Atomic Models: From Cathode Rays to Bohr’s Theory
Cathode Rays: Electrons
The study of electrical discharges through gases was the origin of the discovery of the electron. Gases at atmospheric pressure do not normally conduct electric current; they are almost perfect insulators. For example, an electric spark requires an enormous potential of 30,000 V to jump between two spheres separated by 1 cm. If the distance increases, the potential difference required for the spark to jump also increases. However, gases become increasingly good conductors of electricity as the pressure decreases. When a high potential difference, on the order of 5,000-10,000 V, is applied, and the pressure inside the tube is about 5 mm Hg, a series of lights that fill the entire discharge space between the cathode and the anode is observed. If the gas pressure decreases further, the luminosity inside the tube disappears, the tube becomes dark, and the wall opposite the cathode emits a green light; the glass becomes fluorescent. This green light is produced by a kind of ray coming out of the cathode, hence called cathode rays. They propagate in straight lines, are negatively charged, and have kinetic energy.
Cathode Rays: Protons
In 1886, Goldstein conducted experiments using discharge tubes with a perforated cathode. He observed that some radiation from the anode passed through the perforations of the cathode and struck the wall opposite the anode. These radiations were called positive rays or canal rays. Like cathode rays, these rays are also deflected by electric and magnetic fields. They deviate towards the negative pole of an electric field and towards the south pole of a horseshoe magnet. This shows that these particles have positive charges and a mass greater than that of electrons. In 1911, Thomson calculated the charge-to-mass ratio (q/m) for positive rays and found that this ratio depends on the nature of the gas contained in the tube. When the gas in the tube is hydrogen, the lightest of all gases, the canal rays are formed by hydrogen atoms that have lost an electron. These positive particles are called protons. A proton has a charge equal to that of an electron but opposite in sign, and its mass is 1,841 times greater than that of an electron. The discovery of these elementary constituents of matter suggested to Thomson the idea of the first atomic model.
Thomson Model
Thomson had shown that electrons were constituent particles of the atom and that the atom had to be electrically neutral. He concluded that positively charged particles must also exist. Since the mass of electrons is very small compared to the mass of the atom, the particles that contribute most to the atomic mass had to be positively charged. Thomson developed the first atomic model, assuming that the atom was a sphere of positive charge, approximately 10-10 m in diameter, with electrons submerged within it in sufficient numbers to achieve electrical neutrality.
Rutherford Model
To verify Thomson’s model experimentally, Rutherford directed a beam of alpha particles, coming from the decay of a radioactive source, onto a thin gold foil. Screens coated with zinc sulfide were placed in front of and behind the foil. When alpha particles struck the screen, they produced a fluorescent scintillation. The results were unexpected. Most alpha particles passed through the foil without being deflected, a few were deflected at small angles, and some were even reflected backward. Rutherford showed that these results could be explained if the positive charge and most of the mass of the atom were concentrated in a very small region called the nucleus. The dimensions of the nucleus would be negligible compared to the atom, and therefore, electrons should be at large distances from the nucleus, in an area called the electron cloud. The space between the nucleus and the electrons would be empty.
Classification of Spectra
a) Emission Spectra:
These arise as a result of the emission of light by certain substances. They are divided into:
- Continuous Spectra: These cover all wavelengths, transitioning gradually from one to another. They are obtained when a solid or liquid is heated to incandescence.
- Discontinuous Spectra: These consist of a series of bright lines on a dark background. They are observed when analyzing the light emitted by a gas at low pressure or a substance volatilized in a flame. Each line in the spectrum corresponds to radiation of a specific wavelength and frequency, characteristic of the substance.
b) Absorption Spectra:
Any substance capable of emitting certain radiation can also absorb it. If a red glass is placed between a white light source and the slit of a spectrometer, the glass will absorb certain radiation from the white light, specifically those corresponding to red. If a tube containing hydrogen gas at low pressure is placed in the path of the white light, the observed spectrum is a continuous spectrum with four black lines. This is because hydrogen absorbs four specific radiations from the white light, which coincide with the radiations observed in the emission spectrum of hydrogen. If we superimposed the emission and absorption spectra of hydrogen, we would see that they match because they are complementary. Therefore, the absorption spectrum of a substance is obtained when radiation passes through the substance, and the substance absorbs certain radiations. These absorbed radiations are absent from the spectrum, leaving dark lines in their place, which occupy the same positions as the bright lines in the discontinuous emission spectrum of the substance.
Interpretation of Atomic Spectra
Atomic spectra originate from the radiant energy emitted by atoms when they are excited, that is when they are supplied with thermal energy, light, or any other form of energy. It is experimentally observed that emission spectra comprise a set of lines sufficiently dispersed by a prism so that they can be observed separately. These are, therefore, discontinuous emission spectra, which indicate that the atom has emitted energy discontinuously. Absorption spectra also indicate that atoms only absorb energy of certain frequencies; the absorption of energy by atoms is discontinuous. The fact that atoms can absorb or emit electromagnetic radiation in this specific way cannot be interpreted with the laws of classical physics and, therefore, cannot be explained with Rutherford’s model. According to this model, the atom consists of a nucleus, where the positive charge and almost all the mass are concentrated, and an electron cloud, where electrons move around the nucleus. Since an electron in rotational motion experiences a centripetal force, it will have a centripetal acceleration. According to electromagnetic theory, the electron would lose energy and eventually fall into the nucleus, resulting in unstable atoms. This does not happen; otherwise, matter would not exist.
Bohr Model
In 1913, Bohr, based on Planck’s quantum theory, proposed a different theory about the stability of the hydrogen atom and to interpret its spectrum. The hydrogen atom, with a single proton in the nucleus and only one electron, is the simplest atom. At the end of the 19th century, it was shown that the frequencies of the hydrogen atom’s spectrum could be grouped into sets called spectral series.