CoreChem:Chapter 4 - THE STRUCTURE OF ATOMS
From ChemEd Collaborative
THE STRUCTURE OF ATOMS
4.1 Descriptive Chemistry of Some Groups of Related Elements
4.2 The Periodic Classification of the Elements
4.4 Atomic Structure and Isotopes
4.5 Measurement of Atomic Weights
We have examined the theoretical implications and practical applications of John Dalton’s ideas about atoms in the preceding two chapters. Clearly the atomic theory is a powerful too1 which aids our thinking about how much of one substance can combine with (or be produced from) a given quantity of another. The theory is much less helpful, however, when we try to speculate about what holds the atoms together in molecules such as Br2, HgBr2 and Hg2Br2. As you have seen, techniques are available for experimental determination of the formula of a new compound, but Dalton’s theory is of little value in predicting formulas. Neither does it tell us which elements are likely to combine with which, nor indicate what chemical and physical properties are to be expected of the compounds which form.
The ability to make predictions about chemical reactivity and properties is very important because it guides chemists’ efforts to synthesize new substances which are of value to society at large. Medicines, metals, transistors, plastics, textiles, fertilizers, and many other things that we take for granted today have been made possible by detailed knowledge of chemical and physical properties. Such knowledge also permits greater understanding of how the natural world works and what changes (favorable or detrimental) may be brought about by human activities.
Knowledge of chemical reactivity and properties may be approached on both the macroscopic and microscopic levels. Macroscopically this involves what is called descriptive chemistry. The person who first carries out a chemical reaction describes what happened, usually in terms of a balanced equation, and lists properties of any new substances. This enables other scientists to repeat the experiment if they wish. Even if the work is not carried out again, the descriptive report allows prediction of what would happen if it were repeated.
The microscopic approach uses theory to predict which substances will react with which. During the past century Dalton’s atomic theory has been modified so that it can help us to remember the properties of elements and compounds. We now attribute structure to each kind of atom and expect atoms having similar structures to undergo similar reactions. The additional complication of learning about atomic structure is repaid manyfold by the increased ability of our microscopic model to predict macroscopic properties.
