Atoms, Greek concept

The concept of the atom as the smallest particle of matter (from the Greek word for indivisible) was promulgated by John Dalton about 1803. Within about a century and a quarter of scientific investigation which will be briefly described in this chapter, this concept yielded the idea of the periodic table and the understanding of the periodic table including the nuclear atom, the concept of isotopes, and the discovery of the majority of the isotopes which are used in the studies of the isotope effects. It is appropriate to point out that this book deals with the study of the effect of isotopic substitution on the physical and chemical properties of molecular (or atomic) systems. The book does not deal with the use of isotopes as tracers, a use which usually depends on the assumption that isotope effects are small and can be ignored in tracer studies. 

Over the centuries, many other concepts were proposed to explain the nature of matter— many of them extensions of the Greek concept of an ultimately indivisible and indestructible elementary bit of matter. But it was not until J. J. Thomson proposed his model of the atom, consisting of a sphere with an agglomeration of particles with negative electric charges somehow positioned randomly inside a very small ball of matter, that the modern structure of the atom began to take shape. 

Ancient philosophers in Greece, India, China, and Japan speculated that all matter was composed of four or five elements. The Greeks thought that these were fire, air, earth, and water. Indian philosophers and Aristotle from Greece also thought a fifth element—"aether" or "quintessence"—filled all of empty space. The Greek philosopher Democritus thought that matter was composed of indivisible and indestructible atoms. These concepts are now known as classical elements and classical atomic theory. 

We have shown that the unitary group provides an organizing principle for much of matter - atoms, molecules, solids, nuclei and baryons. Note the particle number, N, is a unitary group-quantum number. Thus, the Greek concept of pure form, i.e. the group, is extended to include substance. The N = 0 space is a space without particle-number and hence without substance. The group theoretical history of our universe is shown in Fig. 12.1. 

In 1808, John Dalton, an English school teacher, used the Greek concept of the atom and the law of definite proportions, the law of conservation of mass, and the law of multiple proportions to develop an atomic theory. Dalton believed that a few kinds of atoms made up all matter. 

Democritus (c. 460-370 bc), the father of atomism, focused on the ultimate components of all substances, and his reasoning went something like this If you cut a piece of, say, copper smaller and smaller, you must eventually reach a particle of copper so small that it can no longer be cut. Therefore, matter is ultimately composed of indivisible particles, with nothing between them but empty space. He called the particles atoms (Greek atomos, uncuttable ). However, Aristotle (384-322 bc) held that it was impossible for nothing to exist, and his influence suppressed the concept of atoms for 2000 years. 

In summary, for Leukipp and Demokrit, the empty space between the atoms was a key assumption in their model, because, if particles were closely packed, they could not move and substances could not be mixed. When asking students to philosophise about the nature of matter, we indeed find parallels to the ancient Greek thinking, both to the so-called atomists and to the continuous ideas of Aristotle and others. For example, Leukipp s and Demokrit s explanation for the specific weight of substances corresponds to one student conception younger students especially tend to explain differences in the specific weight (but also hardness of substances) with differences in the closeness of particles (Fig. 10.6). They seldom take into account that the particles could have a different weight themselves. 

The concept of elements is intimately entwined with the idea of atoms, but each does not demand the other. Plato believed in the four canonical elements of antiquity, but he did not exactly concur with the notion of atoms. Other Greek philosophers trusted in atoms but did not divide all matter into a handful of basic ingredients. 

Leopold May goes back even further in time to outline a variety of atomistic ideas from aronnd the world. His chapter Atomism before Dalton concentrates on conceptions of matter that are more philosophical or religiotts than scientific, ranging from ancient Hindu, to classical Greek, to alchemical notions, before touching on a few concepts from the period of early modem science. May is Professor of Chemistry, Emeritus, at the Catholic University of America in Washington, DC. 

