X-RAYS AND RADIOACTIVITY BASIC INFORMATION AND TUTORIALS

Cathode-ray research had many important spin-offs. In particular, two natural phenomena of immense theoretical and practical significance were discovered in the course of other investigations.

In 1895, Wilhelm Roentgen (1845 1923) noticed that when cathode-ray tubes were operating, certain materials outside the tubes glowed or fluoresced.

He showed that this fluorescence was caused by radiation emitted by the cathode-ray tubes. Because of the unknown nature of this radiation, Roentgen coined the term X-ray. We now recognize the X-ray as a form of high-energy electromagnetic radiation.

Antoine Henri Becquerel (1852 1908) associated X-rays with fluorescence and wondered if naturally fluorescent materials produce X-rays. To test this idea, he wrapped a photographic plate with black paper, placed a coin on the paper, covered the coin with a uranium-containing fluorescent material, and exposed the entire assembly to sunlight.

When he developed the film, a clear image of the coin could be seen. The fluorescent material had emitted radiation (presumably X-rays) that penetrated the paper and exposed the film.

On one occasion, because the sky was overcast, Becquerel placed the experimental assembly inside a desk drawer for a few days while waiting for the weather to clear. On resuming the experiment, Becquerel decided to replace the original photographic film, expecting that it may have become slightly exposed.

He developed the original film and found that instead of the expected feeble image, there was a very sharp one. The film had become strongly exposed because the uranium-containing material had emitted radiation continuously, even when it was not fluorescing. Becquerel had discovered radioactivity.

Ernest Rutherford (1871 1937) identified two types of radiation from radioactive materials, alpha and beta Alpha particles carry two fundamental units of positive charge and have essentially the same mass as helium atoms.

In fact, alpha particles are identical to ions. Beta particles are negatively charged particles produced by changes occurring within the nuclei of radioactive atoms and have the same properties as electrons.

A third form of radiation, which is not affected by electric or magnetic fields, was discovered in 1900 by Paul Villard. This radiation, called gamma rays.

DALTON'S ATOMIC THEORY BASIC INFORMATION AND TUTORIALS

From 1803 to 1808, John Dalton, an English schoolteacher, used the two fundamental laws of chemical combination just described as the basis of an atomic theory. His theory involved three assumptions:

1. Each chemical element is composed of minute, indivisible particles called atoms. Atoms can be neither created nor destroyed during a chemical change.

2. All atoms of an element are alike in mass (weight) and other properties, but the atoms of one element are different from those of all other elements.

3. In each of their compounds, different elements combine in a simple numerical ratio, for example, one atom of A to one of B (AB), or one atom of A to two of

If atoms of an element are indestructible (assumption 1), then the same atoms must be present after a chemical reaction as before. The total mass remains unchanged. Dalton s theory explains the law of conservation of mass. 

If all atoms of an element are alike in mass (assumption 2) and if atoms unite in fixed numerical ratios (assumption 3), the percent composition of a compound must have a unique value, regardless of the origin of the sample analyzed. 

Dalton s theory also explains the law of constant composition.

Like all good theories, Dalton s atomic theory led to a prediction the law of multiple proportions. B (AB2).

If two elements form more than a single compound, the masses of one element combined with a fixed mass of the second are in the ratio of small whole numbers.

John Dalton (1766 1844), developer of the atomic theory. Dalton has not been considered a particularly good experimenter, perhaps because of his color blindness (a condition sometimes called daltonism). 

However, he did skillfully use the data of others in formulating his atomic theory.