Atomic theory of matter

With remarkable accuracy, Democritus in the fifth century B.C. set the stage for modem chemistry. His atomic theory of matter, which he formulated without experimental verification, still stands, more or less intact, and encapsulates the profound truth that nature s stunning wealth boils down to atoms and molecules. As science uncovers the mysteries of the world around us, we stand ever more in awe of nature s ingenious molecular designs and biological systems nucleic acids, saccharides, proteins, and secondary metabolites are four classes of wondrous molecules that nature synthesizes with remarkable ease, and uses with admirable precision in the assembly and function of living systems. 

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. 

The relative molecular mass of a molecule is the sum of the atomic masses of its constituent atoms. The term has replaced molecular weight because weight is a parameter that depends on the magnitude of gravitational attraction. Since relative molecular mass is a ratio (of the mass of the molecule to one-twelfth of the mass of the carbon-12 atom) no units are required. It has, however, become accepted practice to use daltons as a unit of molecular mass, commemorating John Dalton s atomic theory of matter. Relative molecular mass is an approximate indication of size a spherical molecule of 5000 ddtons (or 5 kDa) has a diameter of approximately 2.4 nm. 

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. 

Einstein, A. Investigations in the Theory of the Brownian Movement., Dover, New York, 1956. (Undergraduate level [now ]. A collection of the original papers of Einstein on Brownian movement. Accessible to undergraduates. A window to an era in which atomic theory of matter was still under hot debate.)... 

To Leucippus and Democritus the Greeks and the Western world are indebted for the first clearly defined atomic theory of matter. Leucippus was the teacher of Democritus,... 

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. 

Historically, the observation that volumes of reacting gases always simplified to ratios of small, whole numbers is called Gay-Lussac s Law of Combining Volumes. In the preceding example, the volumes of NO to 02 to N02 fit the pattern of 2 1 2. This observation further strengthened Dalton s argument for an atomic theory of matter. 

The British scientist John Dalton put forward an atomic theory of matter at the beginning of the nineteenth century. This remains a sound basis for understanding the world around us and the actions and reactions of its chemical components. Dalton proposed that all substances are made of matter, which occupies space and has mass, and his theory deals with the nature of this matter. 

John Dalton was a British teacher and self-taught scientist. In 1809, he described atoms as solid, indestructible particles that make up all matter. (See Figure 2.1.) Dalton s concept of the atom is one of several ideas in his atomic theory of matter, which is outlined on the next page. Keep in mind that scientists have modified several of Dalton s ideas, based on later discoveries. You will learn about these modifications at the end of this section. See if you can infer what some of them are as you study the structure of the atom on the next few pages. 

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. 

It is the operational essence of the atomic hypothesis that one can assign properties to atoms and groupings of atoms in molecules and on this basis identify them in a given system or use their properties to predict the behaviour of the system in which they are found. The primary purpose of this section is to demonstrate that the quantum atoms transform this atomic hypothesis into an atomic theory of matter by identifying the atoms of chemistry and defining their properties. This section is not a review of applications, but is rather intended to introduce and illustrate the uses of various atomic properties. 

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

Outline Dalton s atomic theory of matter and describe its experimental basis (Section 1.3). 

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. 

The power of thermodynamics lies in its generality It rests on no particnlar model of the structure of matter. In fact, if the entire atomic theory of matter were to be found invalid and discarded (a very unlikely event ), the foundations of thermodynamics would remain nnshaken. Nonetheless, thermodynamics has some important limitations. Thermodynamics asserts that snbstances have specific mea-snrable macroscopic properties, but it cannot explain why a particular substance has particular numerical values for these properties. Thermodynamics can determine whether a process is possible, but it cannot say how rapidly the process will occur. For example, thermodynamics predicts that diamond is an unstable substance at atmospheric pressure and will eventually become graphite, but cannot predict how long this process will take. 

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. 

John Dalton (1766-1844), an English schoolteacher and chemist, studied the results of experiments by Lavoisier, Proust, and many other scientists. He realized that an atomic theory of matter must explain the experimental evidence. For example, if matter were composed of indivisible atoms, then a chemical reaction would only rearrange those atoms, and no atoms would form or disappear. This idea would explain the law of conservation of mass. Also, if each element consisted of atoms of a specific type and mass, then a compoimd would always consist of a certain combination of atoms that never varied for that compound. Thus, Dalton s theory explained the law of definite proportions, as well. Dalton proposed his atomic theory of matter in 1803. Although his theory has been modified slightly to accommodate new discoveries, Dalton s theory was so insightful that it has remained essentially intact up to the present time. 

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. 

With almost 200 years of hindsight, it may be easy to see how the mass laws could be explained by an atomic model—matter existing in indestructible units, each with a particular mass—but it was a major breakthrough in 1808 when John Dalton (1766-1844) presented his atomic theory of matter in A New System of Chemical Philosophy. 

The works performed by Antoine Francois de Fourcroy (1755-1809), Louis Nicolas Vauquelin (1763-1829), Joseph Louis Proust (1754-1826) and Jons Jakob Berzelius (1779-1848) introduced new concepts in chemistry. Gay-Lussac published his Law of Combining Volumes in 1809, the year after John Dalton (1766-1844) had proposed his Atomic Theory of Matter around 1803. It was left to Amedeo Avogadro (1776-1856) to take the first major step in rationalizing Gay-Lussac s results two years later. 

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],... 

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