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The Atom
How wonderful that we have met with a
paradox. Now we have some hope of
making progress.
- Nils Bohr
Read Trefil and Hazen, Chapter 9
The Greek Atom
Leucippus of Miletus suggested the idea of
atoms in 440 BC. He and his pupil, Democritus
(c460-371 BC) of Abdera, refined and extended
the idea. There are five major points to their
atomic theory.
1. All matter is composed of atoms, which are bits of
matter too small to be seen. These atoms cannot be
further split into smaller portions.
2. There is a void, which is empty space between atoms.
3. Atoms are complete solid.
4. Atoms are homogeneous, with no internal structure.
5. Atoms are different in their sizes, shapes, and
weights.
Democritus was
known as the
Laughing
Philosopher because
of his good humor.
Dalton, Atoms, and Elements
John Dalton, a British Chemist, revived the
Greek idea of the atom in 1803 to explain the
patterns observed in chemistry. His theory
consisted of 3 postulates.
1. all matter was composed of small indivisible
particles termed atoms,
2. atoms of a given element possess unique
characteristics and weight,
3. three types of atoms exist: simple (elements),
compound (simple molecules), and complex
(complex molecules).
Dalton identified chemical elements as a specific type of atom and that
(following Richter) chemical elements combine in integral ratios. Thus, the
chemical hydrogen was composed of hydrogen atoms, helium of helium
atoms, etc. Dalton also introduced the use of chemical symbols, such as
H2 + O  H2O
The Rutherford Atom Can’t Work
The studies of radioactivity and nuclei at the end of the 19th century
led immediately to the identification of Rutherford’s nucleus with
Dalton’s atoms.
Rutherford’s model for the atom was
appealing. It was simple. It was
based on the well known electric
force. It was consistent with all
known experimental evidence. It had
one flaw however that led to its
demise but opened up a grand new
era in physics.
We know from Maxwell’s equations that an accelerating charged particle
emits electromagnetic radiation. We know that an electron in a circular
orbit about a nucleus is accelerating because the direction of its velocity
is constantly changing. Thus, the electron should emit electromagnetic
waves. The electromagnetic radiation should carry away energy and the
electron should slow down. In fact, the electron should lose all its
energy in less than 1 second. Obviously, this does not happen.
Spectroscopy
Spectral Lines
The spectrum of atomic
hydrogen
Gases emit and absorb light at only specific wavelengths, called spectral
lines. For example, in the visible region of the spectrum, hydrogen will absorb
or emit only at 0.656, 0.481, 0.434, and 0.410 microns. Chemicals can be
easily identified by measuring the wavelength of absorbed or emitted light.
The Riddle of the Hydrogen
Spectrum
Spectroscopists discovered some simple formulas relating the wavelengths of
the spectral lines of atomic hydrogen.
1 
 1
 10.97 2  2 

m 
n
1
microns
Where  is the wavelength of the emitted light and n and m are integers.
Pick any two integers n and m (n must be less than m), calculate the
wavelength and hydrogen will have a spectral line at that wavelength.
There are no spectral lines at any wavelengths not given by this equation.
Such a simple and successful formula must mean something, but what?
The Bohr Hydrogen Atom
What is it about an atom that allows it to absorb or
emit radiation at only specific wavelengths? Do the
wavelengths tell us anything about the structure of
the atoms?
Nils Bohr suggested that emitted or absorbed
radiation corresponds to changes in the orbits of
electrons about the nucleus. Moreover, the electrons
can not orbit at any distance from the nucleus, but
exist only in specific “allowed” orbits.
To explain the allowed orbits, Bohr postulated that
electrons had both wave and particle properties.
Nils Bohr (1885-1962) was
The allowed orbits were those for which a
a Danish Physicist. His
wavelength fit perfectly inside the orbit, just as only
certain wavelengths are allowed on a vibrating string. model for the hydrogen
atom was the beginning of
quantum theory.
Quantum
Leaps and
Photons
Multi-electron Atoms
The Periodic Table
Images of Atoms