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Transcript
Unit 4.4 Bohr’s Model of the
Atom
Teacher: Dr. Van Der Sluys
Objectives
• Light and energy
– White light
– Colors
• Line spectra
• Bohr Model of the atom
– Spectral emission lines
Newton and Light
• Isaac Newton found that white light
could be separated into colors and that
the colors could be recombined to form
white light again.
1
Electromagnetic Spectrum
E = hc
λ
Gas Discharge Tubes
• Similar to
Thompson’s
experiment, a gas
discharge tube has
a cathode and an
anode. When a
large voltage is
applied electrons
jump off the cathode
and collide with the
gases in the tube.
Line Spectra of Elements
• When the light from a discharge tube is
passed through of prism (or grating)
the wavelengths of light can be
separated.
Hydrogen
Helium
2
Bohr Model
•
•
•
•
The Bohr model of the atom is
similar to the solar system.
The nucleus is at the center of
the atom and contains the vast
majority of the mass of the
atom.
Electrons orbit the nucleus at
fixed distances (energy levels).
These orbits are assign integer
values (n = 1, 2, 3, etc). These
numbers are called the
principle quantum number.
Electrons can jump from an
occupied energy level to an
empty level by absorbing or
emitting energy (light).
Energy States
• The lowest energy situation for an atom is to have as
many electrons as possible in orbits as close to the
nucleus as possible. This is called the ground state.
• Excited states are any other situation in which one or
more electrons are located in orbits further from the
nucleus than in the ground state.
• The energy levels are said to be quantized with a
specific amount of energy being required for each
level.
• The ionization energy is the energy required to have
the electron jump completely out of the atom.
The Bohr Model and Spectral
Lines
•
•
•
The emission lines
for hydrogen in the
visible portion of the
spectrum are due to
the electron jumping
from levels where n ≥
3 to n = 2 (Balmer
series).
The Lyman series
occurs in the
ultraviolet region of
the spectrum and are
due to the electron
jumping from levels
of n ≥ 2 to n = 1.
The Paschen series
occurs in the infrared
portion of the
spectrum and is due
to the electron
jumping from n ≥ 4 to
n = 3.
3
Elements Other than
Hydrogen
• The spacing between energy levels for
atoms other than hydrogen are not as
systematic and result in much more
complicated line spectra.
• Each element has a unique line
spectrum and the intensity of a line is
proportional to the amount of that
element in the sample.
The Sun and the Stars
• The sun and other stars are powered by
nuclear reactions.
• The sun gives off light with intensities
corresponding to spectral lines of various
elements, predominantly hydrogen helium
and a variety of other elements.
• The Doppler shift can be used to determine if
stars are moving towards or away from earth.
Most stars appear to be moving away from
earth. Conclusion: The universe is
expanding.
Photons and the
Photoelectric Effect
• Energy is quantized.
• Light is a form of energy.
• Therefore, light must be
quantized
• These smallest pieces
of light are called
photons.
• Photoelectric effect?
When photons of the
correct energy strike the
surface of a metal it can
cause ionization.
4