Postulates of the Atomic Theory

Dalton expressed his theory in a series of postulates. Like most great thinkers, Dalton incorporated the ideas of others into his own to create the new theory. As we go through the postulates, which are presented here in modern terms, let s see which were original and which came from others. 

All matter consists of atoms, tiny indivisible particles of an element that cannot be created or destroyed. (Derives from the eternal, indestructible atoms proposed by Democritus more than 2000 years earlier and conforms to mass conservation as stated by Lavoisier.) 

Atoms of one element cannot be converted into atoms of another element. In chemical reactions, the atoms of the original substances recombine to form different substances. (Rejects the earlier belief by alchemists that one element could be magically transformed into another, such as lead into gold.) 

Atoms of an element are identical in mass and other properties and are different from atoms of any other element. (Contains Dalton s major new ideas unique mass and properties for all the atoms of a given element.) 

Compounds result from the chemical combination of a specific ratio of atoms of different elements. (Follows directly from the fact of definite composition.) 

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Dalton s Atomic Theory 37 Postulates of the Atomic Theory 38 How the Theory Explains the Mass... 

In 1808, an English scientist and schoolteacher, John Dalton, developed the atomic model of matter that underlies modem chemistry. Three of the main postulates of modem atomic theory, all of which Dalton suggested in a somewhat different form, are stated below and illustrated in Figure 2.1. 

This success of the atomic theory is not surprising to a historian of science. The atomic theory was first deduced from the laws of chemical composition. In the first decade of the nineteenth century, an English scientist named John Dalton wondered why chemical compounds display such simple weight relations. He proposed that perhaps each element consists of discrete particles and perhaps each compound is composed of molecules that can be formed only by a unique combination of these particles. Suddenly many facts of chemistry became understandable in terms of this proposal. The continued success of the atomic theory in correlating a multitude of new observations accounts for its survival. Today, many other types of evidence can be cited to support the atomic postulate, but the laws of chemical composition still provide the cornerstone for our belief in this theory of the structure of matter. 

Matter possesses mass, and matter is made up of atoms, so atoms possess mass. This property was already recognized in the time of John Dalton, who made it one of the postulates of his atomic theory. 

The approach to be used here is, to be sure, well known in parts of theoretical physics, but is novel as far as chemistry is concerned. It is based on the view that macroscopic matter is to be described by a suitably generalized formulation of quantum mechanics, namely Quantum Field Theory the traditional postulate that matter is made up or composed of microscopic elementary constituents (in the classical building-block sense) is given up, and instead the fundamental postulate of the quantum theory of matter is, to paraphrase Gertrude Stein, Matter is Matter is Matter. Then if our interest is chemistry we have of course to confront the obvious question as to how we may construct the particles we call atoms and molecules i.e. we must establish how the notions of atom and molecule emerge from quantum theory construed in a general and modem way as the theory of matter. This is the subject matter of the next section of the review... 

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. 

Describe the basic postulates of Daltons atomic theory. (Section 2.1)... 

Kinetic-molecular theory (5.6) Description of a gas as a collection of a very large number of atoms or molecules in constant, random motion. The ideal gas law can be derived from the postulates of the kinetic theory. 

Development of the Atomic Theory The search for the fundamental units of matter began in ancient times. The modem version of atomic theory was laid out by John Dalton, who postulated that elements are compost of extremely small particles, called atoms, and that aU atoms of a given element are identical, but they are (hlTerent from atoms of all other elements. 

For example, a clearly defined working concept of an element was essential for the elaboration of the atomic theory by John Dalton (1766-1844). In 1808, this English schoolmaster published A New System of Chemical Philosophy in which he put forth his postulates about the structure of matter. Each element, Dalton argued, is constituted of identical, immutable, and uniquely characteristic atoms. When elements combine to form compounds, then-atoms unite in a fixed ratio that is characteristic of the compound. Because these numbers are fixed, the elementary composition of the compound, by mass, is also constant. This latter property could be quite accurately determined, even in Dalton s time. However, Dalton had no direct way of ascertaining the correct atomic ratio of the elements of any compound. As a consequence, he could not calculate, with confidence, the relative masses of the atoms of the various elements—their atomic masses or atomic weights. The atomic masses of the elements and the atomic ratios characteristic of specific compounds are manifested in the elementary mass composition of the compounds. Once the correct atomic ratio is known, atomic masses can be readily calculated from mass composition. Alternatively, knowledge of atomic masses permits conversion of composition by mass to composition by atomic ratio. However, Dalton faced the dilemma of an equation with two unknowns. 

Artificial transmutations. Everyone is more or less fomiliar with the dreams, shattered by Dalton, of the alchemists who attempted to transform a cheap metal into gold. With the postulation of the nuclear theory of the atom, artificial transmutation came to appear feasible. The first induced transmutation was demonstrated in 1919 by Rutherford, who exposed nitrogen to a particles from radium and detected the production of protons ... 

The law of conservation of mass This states that there is no detectable change in mass in an ordinary chemical reaction. If atoms are conserved in a reaction (postulate 2 of the atomic theory), mass will also be conserved. 

There are three different approaches to a thermodynamic theory of continuum that can be distinguished. These approaches differ from each other by the fundamental postulates on which the theory is based. All of them are characterized by the same fundamental requirement that the results should be obtained without having recourse to statistical or kinetic theories. None of these approaches is concerned with the atomic structure of the material. Therefore, they represent a pure phenomenological approach. The principal postulates of the first approach, usually called the classical thermodynamics of irreversible processes, are documented. The principle of local state is assumed to be valid. The equation of entropy balance is assumed to involve a term expressing the entropy production which can be represented as a sum of products of fluxes and forces. This term is zero for a state of equilibrium and positive for an irreversible process. The fluxes are function of forces, not necessarily linear. However, the reciprocity relations concern only coefficients of the linear terms of the series expansions. Using methods of this approach, a thermodynamic description of elastic, rheologic and plastic materials was obtained. 

Particularly spectra and quantum theory seemed to indicate an order. A planetary model almost suggested itself, but according to classical physics, the moving electrons should emit energy and consequently collapse into the nucleus. The 28-year-old Niels Bohr ignored this principle and postulated that the electrons in these orbits were "out of law". This clearly meant that classical physics could not describe or explain the properties of the atoms. The framework of physical theory came crashing down. Fundamentally new models had to be developed.1... 

Ans. The gas laws work for unbonded atoms as well as for multiatom molecules, and so it is convenient to classify the unbonded atoms as molecules. If these atoms were not classified as molecules, it would be harder to state the postulates of the kinetic molecular theory. For example, postulate 1 would have to be stated "Molecules or unbonded atoms are in constant random motion. ... 

The interaction processes between UV-Vis photons and the outer electrons of the atoms of the analytes can be understood using quantum mechanics theory. In the thermodynamic equilibrium between matter and interacting electromagnetic radiation, according to the radiation laws postulated by Einstein, three basic processes between two stable energy levels 1 and 2 are possible. These processes, which can be defined by their corresponding transition probabilities, are summarised in Figure 1.3. 

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