Dalton, John about

In England, around the same time, John Dalton studied the masses of compounds as they reacted to produce products. After Dalton read about the similar work of other scientists, such as Lavoisier and the British scientist Joseph Priestley, he contacted Gay-Lussac. He described his results and hypotheses to Gay-Lussac. In 1808, both men published their theories. After examining the theories of Dalton and Gay-Lussac, an Italian scientist named Amedeo Avogadro formulated a hypothesis that combined their theories. 

John Dalton s Atomic hypothesis. J. Fraunhofer locates and names Fraunhofer lines A...L in solar spectrum. About the same time, Herschel discovers infrared radiation from the Sun. 

The concept of the atom as the smallest particle of matter (from the Greek word for indivisible) was promulgated by John Dalton about 1803. Within about a century and a quarter of scientific investigation which will be briefly described in this chapter, this concept yielded the idea of the periodic table and the understanding of the periodic table including the nuclear atom, the concept of isotopes, and the discovery of the majority of the isotopes which are used in the studies of the isotope effects. It is appropriate to point out that this book deals with the study of the effect of isotopic substitution on the physical and chemical properties of molecular (or atomic) systems. The book does not deal with the use of isotopes as tracers, a use which usually depends on the assumption that isotope effects are small and can be ignored in tracer studies. 

The molecular mass has units termed daltons (Da), after the famous English chemist John Dalton (1766-1848). Eor example, the molecular mass of insulin might be said to be about 6000 daltons. However, this designation does not add much to understanding the concept of molecular size and we shall delete the dalton unit in all that follows. 

As a child Dalton was sent, along with his older brother Jonathan, to Pardshaw Hall School, about two miles from his home. There he had an excellent teacher in John Fletcher, who did not believe that students should be force-fed Latin, as they were in most English schools, and who encouraged Dalton to pursue his interest in mathematics. Dalton pursued mathematics tenaciously. If he encountered a problem he couldn t solve at first, he continued to work at it until he found an answer. 

People have been thinking about tiny objects for a long time. The ancient Greek philosopher Democritus (ca. 460-370 b.c.e.) believed that properties of matter depended on the shapes of small, indivisible bits of matter called atoms. Although this idea failed to catch on at the time— no one could see these atoms because they were so small—in 1803, the British chemist John Dalton (1766-1844) proposed a similar theory. Dalton s theory was an important advance and helped scientists understand chemical reactions—for example, the reaction of two atoms of hydrogen (H) and one atom of oxygen (O) to form H O—but atoms themselves remained cloaked in mystery. 

In 1803, John Dalton (Figure 2.2) suggested that each element was composed of its own kind of particles, which he called atoms. Atoms are much too small to be seen. We now know that about 20 x 106 of them would stretch over a length of only 1 cm. 

We have already seen in Chapter 2 that everything you see around you is made out of tiny particles called atoms (Figure 3.1). When John Dalton developed his atomic theory, about 200 years ago (1807/1808), he stated that the atoms of any one element were identical and that each atom was indivisible . Scientists in those days believed that atoms were solid particles like marbles. 

Humphry Davy (1778-1829) was a self-made man, a woodcarver s son who later, as the leading British chemist of his day, became president of the Royal Society of London. He made chemistry fashionable in London. Carriages queued up to bring people to his lectures or to read bulletins of his health when he was ill. He was constantly probing the nature and number of chemical elements, and in doing so he made the most dramatic use of an instrument invented by Alessandro Volta. Davy kept returning to questions about the existence and nature of atoms. And he, even more than John Dalton, saw himself as the Newton of chemistry, the one who brought laws and order into the chemical laboratory. 

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. 

John Dalton did not know about subatomic particles when he developed his atomic theory. Even so, the modern atomic theory (shown on the next page) retains many of Dalton s ideas, with only a few modifications. Examine the comments to the right of each point. They explain how the modern theory differs from Dalton s. 

