Molecular Model for Water
Work in Progress; This Exemplar is not ready for use.Vitz 20:46, 8 September 2009 (UTC)
The development of the atomic theory owes much to the work of two men: Antoine Lavoisier, who did not himself think of matter in terms of atoms but whose work laid organization groundwork for thinking about elements, and John Dalton, to whom the atomic theory is attributed. Much of Lavoisier’s work as a chemist was devoted to the study of combustion. He discredited the erroneous "phlogiston theory" which held that combustion was the removal of a fire-like element (phlogiston) from a burning substance by air. When air became completely phlogisticated, it could no longer support combustion or life. But Lavoisier became convinced that when a matter is burned in air, the matter combines with some component of the air, because he noticed that the matter often increases in weight. It couldn't be losing phlogiston! Eventually he realized that the component of air was the dephlogisticated air (pure oxygen) which had been discovered by Joseph Priestly (1733 to 1804) and C.W. Scheele a few years earlier. Lavoisier renamed this substance oxygen . Lavoisier showed that dephlogisticated air supported life; as a matter of fact, Lavoisier’s careful experiments also revealed that animals and burning were both similar forms of combustion, requiring oxygen and releasing heat.
Using closed vessels, he demonstrated that oxygen gas had mass, and that there was no change in mass upon reaction with matter or animals (the gain in weight of the matter equalled the loss in weight of air. Lavoisier hypothesized that this should be true of all chemical changes, and further experiments showed that he was right. This principle is now called the law of conservation of mass.
As Lavoisier continued his experiments with oxygen, he noticed something else. Although oxygen combined with many other substances, it never behaved as though it were itself a combination of other substances. For example, Lavoisier was able to decompose mercuric oxide into mercury and oxygen, but he could find no way to break down oxygen into two or more new substances. Because of this he suggested that oxygen must be an element—an ultimately simple substance which could not be decomposed by chemical changes.
Lavoisier did not originate the idea that certain substances (elements) were fundamental and all others could be derived from them. This had first been proposed in Greece during the fifth century B.C. by Empedocles, who speculated that all matter consisted of combinations of earth, air, fire, and water. These ideas were further developed and taught by Aristotle and remained influential for 2000 years.
Lavoisier did, however, produce the first table of the elements which contained a large number of substances that modern chemists would agree should be classifies as elements. He published it with the knowledge that further research might succeed decomposing some of the substances listed, thus showing them not to be elements. One of his objectives was to prod his contemporaries into just that kind of research. Sure enough the “earth substances” listed at the bottom were eventually shown to be combinations of certain metals with oxygen. It is also interesting to note that not even Lavoisier could entirely escape from Aristotle’s influence. The second element in his list is Aristotle’s “fire,” which Lavoisier called “caloric,” and which we now call “heat.” Both heat and light, the first two items in the table, are now regarded as forms of energy rather than of matter.
Although his table of elements was incomplete, and even incorrect in some instances, Lavoisier’s work represented a major step forward. By classifying certain substances as elements, he stimulated much additional chemical research and brought order and structure to the subject where none had existed before. His contemporaries accepted his ideas very readily, and he became known as the father of chemistry.
John Dalton (1766 to 1844) was a generation younger than Lavoisier and different from him in almost every respect. Dalton came from a working class family and only attended elementary school. Apart from this, he was entirely self-taught. Even after he became famous, he never aspired beyond a modest bachelor’s existence in which he supported himself by teaching mathematics to private pupils. Dalton made many contributions to science, and he seems not to have realized that his atomic theory was the most important of them. In his “New System of Chemical Philosophy” published in 1808, only the last seven pages out of a total of 168 are devoted to it!
The postulates of the atomic theory are given in the following table. The first is no advance on the ancient Greek philosopher Democritus who had theorized almost 2000 years earlier that matter consists of very small particles.
The Postulates of Dalton's Atomic Theory
1 All matter is composed of a very large number of very small particles called atoms.
2 For a given element, all atoms are identical in all respects. In particular all atoms of the same element have the same constant mass, while atoms of different elements have different masses.
3 The atoms are the units of chemical changes. Chemical reactions involve the combination, separation, or rearrangement of atoms, but atoms are neither created, destroyed, divided into parts, or converted into atoms of any other kind.
4 Atoms combine to form molecules in fixed ratios of small whole numbers.
The second postulate, however, shows the mark of an original genius; here Dalton links the idea of atom to the idea of element. Lavoisier’s criterion for an element had been essentially a macroscopic, experimental one. If a substance could not be decomposed chemically, then it was probably an element. By contrast, Dalton defines an element in theoretical, microscopic terms. An element is an element because all its atoms are the same. Different elements have different atoms. There are just as many different kinds of elements as there are different kinds of atoms.
Now look at a molecular model of hydrogen, we see that hydrogen is an element, because it consists of only one type of atom, even though the gas consists of molecules with two identical atoms bonded together. Although atoms are drawn as spheres in the figure, it would be more common today to represent them using chemical symbols, in this case H, so each molecule would look like H - H. Note that Dalton's fourth hypothesis suggests that atoms may combine to form molecules, although it was not clear to early chemists whether hydrogen was diatomic or monatomic. When identical atoms bond, the molecule is still an element; if different atoms bond, for example if two hydrogen atoms bond to one oxygen atom to make water, H-O-H, the molecule represents a compound.
The molecular model for oxygen would look very similar, because oxygen is also a diatomic gas, consisting of O-O molecules. Many elements exist as simple atoms, while some may exist as triatomic or even more complicated molecules. Mercury, for example, exists as simple atoms:
The chemical symbols for all the currently known elements are listed below in the table, which also includes atomic weights.
Names, Chemical Symbols, and Atomic Weights of the Element
|Name||Symbol||Atomic Number||Atomic Weight||Name||Symbol||Atomic Number||Atomic Weight|
The chemical symbol for an element (or an atom of that element) is a one- or two-letter abbreviation of its name. Usually, but not always, the first one or two letters are used. To complicate matters further, chemical symbols are sometimes derived from a language other than English. For example the symbol for Hg for mercury comes from the first and seventh letters of the element’s Latin name, hydrargyrum.
These symbols are the basic vocabulary of chemistry because the atoms they represent make up all matter. You will see symbols for the more important elements over and over again, and the sooner you know what element they stand for, the easier it will be for you to learn chemistry. These more important element have been indicated in the above table by colored shading around their names.
The composition of a molecule is indicated by a chemical formula. A subscript to the right of the symbol for each element tells how many atoms of that element are in the molecule. For example, the atomic weights table gives the chemical symbol H for hydrogen, but each molecule contains two hydrogen atoms, and so the chemical formula is H2. According to Dalton’s fourth postulate, atoms combine in the ratio of small whole numbers, and so the subscripts in a formula should be small whole numbers.