THE ATOMIC THEORY OF MATTER

Philosophers from the earliest times speculated about the nature of the fundamental stuff from which the world is made. Democritus (460-370 Bc) and other early Greek philosophers described the material world as made up of tiny indivisible particles they called atomos, meaning indivisible or uncuttable. Later, however, Plato and Aristotle formulated the notion that there can be no ultimately indivisible particles, and the atomic view of matter faded for many centuries during which Aristotelean philosophy dominated Western culture. 

The notion of atoms reemerged in Europe during the seventeenth century. As chemists learned to measure the amounts of elements that reacted with one another to form new substances, the ground was laid for an atomic theory that linked the idea of elements with the idea of atoms. That theory came from the work of John Dalton during the period from 1803 to 1807. Dalton s atomic theory was based on the four postulates given in T FIGURE 2.1. 

Dalton s theory explains several laws of chemical combination that were known during his time, including the law of constant composition — (Section 1.2), based on postulate 4  

In a given compound, the relative numbers and kinds of atoms are constant. 

It also explains the law of conservation of mass, based on postulate 3  

The total mass of materials present after a chemical reaction is the same as the total mass present before the reaction. 

A good theory explains known facts and predicts new ones. Dalton used his theory to deduce the law of multiple proportions  

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It may be remarked that there is no call for an atomic theory of energy, analogous to the atomic theories of matter and electricity, as the discontinuity arises from the peculiar character of the system (cf. Planck, 45, 5, 1912). 

The sub-micro level is real, but is not visible and so it can be difficult to comprehend. As Kozma and Russell (1997) point out, understanding chemistry relies on making sense of the invisible and the untouchable (p. 949). Explaining chemical reactions demands that a mental picture is developed to represent the sub-micro particles in the substances being observed. Chemical diagrams are one form of representation that contributes to a mental model. It is not yet possible to see how the atoms interact, thus the chemist relies on the atomic theory of matter on which the sub-micro level is based. This is presented diagrammatically in Fig. 8.2. The links from the sub-micro level to the theory and representational level is shown with the dotted line. 

Johnstone (2000) emphasises the importance of beginning with the macro and symbolic levels (Fig. 8.3) because both comers of the triangle are vistrahsable and can be made concrete with models (p. 12). The strb-micro level, by far the most difficult (Nelson, 2002), is described by the atomic theory of matter, in terms of particles such as electrorrs, atoms and molecules. It is commorrly referred to as the molecular level. Johnstone (2000) describes this level simirltaneorrsly as the strength and weakness of the subject of cherrristry it provides strength through the intellectual basis for chemical explanatiorrs, but it also presents a weakness when novice students try to learn and rmderstand it. 

Brush, Stephen G. (1983), Statistical Physics and the Atomic Theory of Matter from Boyle and Newton to Landau and Onsager, Princeton University Press Princeton, NJ. 

Pyle, Andrew (1995), Atomism and Its Critics Problem Areas Associated with the Development of the Atomic Theory of Matter from Democritus to Newton, Thoemmes Press Bristol. 

Probably the concept of atomism could have gone little further than with Democritus so long as exact experimental means of questioning nature were not employed. The atomic theory of matter and indeed the effort to account for the phenomena of nature by physical causes were to lose in interest to the ancient philosophers through the influence of the two greatest philosophers of ancient times, Plato and Aristotle. 

A unit of mass very nearly equal to that of a hydrogen atom. Named after John Dalton (1766-1844), who developed the atomic theory of matter. 

In 1808, Dalton published A New System of Chemical Philosophy, in which the following five postulates comprise the atomic theory of matter ... 

Dalton s 1808 version of the atomic theory of matter included five general statements (see Section 1.3). According to modem understanding, four of those statements require amendment or extension. List the modifications that have been made to four of the five original postulates. 

In the introduction to his Lectures on Physics,5 Richard P. Feynman asserts that the atomic theory of matter is the most important of scientific theories since it underpins our explanation of the material world. He wrote... 

Boyle, Robert. (1627-1691). A native of Ireland, Boyle devoted his life to experiments in what was then called natural philosophy, i.e., physical science. He was influenced early by Galileo. His interest aroused by a pump that had just been invented, Boyle studied the properties of air, on which he wrote a treatise (1660). Soon thereafter, he stated the famous law that bears his name (see following entry). Boyle s group of scientific enthusiasts was known as the invisible college , and in 1663 it became the Royal Society of London. Boyle was one of the first to apply the principle that Francis Bacon had described as the new method —namely, inductive experimentation as opposed to the deductive method of Aristotle—and this became and has remained the cornerstone of scientific research. Boyle also investigated hydrostatics, desalination of seawater, crystals, electricity, etc. He approached but never quite stated the atomic theory of matter however, he did distinguish between compounds and mixtures and conceived the idea of particles becoming associated to form molecules. 

Statistical Physics and the Atomic Theory of Matter by S. G. Brush, Princeton University Press, Princeton New Jersey, 1983. The interplay between statistical mechanics and thermodynamics is perhaps the greatest single success story in the type of modeling described in this chapter. Brush s book makes for fascinating and instructive reading (as do his other books) since they illustrate the intense growing pains that were suffered in order to convert profound science into what are now little more than textbook exercises. 

Democritus, 460-370 B.C., was a philosopher who proposed that the world is made up of empty space and tiny particles called atoms. Democritus thought that atoms are the smallest particles of matter and that different types of atoms exist for every type of matter. The idea that matter is made up of fundamental particles called atoms is known as the atomic theory of matter. 

One of the interesting things about thermodynamics is that although it deals with matter, it makes no assumptions about the microscopic nature of that matter. Thermodynamics deals with matter in a macroscopic sense it would be valid even if the atomic theory of matter were wrong. This is an important quality, because it means that reasoning based on thermodynamics is unlikely to require alteration as new facts about atomic structure and atomic interactions come to light. 

