I 566 MAN IS MASTERING HIS MATERIAL WORLD wheat for protein, for example, and the baker usually checks this analysis to make sure that he receives what he has paid for. A carload of iron, copper, silver, or some other ore is paid for on the basis of analysis. A small change in the decimal place of the atomic weight of an element might make a difference of thousands of dollars in the amount paid to a large mine each year, for all calculations based on analyses employ atomic weights. It is exceedingly important, therefore, that atomic weights be known as accurately as human ingenuity will permit. 1. Define atom and molecule. STUDY QUESTIONS 2. State the main points of Dalton's atomic theory. 3. State the law of conservation of mass. 4. What does Dalton's atomic theory explain? 5. State Avogadro's number. What does it mean? 6. What do chemical equations represent? Of what value are chemical equations? 7. What is (1) a molecular weight, (2) a gram-molecular weight? 8. What is a combining weight of an element? 9. What is the relation between the combining weight of an element and its atomic weight? 10. What is valence? What information is required in order to obtain the valence of an element in a given compound? IL State the law of definite composition.
UNIT VIII SECTION 3 ATOMS ARE COMPLEX AND MAY BE TRANSFORMED BY NUCLEAR CHANGES We find ourselves, in consequence of the progress of physical science, at the pinnacle of one ascent of civilization, taking the first step upward out on the lowest plane of the next. Above us rises indefinitely the ascent of physical power — far beyond the dream of mortals in any previous system of philosophy. — Frederick Soddy. Introduction. William Prout, having observed that many atomic weights are whole numbers and having assumed that all atomic weights would prove to be whole numbers, advanced the hypothesis that the atoms of all elements are simple multiples of the atoms of smallest mass, those of hydrogen. Prout was on the track of a great idea, but it was not until some of the information presented in this Section became available that his idea that atoms are complex in nature received confirmation. The Periodic Law Suggests Atomic Complexity. In 1865 John Newlands observed that the elements seem to exist as Today, for example, we recognize that the inert gases of the families. atmosphere — helium, neon, argon, krypton, and xenon — are so much alike chemically that it is difficult to separate them. These elements had not been discovered in Newlands' time, but he observed other families with similar characteristics. For example, the alkali group consists of lithium, sodium, potassium, rubidium, and cesium, all very active metals with a combining power of one. Another such family is the alkaline earth group, consisting of magnesium, calcium, strontium, and barium, which are metals which pccur in similar types of compounds and exhibit a combining power of two. The halogens are a group of very active nonmetals with a combining power of one. This group consists of fluorine, chlorine, bromine, and iodine. Newlands arranged the elements in the increasing order of their atomic weights and observed that every eighth element seemed to belong to the same 567