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 ElementsJohn 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

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.

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.

Spectroscopy

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 spectrum of atomic hydrogen

Spectral Lines

The Riddle of the Hydrogen Spectrum

Spectroscopists discovered some simple formulas relating the wavelengths of the spectral lines of atomic hydrogen.

microns11

97.101

22

mn

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 AtomWhat 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. The allowed orbits were those for which a wavelength fit perfectly inside the orbit, just as only certain wavelengths are allowed on a vibrating string.

Nils Bohr (1885-1962) was a Danish Physicist. His model for the hydrogen atom was the beginning of quantum theory.

Quantum Leaps and Photons

Multi-electron Atoms

The Periodic Table

Images of Atoms