Lecture 23 Models of the Atom

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Lecture 23Models of the Atom
Chapter 28.1 ? 28.4
Outline

• The Thomson and Rutherford Models
• Atomic Spectra
• The Bohr Model

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Rutherford Model of the Atom
J.J. Thomson discovered electron in 1897. He
suggested an atomic model in which electrons were
embedded in spread positive charge.
Ernest Rutherford attempted to test this model by
bombarding a thin gold foil with
alpha(?)-particles.
A significant scattering of the ?-particles was
detected. Rutherford suggested a new model of the
atom which resembled a mini solar system.
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Atomic Spectra
When an electric current is passed through a gas,
electrons in the gas atoms absorb energy from the
current.
The excited this way gas emits colored light.
If we disperse the emitted light into different
frequencies, we will see series of bright lines,
some of which are more intense. The color of the
most intense lines gives the excited gas its
color (red to neon). This is an emission spectrum.
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Absorption Spectra

• Absorption spectra occur when light from a hot
  source passes through a cool gas before entering
  the spectroscope.

The light source alone would give a continuous
spectrum, but atoms of the gas absorb certain
frequencies from the light.
The lines in the emission and absorption spectrum
of the same chemical element have the same
frequencies.
Frequencies in the spectrum of an element fall
into sets called spectral series.
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The Bohr Model
In 1913 Niels Bohr proposed a theory of the
hydrogen atom that could account for its
stability and for the frequencies of its spectral
lines.
Bohr proposed than an electron can circle the
nucleus without losing energy only in certain
specific orbits. The energy of the electron
depends on which orbit it is in.
Thus Bohr suggested that atomic electrons can
have only certain particular energies.
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The Bohr Model
An electron in the innermost orbit has the least
energy. The larger the orbit, the more the
electron has energy.
The orbits are identified by a quantum number,
n. Each orbit has an energy level En ?13.6/n2
eV.
An electron can absorb only those photons whose
energy permit it to jump from one orbit (ni) to
another, farther out (nf).
When an electron jumps to an orbit, closer to the
nucleus, it emits a photon of a wavelength ?.
1 1 1 ? RH ? ? ? ?
nf2 ni2
RH ? Rydberg constant
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Electron Waves and Orbits
Why does an atomic electron follow certain orbits
only?
The de Broglie wavelength of the electron is
exactly equal to the circumference of its ground
state (the innermost orbit with n1).
If we consider the vibrations of a wire loop, we
find that their wavelengths always fit a whole
number of times into the loops circumference.
An electron can circle a nucleus only in orbits
that contain an integral number of de Broglie
wavelengths.
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Quantum Theory of the Atom
The Bohrs model has some severe limitations. It
correctly predicts the spectral series for
hydrogen, but fails predicting the same for atoms
with 2 or more electrons.
A more general approach was developed in 1925/6
by Erwin Schrodinger, Werner Heisenberg, and
others, and is called quantum mechanics.
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Classical versus Quantum Mechanics
Classical mechanics takes such quantities as
position, mass, velocity, and acceleration for
granted.
Quantum mechanics uses the uncertainty principle
instead and explores probabilities. It deals only
with quantities that can actually be measured.
The measurable quantities are mass of the
electron, its electric charge, frequencies of
spectral lines, etc. But we cannot measure the
precise diameter of an electrons orbit.
Quantum mechanics includes Newtonian mechanics as
a special case.
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Quantum Numbers
In the quantum theory of atom, an electron has no
fixed orbit but is free to move about 3
dimensions.
It circulates in a probability cloud and can be
found where the cloud is the most dense.
Three quantum numbers determine the size and
shape of the probability cloud of an atomic
electron. n ? the principal quantum number l ?
the orbital quantum number ml ? the magnetic
quantum number
The 4th, spin quantum number (ms) determines the
maximum number of electrons allowed on an orbit.
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Summary
The Bohr model correctly explained properties of
only hydrogen atoms. Quantum theory of atom is a
probabilistic approach, which enlarges
applications of the classical mechanics. Quantum
mechanics shows that four quantum numbers are
needed to specify the physical state of each
atomic electron.