The English scientist John Dalton did a very thorough analysis of the atmosphere. He concluded that it comprised about 79% nitrogen and 21% oxygen. (See Table 11.2 to find out how close he really was.) Dalton noticed that the water vapour, however, seemed quite variable, so he did further experiments. He obtained some very dry air, and measured the pressure in the container. He then introduced some water vapour. The pressure increased Dalton repeated and adjusted his experiment time after time, always with the same results. He concluded that the total pressure of a mixture of gases is the sum of the pressures of each of the individual gases. This is called Dalton s Law of partial pressures. (See Figure 11.27 below.)... 

In 1808, John Dalton published A New System of Chemical Philosophy, which proposed his hypotheses about the nature of matter. Dalton s atomic theory explained that... 

Proust s discovery stimulated John Dalton (1766-1844), an English schoolteacher (Fig. 2.2), to think about atoms. Dalton reasoned that if elements were composed of tiny individual particles, a given compound should always contain the same combination of these atoms. This concept explained why the same relative masses of elements were always found in a given compound. 

In the past 200 years a great deal of experimental evidence has accumulated to support the atomic model. This theory has proved to be both extremely useful and physically reasonable. When atoms were first suggested by the Greek philosophers Democritus and Leucippus about 400 B.c., the concept was based mostly on intuition. In fact, for the following 20 centuries, no convincing experimental evidence was available to support the existence of atoms. The first real scientific data were gathered by Lavoisier and others from quantitative measurements of chemical reactions. The results of these stoichiometric experiments led John Dalton to propose the first systematic atomic theory. Dalton s theory, although crude, has stood the test of time extremely well. 

FIG. 7-i. Atomic symbols and molecular formulas used by John Dalton, about 1803. 

English scientist John Dalton was by no means the first person to propose the existence of atoms as we have seen, speculations about them date back to Greek times (the word atom is derived from Greek a- [ not ] plus tomos [ cut ], meaning not divisible ). Dalton s major contribution to chemistry was to marshal the evidence for the existence of atoms. He showed that the mass relationships found by Lavoisier and Proust could be interpreted most simply by postulating the existence of atoms of the various elements. 

Some of the first steps toward understanding how the basic corpuscles of matter behaved came from what might seem like an unlike source meteorology. John Dalton (1766-1844), bom in Manchester, began to study weather in 1787. He also studied Newton s Principia and as a result was conversant with the concept of the inverse square law. Newton had used it to show that the attraction of gravity decreased as the square of the distance between objects. This mathematical relationship also applied to a range of other physical phenomena, such as luminosity. When considering questions about precipitation— why there was fog, rain, and snow—he discovered that the quantity of water vapor in a gas was independent of the type of gas but dependent on the temperature of the gas. 

By the early 1800s, the Law of Conservation of Matter (Section 1-1) and the Law of Definite Proportions (Section 1-5) were both accepted as general descriptions of how matter behaves. John Dalton (1766-1844), an English schoolteacher, tried to explain why matter behaves in such systematic ways as those expressed here. In 1808, he published the first modern ideas about the existence and nature of atoms. Dalton s explanation summarized and expanded the nebulous concepts of early philosophers and scientists more importantly, his ideas were based on reproducible experimental restdts of measurements by many scientists. These ideas form the core of Dalton s Atomic Theory, one of the highlights in the history of scientific thought. In condensed form, Dalton s ideas may be stated as follows ... 

Modern atomic theory is credited to the work of John Dalton published in 1803-1807. Observations made by him and others about the composition, properties, and reactions of many compounds led him to develop the following postulates ... 

At about the same time (1801), John Dalton observed that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component ... 

Speculations about the nature of matter date back to ancient Greek philosophers like Thales, who lived in the sixth century b.c.e., and Democritus, who lived in the fifth century b.c.e., and to whom we credit the first theory of atoms. It has taken two and a half millennia for natural philosophers and, more recently, for chemists and physicists to arrive at a modern understanding of the nature of elements and compounds. By the 19th century, chemists such as John Dalton of England had learned to define elements as pure substances that contain only one kind of atom. It took scientists like the British physicists Joseph John Thomson and Ernest Rutherford in the early years of the 20th century, however, to demonstrate what atoms are—entities composed of even smaller and more elementary particles called protons, neutrons, and electrons. These particles give atoms their properties and, in turn, give elements their physical and chemical properties. 

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