. ; 560 MAN IS MASTKRING HIS MATERIAL WORLD No thought of beauty or convenience in use was shown in naming such elements as dysprosium, molybdenum, neodymium, manganese, praseodymium, ytterbium, xenon, and yttrium. A great many of the elements have names derived from the Greek — for example, hydrogen (water-producing) ; chlorine (yellowish-green) helium (sun); neon (new). Some elements are named after the particular localities in which they were discovered or occur — for example, strontium, from Strontian in Scotland. The Use of Symbols and Formulas Saves Time. A symbol is the representation of an element; thus H is a symbol for hydrogen, and O is a symbol for oxygen. Many of the elements are known by symbols derived from their former (usually Latin) names. For example, the symbol for sodium is Na (from Natrium), and the symbol for mercury is Hg (from Hydrargyrum) Inasmuch as compounds are composed of elements, the representation of a compound, called 3. formula, is composed of the symbols of the elements in the compound. The formula of copper oxide is CuO because it is composed of copper and oxygen. 1. Define element and compound. STUDY QUESTIONS 2. What are the three types of elements? 3. How many elements are there? 4. How were the elements named? 5. What was the main idea of the alchemists? 6. Differentiate between mixtures, compounds, and elements. 7. How does a substance differ from a mixture? What are the two types of substances? 8. Give an outline for the classification of matter. 9. Differentiate between chemical and physical changes. 10. What is analysis? 11. Name the four most abundant elements on the earth's surface.
UNIT VIII THE ATOM IS SECTION 2 THE UNIT OF CHEMICAL CHANGE All science has one aim, namely to find a Theory of Nature. — Emerson. Introduction. In the previous Section it was pointed out that all of the matter in the universe is composed of at least ninety-two elements, which combine to form compounds. In this Section we shall trace the development of the idea of the concept of atoms as the smallest portions of elements which act as units in chemical changes. The Greeks Taught the Existence of Atoms. The idea that all matter is composed of ultimate divisions, which we call atoms, may be traced at least as far back as 1000 B.C. The Greek philosophers Leucippus , Democritus, and Epicurus advanced a doctrine in the fifth and fourth centuries B.C. which held that matter is made up of individual particles called atoms, or the first beginnings of things, which are immutable and eternal. These particles were supposed to be in constant motion. On the other hand, most of the other Greek philosophers held that matter is continuous — that it is capable of infinite subdivision. Aristotle also believed in the continuity of matter, and the long-continued intellectual dominance of his philosophy caused the atomistic hypothesis to fall into oblivion. In the sixteenth century the physicist, Gassendi, revived the atomic hypothesis, and in the following century Newton gave it powerful support. His views were expressed in the following quotation: It seems probable to me that God, in the beginning, formed matter in solid, massy, hard, impenetrable, movable particles, of such sizes and figures, and with such other properties, and in such proportion to space, as must conduce to the end for which He formed them; and that those primitive particles, being solids, are incomparably harder than any porous bodies compounded of them; even so very hard as never to wear or break in pieces; no ordinary power being able to divide what God Himself made one in the first creation. 561