The Atomic Theory Today

For over 200 years, scientists have known that all matter consists of atoms, and they have learned astonishing things about them. Dalton s tiny indivisible particles have given way to atoms with fuzzy, indistinct boundaries and an elaborate internal architecture of subatomic particles. In this section, we examine our current model and begin to see how the properties of subatomic particles affect the properties of atoms. 

---

The Atomic Theory Today 42 Structure of the Atom 42 Atomic Number, Mass Number,... 

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. 

The atomic theory is so rooted in today s science that we readily accept the existence of atoms. Even though few of us have observed them directly, we have faith that everything around us is made of them. Yet we continue to probe the structure of the atom. It is interesting that we continue to seek greater knowledge about the atom and its smallest constituents in order to understand the largest entity we can think of— the universe. Just as Democritus quest was to build a philosophical system to explain nature, we continue to probe the atom in hope of understanding the universe and our place in it. 

In a preceding section it has been stated that the atomic theory provides a simple explanation of the laws of stoichiometry, and that there is now no question as to the reality of atoms. The thoughtful student may ask what the meanings of the words explanation and reality are, in these statements. These questions are not easy to answer the great philosophers and scientists of past ages and of today have been deeply interested in them. The following discussion is far from exhaustive. 

In 1804, the English scientist John Dalton formulated the atomic theory, which set out some fundamental characteristics of matter, and which is still used today. According to this theory, matter is composed of extremely small particles called atoms, which can be neither created nor destroyed. Atoms can, however, attach themselves (bond) to each other in various arrangements to form molecules. A material composed entirely of atoms of one type is an element, and different elements are made of different atoms. A material composed entirely of molecules of one type is a compound, and different compounds are made of different molecules. Pure elements and pure compounds are often referred to collectively as pure substances, as opposed to a mixture in which atoms or molecules of more than one type are jumbled together in no particular arrangement. 

It is true that the ancient Greeks were concerned with knowledge for its own sake. They even had a well-developed atomic theory which was in many ways similar to atomic theory today. But the Greeks were not fond of doing experimental work, so their theory just stayed on the books and was never developed. 

Coordination compounds are formed by the union of two or more compounds which are themselves stable and capable of independent existence. Some of them dissociate on heating, but others are extremely stable. Examples of coordination compounds are NH3BF3 (Experiment 15), A(BF3)2, Fe(CO)6, Co(NH3)3(N02)3, and ionized salts such as Cu(NH3)4S04 (Experiment 5), K4Fe(CN)6, the hundreds of cobaltammines, NH4CI, and H2SiFe. These compounds were a puzzle to the older theories of valence, since their formation seemed to involve residual valences or valences in excess of the normal valences of the atoms concerned. Today, however, they are believed to be bound by means of the coordinate link, which is simply a special kind of covalent bond. 

These concepts, which seem so familiar today, only became clear with the atomic theory of Le Bel and van t Hoff which provided a solid theoretical basis for the understanding of the three-dimensional structure of molecular objects. 

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],... 

At the beginning of the 19 century the description of matter attained, what one would call today, a scientific basis. Dalton supported the atomic theory with experiments permitting the development of modem chemistry. The book A New System of Chemical Philosophy, describes this new approach. In Fig. 1.1, an excerpt is displayed. Chapters 1 and II give a summary of the contemporary understanding of nature by analyzing heat and mass, the two basic building-blocks of any material. Chapter 1 displays the theory of the caloric as it was generally... 

The first quantitative analytical fields to be developed were for quantitative elemental analysis, which revealed how much of each element was present in a sample. These early techniques were not instrumental methods, for the most part, but relied on chemical reactions, physical separations, and weighing of products (gravimetry), titrations (titrimetry or volumetric analysis), or production of colored products with visual estimation of the amount of color produced (colorimetry). Using these methods, it was found, for example, that dry sodium chloride, NaCl, always contained 39.33% Na and 60.67% Cl. The atomic theory was founded on early quantitative results such as this, as were the concept of valence and the determination of atomic weights. Today, quantitative inorganic elemental analysis is performed by atomic absorption spectrometry (AAS), AES of many sorts, inorganic MS (snch as ICP-MS), XRF, ion chromatography (1C), and other techniques discussed in detail in later chapters. 