The notion of atoms arrived in the East, ancient India, prior to its appearance in the West, the ancient Mediterranean (Greek) world. Both societies were polytheistic, and philosopher-chemists dominated the study of chemistry. Atomic concepts were based upon philosophical considerations and not experimental observations. No exchange on atomism between these two regions in this ancient time has been detected, indicating that these concepts were developed independently ). These developments occurred dming Period I of the Ancient Regime of Chemistry (—10,000 BCE - —100 BCE), which may be called the... 

The contemporary science student is probably aware that the concept of the atom is traceable to early Greek philosophers, notably Democritus. More than likely, however, few have bothered to follow through the hypothetical subdivision process that led to the original concept of an atom. The time has come to remedy this situation since, as mentioned above, the colloidal size range lies between microscopic chunks of material and individual atoms. 

Sala was also an important champion of the introduction of the chemical medicines. Sala s description of fermentation, as an intimate movement of elementary particles which tend to group themselves in a different order to make new compounds, is evidence of a concept doubtless derived from the atomic theory of the Greeks, and differs from the concept of chemical action in the nineteenth century mainly by lacking qualitative and quantitative definition. 

In the past 200 years a great deal of experimental evidence has accumulated to support the atomic model. This theory has proved to be both extremely useful and physically reasonable. When atoms were first suggested by the Greek philosophers Democritus and Leucippus about 400 B.c., the concept was based mostly on intuition. In fact, for the following 20 centuries, no convincing experimental evidence was available to support the existence of atoms. The first real scientific data were gathered by Lavoisier and others from quantitative measurements of chemical reactions. The results of these stoichiometric experiments led John Dalton to propose the first systematic atomic theory. Dalton s theory, although crude, has stood the test of time extremely well. 

The Greek philosopher Democritus (460-370 b.c.) proposed the concept of the atom more than two thousand years ago. 

The Greek philosopher Aristotle (384-322 B.c.) was influential In the rejection of the concept of the atom. 

I he atom is the most fundamental concept in the science of chemistry. A chem- I ical reaction occurs by regrouping a set of atoms initially found in those molecules called reactants to form those molecules called products. Atoms are neither created nor destroyed in chemical reactions. Chemical bonds between atoms in the reactants are broken, and new bonds are formed between atoms in the products. We have traced the concept of the atom from the suppositions of the Greek philosophers to the physics experiments of Thomson and Rutherford and we have arrived at the planetary model of the atom. We have used the Coulomb force and potential energy laws describing the interactions among the nucleus and the electrons in the planetary atom to account for the gain and loss of electrons by atoms,... 

Pauling developed the concept of hybrid orbitals to describe the bonding in molecules containing second-period atoms with steric numbers 2, 3, and 4. Let s discuss these hybridization schemes in sequence, starting with BeH2. We will use the lowercase Greek letter chi, x, to represent hybrid orbitals. 

Dalton, John. (1766-1844). The first theorist since the Greek philosopher Democritus to conceive of matter in terms of small particles. The founder of the atomic theory on which all succeeding chemical investigation has been based (1807). His essential concept of the indivisibility of the atom was not called into question until 1910 when radioactive decay was established by Rutherford. Dalton s theories relating to pressures of gases and atomic combinations led to the basic generalizations stated in the law of multiple proportions, the law of constant composition, and the law of conservation of matter. 

Democritus. A Greek philosopher (approximately 465 bc). The first thinker of record to conceive of matter as existing in the form of small indivisible particles, which he called atoms. However, this concept was overshadowed by Aristotle s theories, and it was not until some 2000 years later that it was developed by John Dalton in England— an astonishing length of dormancy for one of the most creative ideas in the history of science. 

The Greek philosopher Democritus (470-400 Bc) suggested that all matter is composed of tiny, discrete, indivisible particles that he called atoms. His ideas, based entirely on philosophical speculation rather than experimental evidence, were rejected for 2000 years. By the late 1700s, scientists began to realize that the concept of atoms provided an explanation for many experimental observations about the nature of matter. 

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