Atoms - Part 3 - Rutherford & Einstein
Around 1900 it was determined that atoms contain electrically charged particles. J.J. Thompson determined that most of the mass of the atom was in the positive particle. In 1906 Ernest Rutherford bombarded gold foil with alpha particles (positive helium nuclei). Most of the particles passed though the foil with little deflection, but a very small number were deflected (Figure 4). Some of these were deflected by more than 90°. Rutherford concluded that most of the atom is empty space and that the mass of the atom was concentrated in a tiny positive nucleus at the center.
Figure 4. Rutherford's scattering experiment.
It became apparent that classical electrodynamics and mechanics could not account for many phenomena, particularly the absorption and emission of electromagnetic radiation. The hydrogen atom consists of a single electron revolving around the nucleus. According to classical electrodynamics an electron accelerated in this manner should radiate continuously, gradually lose energy and finally plunge into the nucleus. Yet the hydrogen atom emits only at certain frequencies. The orbit is very stable. The electron maintains the same energy in spite of frequent collisions between atoms. By analogy, suppose a star passed near the solar system. The planetary orbits around the sun could not remain stable under these conditions. They would be permanently altered. There was no way to account for the consistency in the energy of electrons.
Another dilemma was the existence of maxima in the intensity of "black-body radiation" as a function of wavelength. The intensity at any temperature begins to decrease in the ultra-violet region. Classical theory predicted a continuous increase in intensity into the high-energy ultra-violet region. Max Planck was able to solve this problem by postulating discrete, finite quanta of energy. From this came the relationship between energy and frequency, the proportionality constant h called Planck’s constant.
Einstein took this a step further and postulated that light consists of quanta (corpuscles) of energy hn, Planck's constant times the frequency. The energy E of a photon is proportional to the frequency n. In 1905 he successfully explained the photoelectric effect with the light quanta concept. If ultra-violet light falls on a alkali metal surface in high vacuum, the metal becomes positively charged by giving off electrons.
The energy of the electrons can be measured. The velocity of the electrons depends on the frequency of the light, not the intensity. The energy was found to be proportional to the frequency minus a constant characteristic of the metal. This constant was the amount of energy required to remove it from the metal. This is expressed as
where A is the energy with which the electron was held in the atom. This is called the ionization energy.
Electromagnetic radiation is represented as a sine wave (Figure 5). The energy E of a photon is proportional to the frequency n. The frequency is the number of cycles per time period (sec.). The frequency n is related to the wavelength,n = c/l where c is the speed of light.
Figure 5.