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.

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Metals and nonmetals

1. There are some distinctions between metals and nonmetals. Metals are distinguished from nonmetals by their high conductivity for heat and electricity, by metallic lustre and by their resistance to electric current. Their use in industry is explained not only by those properties, but also by the fact that their properties, such as strength and hardness, can be greatly improved by alloying them with other metals.

2. There are several important groups of metals and alloys. The common metals such as iron, copper, zinc, etc. are produced in great quantities.

3. The so-called precious metals include silver, gold, platinum and palladium. The light metals are aluminium, berillium and titanium. They are important in aircraft and rocket construction.

4. Many elements are classified as semimetals (bismuth, for example) because they have much poorer conductivity than common metals.

5. Nonmetals (carbon, silicon, sulphur) in the solid state are usually brittle materials without metallic lustre and are usually poor conductors of electricity. Nonmetals show greater variety of chemical properties than common metals do.

6. Metals can undergo corrosion, changing in this case their chemical and electromechanical properties. In order to protect metals from corrosion the products made of metals and steel are coated by some films (coatings). Organic coatings protect metals and steel from corrosion by forming a corrosion-resistant barrier between metal or steel and the corrosive environment.

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Emulsions

An emulsion represents a disperse system in which the phases are immiscible or partly immiscible liquids.

In nearly all emulsions, one of the phases is aqueous and the other is an oil. If the oil is the disperse phase, the emulsion is termed an oil in water (o/w) one, if the aqueous medium is the disperse phase, the emulsion is termed a water in oil (w/o) one.

If one shakes vigorously a vessel containing two immiscible liquids, both liquids are broken up into droplets whose size depends upon the viscosity of the liquid, surface, interfacial tensions and the vigor of the shaking. As soon as the mechanical dispersive action ceases, the droplets begin to coalesce in order that the total surface free energy may be reduced. Most often, particularly in the case of two pure liquids, the coalescence process is rapid, and within a very few minutes the system consists only of two liquid layers. In the presence of small amounts of additional components, termed emulsifiers, the rate of coalescence of the droplets may be greatly reduced. Emulsions are intrinsically unstable, thus resembling lyophobic colloids. Three distinct kinds of instability are found to exist, each may be of great, importance in industrial products.

Emulsions may "cream", i.e. separate into layers of aqueous phase with a concentrated layer of oil droplets floating on top, the rate depending primarily on the viscosity of the aqueous phase, the size of droplets, and the density difference between the aqueous phase and the droplets. They may also flocculate as do other lyophobic colloids. The floes, being larger than the individual drops, have a higher creaming rate.