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Section 2: Quantum Theory and the Atom
Wavelike properties of electrons help relate atomic emission
spectra, energy states of atoms, and atomic orbitals.
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What I Know
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What I Want to Find Out
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What I Learned
• 6(A) Understand the experimental design and conclusions used
in the development of modern atomic theory, including Dalton’s
Postulates, Thomson’s discovery of electron properties,
Rutherford’s nuclear atom, and Bohr’s nuclear atom.
• 6(C) Calculate the wavelength, frequency, and energy of light using
Planck's constant and the speed of light.
• 3(F) Research and describe the history of chemistry and
contributions of scientists.
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Quantum Theory and the Atom
Essential Questions
• How do the Bohr and quantum mechanical models of the atom
compare?
• What is the impact of de Broglie’s wave-particle duality and the
Heisenberg uncertainty principle on the current view of
electrons in atoms?
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Quantum Theory and the Atom
Vocabulary
Review
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ground state
quantum number
de Broglie equation
Heisenberg uncertainty principle
quantum mechanical model of
the atom
atomic orbital
principal quantum number
principal energy level
energy sublevel
Quantum Theory and the Atom
Bohr’s Model of the Atom
Einstein’s theory of light’s dual nature accounted for several unexplainable
phenomena but not why atomic emission spectra of elements were discontinuous
rather continuous. In 1913, Niels Bohr, a Danish physicist working in Rutherford’s
laboratory, proposed a quantum model for the hydrogen atom that seemed to
answer this question.
Bohr correctly predicted the frequency lines in hydrogen’s atomic emission
spectrum. The lowest allowable energy state of an atom is called its ground
state. When an atom gains energy, it is in an excited state.
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Quantum Theory and the Atom
Bohr’s Model of the Atom
Bohr suggested that an electron moves around the nucleus only in certain
allowed circular orbits.
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Quantum Theory and the Atom
Bohr’s Model of the Atom
Each orbit was given a number, called the quantum number.
Hydrogen’s single electron is in the n = 1 orbit in the ground state. When energy
is added, the electron moves to the n = 2 orbit.
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Quantum Theory and the Atom
Bohr’s Model of the Atom
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Quantum Theory and the Atom
Electron Transitions
Energy levels are similar to the rungs
of a ladder. The four visible lines
correspond to electrons dropping
from a higher n to the orbit n = 2. As
n increases, the hydrogen atom’s
energy levels are closer to each
other.
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Quantum Theory and the Atom
The Limits of Bohr’s Model
Bohr’s model explained the hydrogen’s spectral lines, but failed to explain any
other element’s lines. The behavior of electrons is still not fully understood, but
substantial evidence indicates they do not move around the nucleus in circular
orbits.
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Quantum Theory and the Atom
The Quantum Mechanical Model of the Atom
Louis de Broglie (1892–1987) hypothesized that particles, including electrons,
could also have wavelike behaviors. The figure illustrates that electrons orbit
the nucleus only in whole-number wavelengths.
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Quantum Theory and the Atom
The Quantum Mechanical Model of the Atom
The de Broglie equation predicts that all moving particles have wave
characteristics.
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Quantum Theory and the Atom
The Quantum Mechanical Model of the Atom
Heisenberg showed it is impossible to take any measurement of an object
without disturbing it. The Heisenberg uncertainty principle states that it is
fundamentally impossible to know precisely both the velocity and position of a
particle at the same time. The only quantity that can be known is the
probability for an electron to occupy a certain region around the nucleus.
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Quantum Theory and the Atom
The Quantum Mechanical Model of the Atom
Schrödinger treated electrons as waves in a model called the quantum
mechanical model of the atom. Schrödinger’s equation applied equally well
to elements other than hydrogen. The wave function predicts a threedimensional region around the nucleus called the atomic orbital.
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Quantum Theory and the Atom
Hydrogen Atomic Orbitals
Principal quantum number (n) indicates the relative size and energy of
atomic orbitals. n specifies the atom’s major energy levels, called the principal
energy levels. Energy sublevels are contained within the principal energy
levels.
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Quantum Theory and the Atom
Fireworks
Concepts in Motion
Fireworks.
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Quantum Theory and the Atom
Hydrogen Atomic Orbitals
Each energy sublevel relates to orbitals of different shape.
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Quantum Theory and the Atom
Hydrogen Atomic Orbitals
Copyright © McGraw-Hill Education
Quantum Theory and the Atom
Review
Essential Questions
• How do the Bohr and quantum mechanical models of the atom
compare?
• What is the impact of de Broglie’s wave-particle duality and the
Heisenberg uncertainty principle on the current view of electrons in
atoms?
Vocabulary
•
•
•
•
ground state
quantum number
de Broglie equation
Heisenberg
uncertainty principle
Copyright © McGraw-Hill Education
• quantum mechanical
model of the atom
• atomic orbital
• principal quantum
number
• principal energy
level
• energy sublevel
Quantum Theory and the Atom