that an electron is found only in specific circular paths, or orbits, around the nucleus. 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 of an atom. Each energy sublevel corresponds to one or more orbitals of different shapes, which describe where the electron is likely to be found.

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5.1 Revising the Atomic Model > Glossary Terms

• energy level: the specific energies an electron in an atom or other system can have • quantum: the amount of energy needed to move an electron from one energy level to another

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• quantum mechanical model: the modern description, primarily mathematical, of the behavior of electrons in atoms • atomic orbital: a mathematical expression describing the probability of finding an electron at various locations; usually represented by the region of space around the nucleus where there is a high probability of finding an electron

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5.1 Revising the Atomic Model >

END OF 5.1

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