Development of the Atomic Theory

Ancient Greeks:

• Problem: why do different materials have different properties?
• Early atomists (Parmenides, Empedocles, and Epikouros)
• samples can't be subdivided without limit
• tiny, discrete, indestructible units of matter are atoms
• atoms in constant motion through empty space
• sizes and shapes of atoms determine all material properties
• atoms have mass

Dalton's atomic theory:

1. All matter consists of tiny particles. The existence of atoms was first suggested more that 2000 years before Dalton's birth. Atoms remained pure speculation through most of this time, although Newton used arguments based on atoms to explain the gas laws in 1687. (Newton's speculations about atoms in the Principia were carefully copied by hand into Dalton's notebooks.)
2. Atoms are indestructible and unchangeable. Atoms of an element cannot be created, destroyed, broken into smaller parts or transformed into atoms of another element. Dalton based this hypothesis on the law of conservation of mass and on centuries of experimental evidence.

With the discovery of subatomic particles after Dalton's time, it became apparent that atoms could be broken into smaller parts. The discovery of nuclear processes showed that it was even possible to transform atoms from one element into atoms of another. But we don't consider processes that affect the nucleus to be chemical processes. The postulate is still useful in explaining the law of conservation of mass in chemistry. A slightly more restrictive wording is "Atoms cannot be created, destroyed, or transformed into other atoms in a chemical change".

3. Elements are characterized by the mass of their atoms. All atoms of the same element have identical weights, Dalton asserted. Atoms of different elements have different weights.

With the discovery of isotopes, however, the postulate was amended to read, "Elements are characterized by their atomic number".

4. When elements react, their atoms combine in simple, whole-number ratios. This postulate suggested a practical strategy for determining relative atomic weights from elemental percentages in compounds. Experimental atomic weights could then be used to explain the fixed mass percentages of elements in all compounds of those elements!

This effectively explained both the law of definite proportions and the law of multiple proportions.

Some of the details of Dalton's original atomic theory are now known to be incorrect. But the core concepts of the theory (that chemical reactions can be explained by the union and separation of atoms, and that these atoms have characteristic properties) are foundations of modern physical science.

J. J. Thomson's cathode ray experiment:

• "Cathode Rays" pass from negative electrode towards positive electrode in an evacuated tube
• Hypothesis: cathode rays are streams of electrons
• Calculated mass to charge ratio for electrons by observing bending of cathode rays in electric and magnetic fields
• Proposed the plum pudding model of the atom

Table: Hypothetical properties of the electron. How J. J. Thomson used properties of cathode rays to hypothesize properties of the electron.

Ernest Rutherford's scattering experiment:

• Hypothesis: If the plum pudding model of the atom is correct, atoms have no concentration of mass or charge (atoms are 'soft' targets)
• Experiment to test hypothesis:
• Fire massive alpha particles at the atoms in thin metal foil
• Alpha particles should pass like bullets straight through soft plum pudding atoms
• Observation: A few alpha particles ricocheted!
• New hypotheses:
• All of the positive charge and nearly all of the mass of the atom is concentrated in a tiny, incredibly dense 'nucleus', about 10^-14 m in diameter
• Electrons roam empty space about 10^-10 m across, around the nucleus

The Bohr Model

In 1913 Niels Bohr came to work in the laboratory of Ernest Rutherford. Rutherford, who had a few years earlier, discovered the planetary model of the atom asked Bohr to work on it because there were some problems with the model: According to the physics of the time, Rutherford's planetary atom should have an extremely short lifetime. Bohr thought about the problem and knew of the emission spectrum of hydrogen. He quickly realized that the two problems were connected and after some thought came up with the Bohr model of the atom. Bohr's model of the atom revolutionized atomic physics.

The Bohr model consists of four principles:

With these conditions Bohr was able to explain the stability of atoms as well as the emission spectrum of hydrogen. According to Bohr's model only certain orbits were allowed which means only certain energies are possible. Unfortunately, Bohr's model worked only for hydrogen. Thus the final atomic model was yet to be developed.