What were the experiments that scientists designed to come up with the atom models of their time?
John Dalton is considered to be the father of atomic theory. In 1803, he published his theory of the atom which consisted of the following points:
- Elements are composed of tiny particles called atoms.
- All of the atoms of a given element are identical.
- The atoms of a given element are different from the atoms of other elements.
- Atoms can combine to form compounds.
- Atoms cannot be divided into smaller parts.
The last point, shown in bold, was later shown to be untrue. John Dalton did not perform experiments to support his theory.
J. J. Thomson
In 1889, J. J. Thomsom performed several experiments using cathode ray tubes. Normally, a cathode ray will travel straight through a cathode ray tube. Thomson discovered that the cathode ray would bend when charged or magnetic plates were placed on either side of it. Since the cathode ray bent towards the positively charged plate, Thomson inferred that the atoms in the cathode ray must contain negatively charged particles. These negatively charged particles were later called electrons.
Thomson’s model of the atom contained diffuse positive charge and negatively charged electrons dispersed throughout the atom. Thomson named his model the “Plum Pudding Model”.
In 1911, Ernest Rutherford shot positively charged alpha particles through a thin piece of gold foil. Most of the particles passed through the gold atoms in the gold foil. Occasionally, the alpha particles bounced back at various angles. Rutherford concluded that the atom must contain a small, dense, positively charged nucleus at the center of the atom.
Rutherford’s model of the atom contains a positively charged nucleus surrounded by tiny negatively charged electrons. The electrons are so tiny, that most of the atom consists of empty space.
Niels Bohr used the mathematical relationships between the spectral lines of hydrogen atoms to suggest that electrons orbited the nucleus in specific energy levels.
Bohr’s model of the atom contains a positively charged nucleus surrounded by electrons located in energy levels.
Robert Millikan experimented with charged oil drops. From his experiments, he was able to determine the exact charge on the electron.