LIQUIDS OR SOLIDS IN MANY WAYS. MOLECULAR MOTION IN GASES IS TOTALLY RANDOM, AND THE FORCES OF ATTRACTION BETWEEN GAS MOLECULES ARE SO SMALL THAT EACH MOLECULE MOVES EREELY AND ESSENTIALLY INDEPENDENTLY OF OTHER MOLECULES. SUBJECTED TO CHANGES IN TEMPERATURE AND PRESSURE, GASES BEHAVE MUCH MORE PREDICTABLY THAN DO SOLIDS AND LIQUIDS. ThE LAWS THAT GOVERN THIS BEHAVIOR HAVE PLAYED AN IMPORTANT ROLE IN THE DEVELOPMENT OF THE ATOMIC THEORY OF MATTER AND THE KINETIC MOLECULAR THEORY OF GASES. 

Chemists and physicists have used the observed properties of matter to develop models of the individual units of matter. These models collectively make up what we now know as the atomic theory of matter. 

The atomic theory of matter, which was conjectured on qualitative empirical grounds as early as the sixth century BC, was shown to be consistent with increasing experimental and theoretical developments since the seventeenth century AD, and definitely proven by the quantitative explanation of the Brownian motion by Einstein and Perrin early in the twentieth century [1], It then took no more than a century between the first measurements of the electron properties in 1896 and of the proton properties in 1919 and the explosion of the number of so-called elementary particles - and their antiparticles - observed in modern accelerators to several hundred (most of which are very short lived and some, not even isolated). Today, the standard model assumes all particles to be built from three groups of four basic fermions - some endowed with exotic characteristics - interacting through four basic forces mediated by bosons - usually with zero charge and mass and with integer spin [2],... 

Other more recent, important events could also be mentioned here, although it is really during these two centuries that the fundamental basis of electrochemistry was shaped. It is interesting to note that most concepts relating to the existence of ions and the reactions involving the exchange of charge were put forward before the atomic theory of matter was fully accepted. It was in 1803 that Dalton reintroduced the concept of the atom, which had been previously buried for centuries. Thomson s work on the electron was carried out in 1887, and the introduction of the Bohr model dates back to 1913. 

THE ATOMIC THEORY OF MATTER We begin with a brief history of the notion of atoms—the smallest pieces of matter. 

THE ATOMIC THEORY OF MATTER THE DISCOVERY OF ATOMIC STRUCTURE (SECTIONS 2.1 AND 2.2) Atoms are the basic building blocks of matter. They are the smallest units of an element that can combine with other elements. Atoms are composed of even smaller particles, called subatomic particles. Some of these subatomic particles are charged and foUow the usual behavior of charged particles Particles with the same charge repel one another, whereas particles with unlike charges are attracted to one another. 

The Atomic Theory of Matter and the Discovery of Atomic Structure (Sections 2.1 and 2.2)... 

This objection held chemistry back for over 50 years, until Stanislao Cannizzaro (Figure 4.R.2), another Italian chemist, presented in a chemistry conference in Karlsruhe in 1860 and explained how the use of Avogadro s hypothesis resolves all the inconsistencies in the atomic theory of matter. Cannizzaro invented the term mole and also determined the volume of 1 mole of gas as 22.4 L. Thus, 1 mole of H2 weighs 2.01158 g/mol and occupies at room temperature a volume of 22.4L. The volumes of 1 mole of O2, N2, CO2, etc., are all identical, with a value of 22.4 L. If you try to calculate the average space occupied by each molecule in the gas, you will find that large distances, much larger than their own size, separate the molecules. 

The atomic theory of matter infers that we can understand the world around us in terms of its smallest chemical components, atoms, whose structure of a massive, positively charged nucleus surrounded by lighter, fast moving electrons is widely taught. Several details of this model are important in building our knowledge of chemistry—... 

Gases, the subject of this chapter, are simpler than liquids and solids in many ways. Molecular motion in gases is totally random, and the forces of attraction between gas molecules are so small that each molecule moves freely and essentially independently of other molecules. Subjected to changes in temperature and pressure, it is easier to predict the behavior of gases. The laws that govern this behavior have played an important role in the development of the atomic theory of matter and the kinetic molecular theory of gases. 

Aristotle rejected the atomic theory of matter. Democritos had maintained that atoms were in perpetual motion in a void, and Aristotle criticised both the concepts of atomic motion and of the void. Aristotle accounted for motion in terms of the natural tendencies of bodies, and the perpetual random motion of atoms did not concur with his ideas. Aristotle rejected the idea of the void partly as a result of erroneous beliefs concerning falling bodies. He believed that the speed with which a body falls to earth is proportional to its weight he had no understanding of the acceleration of falling bodies. From the observation that bodies fall more slowly in water than in air, Aristotle also concluded that the speed of fall was inversely proportional to the thickness or resistance of the medium. Since a vacuum would have a resistance of zero, the speed of fall of a body in it would be infinite. This was impossible, so Aristotle concluded that a vacuum could not exist. 

Once the Epicureans had suggested a reasonable explanation for a natural phenomenon, they took their enquiry no further. They were content to show that there was no need to involve any Deities (who might have to be propitiated). They were not troubled if two or more explanations could be advanced for a particular phenomenon, and they made no attempt to decide between such alternative theories. An account of Epicurean philosophy was given by the Roman Lucretius (c. 100-55 BC) in his poem De Rerum Natura. It was through Lucretius s work that Pierre Gassendi (1592-1655, Chapter 3) learnt of the Epicurean philosophy and became an early European advocate of the atomic theory of matter. 

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