Matter Consists of Atoms and Molecules

Elements are composed of extremely small particles called atoms. All atoms of a given element are identical, having the same size, mass, and chemical properties. The atoms of one element are different from the atoms of aU other elements. 

Compounds are composed of atoms of more than one element. In any compound, the ratio of the numbers of atoms of any two of the elements present is either an integer or a simple fraction. 

A chemical reaction involves only the separation, combination, or rearrangement of atoms it does not result in the creation or destruction of atoms. 

Dalton recognized that atoms of one element were different from atoms of all other elements (the first hypothesis), but he made no attempt to describe the stracture or composition of atoms because he had no idea what an atom was really Uke. He did realize, however, that the different properties shown by elements such as hydrogen and oxygen could be explained by assuming that hydrogen atoms were not the same as oxygen atoms. 

The second hypothesis suggests that, to form a certain compound, we need not only atoms of the right kinds of elements, but specific numbers of these atoms as 

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Avogadro s principle rather than law, because it is based not on observation alone but also on a model of matter—namely that matter consists of molecules. Even though there is no longer any doubt that matter consists of atoms and molecules, it remains a principle rather than a law. 

Multipolar Properties of Molecules.- Matter consists of atoms and molecules or their ions, as well as of macromolecules and colloid particles, i.e. quite generally of microsystems. These are dynamical systems having an electromagnetic structure, which we are in some cases able to describe in terms of classical or, more strictly, quantum methods. Since, for our present aims, the quantum-mechanical structure of microsystems is not essential, we shall treat the latter classically, as electrostatic systems presenting a ffistribution of negative and positive electric charges. 

During the nineteenth century it was established that matter consists of atoms and chemically bound aggregates of atoms called molecules. At first, it was thought that atoms were structureless. However, by about the turn of the century, it was shown that atoms were miniature solar systems consisting of a positively charged nucleus, whose structure is irrelevant for chemical phenomena, which contains nearly all the atomic mass, and negatively charged planetary electrons which orbit the nucleus. 

Interstellar matter is comprised of both gas and dust. The gas consists of atomic and polyatomic ions and radicals, and also of molecules. It is the... 

Summarizing, the resolution of these different instruments varies, as does the information that can be obtained with each. Each offers a specialized window on the atomic and molecular world each also involves special difficulties in interpreting the images obtained. One of the greatest problems in proximal probe imaging and manipulation of atoms and molecules lies in the difficulty in obtaining tips of consistent shape and known character. Even the tiniest variations in tip shape and composition — often a matter of just a few atoms — can cause variations in the results obtained. 

The idea that all substances can be separated into tiny indivisible particles called atoms, molecules and ions is widely accepted. Students are familiar with atoms and molecules per favour of the popular media and stylised atoms are the logo for several science TV programs (Johnston, 1990). Children are aware of atoms and molecules well before particle theory is taught in school (Lee et al., 1993). But this is where the similarity between science and student preconceptions ends because students consistently attribute the macroscopic properties of matter to its sub-microscopic particles (Albanese Vicenti, 1997). Similar alternative conceptions, which have given rise to the naive or attribution theory, have been reported in several studies. Seven features, that comprise commonly held alternative conceptions by school students, are now described. 

All science is based on a number of postulates. Quanmm mechanics has also elaborated a system of postulates that have been formulated to be as simple as possible and yet to be consistent with experimental results. Postulates are not supposed to be proved-their justification is efficiency. Quantum mechanics, the foundations of which date from 1925 and 1926, still represents the basic theory of phenomena within atoms and molecules. This is the domain of chemistry, biochemistry, and atomic and nuclear physics. Further progress (quantum electrodynamics, quantum field theory, and elementary particle theory) permitted deeper insights into the structure of the atomic nucleus but did not produce any fundamental revision of our understanding of atoms and molecules. Matter as described by non-relativistic quantum mechanics represents a system of electrons and nuclei, treated as pointlike particles with a definite mass and electric... 

Dalton atomic theory John Dalton, an English schoolmaster, was the first person to formulate a theory of matter. In 1808 he made the following assertions. Matter consists of atoms, which are tiny indivisible particles. Atoms cannot be created or destroyed. The atoms of one element are all identical, particularly in mass, and are different from atoms of other elements. Compound atoms (now called molecules) are formed when small numbers of atoms combine chemically. Compound atoms within a compound are identical and differ from those of other compounds. Modern atomic theory has superseded this theory. 

