Text g. Nuclear chemistry

Atoms are known to undergo certain changes, however, which cannot be explained by changes in the configuration of atomic electrons.

H. Becquerel was the first to notice that a crystal of a salt of uranium placed on a photographic plate in the dark affected the plate so that an image of the crystal appeared. He concluded this effect to have been caused by the emission of some kind of ray from uranium.

Shortly after his discovery, the Curies found the intensity of the rays emitted by the pitchblende ore from which ura­nium is obtained to be greater than would be expected from a knowledge of the uranium content.

The enhanced activity was proved to be due to a prev­iously unknown element radium, which occupies the position below barium in Group II of the periodic table.

It was shown that the rays emitted by radium consist of two kinds of particles, called A-particles and B-particles, and an electromagnetic radiation called Y-rays, having a wave length of the same order as that of X-rays. A-particles have been shown to be the nuclei of helium atoms. They are emitted from radium with a speed of about 15,000 miles per second and are able to penetrate a few cm of air, or very thin aluminium foil.

B-rays are electrons; their speed is about 100,000 miles per second.

Text h. Organometallic compounds

For the present purposes organometallic compounds may be defined as those having carbon metal bonds, the word bond being taken to include all types of chemical combina­tion or linkage that do not involve in intermediate atom.

This definition excludes, for example, salts or organic acids, amides, esters or ethers of the amphoteric elements, for in all these carbon is bonded to the metal through a third element.

The formation of organic derivatives is a very general phenomenon.

There are a few elements for which no organic compounds have been isolated, but there appears to be no theoretical reason why such elements should not form organic deriva­tives.

Classifying the very large number of known organometal­lic compounds into a few distinct types is not a simple mat­ter. The properties of an organometallic compound depend both on the nature of the central atom or "parent element" and on the nature of the organic group or groups attached to that atom.

Although a knowledge of the behaviour of the carbon-metal bond is essential to understanding the chemistry of organometallic compounds other factors also are of impor­tance.

A few broad generalizations can be made concerning the trends in chemical behaviour of organometallic compounds. The compounds of the heavier elements are observed to be less stable toward thermal decomposition than those of the

light elements.

Text I. Photochemistry

Many chemical reactions are caused to proceed by the effect of light. For example, a dyed cloth may fade when exposed to sunlight, because of the destruction of molecules of the dye under the influence of the sunlight. Reactions of this sort are said to be called photochemical reactions. A very important photochemical reaction is known to be the conversion of carbon dioxide and water into carbohy­drate and oxygen in the leaves of the plant, where the green substance chlorophyll serves as a catalyst. One law of pho­tochemistry, discovered in 1818, is that only light which is absorbed is photochemically effective. Hence a colored substance must be present in a system that shows photochem­ical reactivity with visible light.

The second law of photochemistry, formulated in 1912 by A. Einstein, is that one molecule of reacting substance may be activated and caused to react by the absorption of one light quantum, a light quantum being the smallest amount of energy that can be removed from a beam of light by any material system. Its magnitude depends on the frequency of the light. In some simple systems the absorption of one quantum of light proved to result in the reaction of decomposition of one molecule.