Today, the evidence for the atomic theory is overwhelming. Recent advances in microscopy have allowed scientists not only to image individual atoms but also to pick them up and move them ( Figure 4.2). Matter is indeed composed of atoms. 

A little over two hundred years ago the atomic theory was broadly accepted for the first time in history (in part because of John Dalfon). Today we can "fake picfures" of them. In 1986 Gerd Binnig of Germany and Heinrich Rohrer of Switzerland shared the Nobel Prize for fheir discovery of the scanning tunneling microscope (STM), a microscope that can image and move individual atoms and molecules. Figure 1-12 shows "NANO USA" written with 112 individual molecules. The ability to see and move individual atoms has created fantastic possibilities. It may be possible some day to construct microscopic... 

The atomic theory of matter, which forms the basis of modem chemistry, was the work of the British chemist John Dalton (1766-1844). Throughout his life, Dalton maintained an interest in the science of weather and climate. This interest led Dalton to study the atmosphere and to speculate on its fundamental stmcture, which eventually led him to his atomic theory. If Dalton were alive today, he might well be conducting experiments designed to answer some current questions we have about atmospheric chemistry How dramatically is the earth s climate being changed by human activity What are the chemical processes involved in the depletion of ozone (a form of oxygen) over the poles and how will this depletion affect life on earth ... 

The law of partial pressures, Eq. (3.3.7), was observed in 1801 by John Dalton (1766-1844), a British chemist and physicist. He worked on the constitution of mixed gases, on the vapor pressure of liquids, and on the thermal expansion of gases. His most important investigations are those concerned with the atomic theory in chemistry, which can be summarized as follows (i) Elements are made of tiny particles called atoms, (ii) Atoms of a given element are identical and different from those of other elements, (iii) Atoms of one element can combine with atoms of other elements to form compounds that always have the same relative numbers of types of atoms, (iv) Atoms cannot be created, divided into smaller particles, or destroyed in the chemical process. These statements of Dalton s theory are to a large extent still true. Today we know that his statement Atoms cannot be created, divided. .. is inconsistent with nuclear fusion and fission, and his statement All atoms of a given element are identical is also not precisely true, as there are different isotopes of an element. Dalton also did research into color blindness, which is sometimes called Daltonism in his honor. 

John Dalton (1766-1844) was an English chemist, mathematician, and philosopher [Figure 2.1(a)]. In addition to the atomic theory, he also formulated several gas laws and gave the first detailed description of the type of color blindness from which he suffered. This type of color blindness is known today as Daltonism. He was described by his friends as awkward and without social grace, and in fact, he spent much of his time studying and tutoring students. 

FIGURE 9.6 John Dalton (1766-1844). In 1803, Dalton restated the atomic theory of Democritus (fourth century b.c.) in a more modern form that with only slight modification is still considered valid today. In his honor, another name for an atomic mass unit is the dalton. Also in his honor, because he was the first person to write a description of color blindness, this affliction is sometimes referred to as daltonism. The original records of his experiments were destroyed by bombing in World War 11. 

As we discussed in Chapter 1, it was only 200 years ago that John Dalton proposed his atomic theory. Today we can image atoms, move them, and even build tiny machines out of just a few dozen atoms (an area of research called nanotechnology). These atomic machines, and the atoms that compose them, are almost unimaginably small. 

Today, the evidence for the atomic theory is overwhelming. Matter is indeed composed of atoms. 

The modern atomic theory of matter is almost two centuries old. It was in the early nineteenth century that Dalton s work (John Dalton, England, 1766-1844) advanced the proposal that matter is not continuously divisible and that there is some fundamental type of particle, the atom. The line of thought that began with the atomic theory of matter took its next major step in the early twenheth century when experiments pointed to the existence of subatomic particles. In a few more decades, it became clear that there are even smaller particles. Even today, the search for exotic subatomic particles continues. As matter is viewed using more and more powerful techniques, we can see that all matter is composed of discrete building blocks (parhcles) rather than continuous materials. 

---