Atomic Structure, Radiochemistry and Chemical Bonding

In previous chapters we have seen how Dalton s atomic theory encountered difficulties, especially during the first half of the nineteenth century. After 1860 the chemical atom became so useful, especially to organic chemists, that only a few dissenting voices were heard in this branch of the science. However, to many physicists the atomic concept seemed unnecessary, and some of the great advances in nineteenth century physics had been made without reference to atomism. Then, in a series of discoveries which were made in rapid succession around the turn of the century, the existence of atoms was established to the satisfaction of everyone. All atoms were shown to contain identical subatomic particles, which were called electrons. Furthermore, certain atoms were shown to be undergoing spontaneous and continuous transmutation into others. The atomic concept may have been vindicated, but at the cost of disproving Dalton s tenets of the indivisibility and immutability of atoms. Further advances placed the periodic table on a firm theoretical basis, and provided an explanation of the forces involved in chemical bonding. 

Supporters of the wave theory had argued that the negative particles ejected from the cathode were distinct from the rays which caused the phosphorescence on the glass. Thomson built a tube containing a collecting cylinder connected to an electrometer. When the phosphorescent spot was deflected by a magnet into the cylinder, the electrometer detected a charge in the cylinder. 

Having established that the electrical and phosphorescent effects of cathode rays could not be separated, Thomson proceeded to demolish another objection to the charged-particle theory. This was that iio deflection of the cathode ray beam had been observed when it was passed between two plates maintained at a small potential difference. Thomson argued that the passage of the rays rendered the 

Not surprisingly, Thomson s suggestion of the existence of subatomic particles common to all atoms was received with some scepticism, but in 1904 he proposed a model for the structure of the atom to fit the available evidence. He suggested that the atom was a sphere of uniform positive charge within which rings of 

Becquerel discovered that the radiation emitted by uranium and its salts had the property of rendering air a conductor of electricity, an effect which Rutherford subsequently explained as being due to the ionisation of the air. It was therefore possible to use an electrometer to measure the intensity of the radiation. Using this technique Marie Curie (1867-1934) embarked in 1898 on a systematic investigation of the new phenomenon. 

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Atomic Structure, Radiochemistry and Chemical Bonding NUCLEAR CHARGE AND ATOMIC NUMBER... 

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