So, at this point, we are fairly sure that matter consists of individual atoms. The nature of these atoms is quite complex, and the components of atoms don t behave much like the objects we see in the world of our experience. We call this world the macroscopic world— Ihe world of cars, tables, baseballs, rocks, oceans, and so forth. One of the main jobs of a scientist is to delve into the macroscopic world and discover its parts. For example, when you view a beach from a distance, it looks like a continuous solid substance. As you get closer, you see that the beach is really made up of individual grains of sand. As we examine these grains of sand, we find that they are composed of silicon and oxygen atoms connected to each other to form intricate shapes (Fig. 1.2). One of the main challenges of chemistry is to nnderstand the connection between the macroscopic world that we experience and the microscopic world of atoms and molecules. To truly understand chemistry, yon mnst learn to think on the atomic level. We will spend much time in this text helping yon learn to do that. 

A fundamental concept in chemistry is that matter is ultimately composed of atoms and molecules. As such, any model in chemistry must be consistent with that concept. Here, we will consider how thermodynamics is impacted by the atomic theory. 

Going to Chapter III in Dalton s book, one finds his biggest contribution to present day knowledge, namely the clear, experiment backed, statement "Chemical analysis and synthesis go no further than to the separation of particles fi om one another, and to their reunion. No new creation or destruction of matter is within the reach of chemical agency." With the well-known rules of relative combining weights he could then prove the structure of compound bodies in terms of their elementary particles and set up the typical chemical equations aA + bB cC + dD. Today one would say that the molecules of matter consist of atoms held together by chemical bonds. The molecules are the basis of the states of matter. 

Only a small portion of matter consists of elements in uncombined atomic states. Hydrogen, which comprises over 90% of the universe, exists as molecular hydrogen, Likewise, the nitrogen and oxygen in the air we breath exist as the diatomic molecules and O. Practically all substances consist of aggregates of atoms in the form of compounds and molecules. The fact that matter exists as combinations of atoms leads naturally to several basic questions ... 

CHEMICAL COMPOSITION. Matter is composed of the chemical elements, which may be in the free or elementary state, or in combination. In the former case, as exemplified by iron, tin, lead, sulfur, iodine, and the rare gases, matter commonly exhibits the properties of the atoms of the particular element, including the chemical properties whereby they combine to form molecules, Molecules may (1) be monoatomic (2) they may consist of atoms of one element only, such as nitrogen or hydrogen molecules (Nj or H2), (3) they may be composed of atoms of more than one element, called compounds, which usually have distinctive properties. 

A familiarity with intermolecular forces is crucial to building insight into matter. We saw in Chapter 4 that a major goal in chemistry is to trace the connection between individual atoms and molecules and the bulk substances they form. There we dealt with gases, in which intermolecular forces play only a minor role. Here we deal with liquids and solids, for which the forces that hold molecules together are of crucial importance. Individual water molecules, for instance, are not wet, but bulk water is wet. Individual water molecules neither freeze nor boil, but bulk water does. We have to refine our atomic and molecular model of matter to see how properties like these, which we observe when we examine samples consisting of huge numbers of molecules, can be interpreted in terms of the properties of individual molecules, such as their size, shape, and polarity. 

Modern explanations of the chemical properties of matter are based on well tested concepts of the atom and molecule. All matter is presumed to consist of exceedingly small fundamental particles (atoms), which retain their identity in all chemical changes. Atoms may form relatively stable combinations with one another (molecules), and chemical changes or reactions involve changes in these molecules. An important part of chemistry, then, is concerned with the questions Which combinations of atoms are stable , What are the circumstances required for their stability , and What conditions make possible the change of one kind of atomic combination into another In other words, we ask about the identity the stability, and the reactivity of molecules. 

Thermodynamics is based on the atomistic view, that is, that matter consists of elementary particles such as atoms and molecules that cannot be divided into smaller units. The three different states of matter are the result of the simultaneous interaction of a very large number, usually N = Na =6.02x 1023, of elementary particles. Thus, the macroscopic behavior of an ensemble of particles can be mathematically described as a state function that can be related to the individual behavior on a molecular scale, leading to the scientiLcally rigorous framework of statistical thermodynamics (Gcpel and Wiemhcfer, 2000). 

A negative muon can participate in a variety of atomic and molecular processes. A muonic atom is formed when a muon stops in matter replacing an electron. A muonic atom interacting with ordinary atoms or molecules can form a muonic molecule. The latter in turn can result in fusion reactions between the nuclei if the target consists of hydrogen isotopes, a phenomenon known as muon catalyzed fusion (pCF) [6]. 

The basic law of viscosity was formulated before an understanding or acceptance of the atomic and molecular structure of matter although just like Hooke s law for the elastic properties of solids the basic equation can be derived from a simple model, where a flnid is assumed to consist of hypothetical spherical molecules. Also like Hooke s law, this theory predicts linear behavior at low rates of strain and deviations at high strain rates. But we digress. The concept of viscosity was first introduced by Newton, who considered what we now call laminar flow and the frictional forces exerted between layers within a fluid. If we have a fluid placed between a stationary wall and a moving wall and we assume there is no slip at the walls (believe it or not, a very good assumption), then the velocity profile illustrated in Figure... 

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