Laws of chemical combination

The dawn of the nineteenth century saw a drastic shift from the dominance of French chemistry to first English-, and, later, German-influenced chemistry. Lavoisier s dualistic views of chemical composition and his explanation of combustion and acidity were landmarks but hardly made chemistry an exact science. Chemistry remained in the nineteenth century basically qualitative in its nature. Despite the Newtonian dream of quantifying the forces of attraction between chemical substances and compiling a table of chemical affinity, no quantitative generalization emerged. It was Dalton s chemical atomic theory and the laws of chemical combination explained by it that made chemistry an exact science. 

The classical, or Daltonian, laws of chemical combination depend on strong, primary forces between atoms. Covalency, for instance, nearly always produces discrete molecules, with a well defined structure, and a fixed composition. Only in special cases do the weaker secondary forces give such clear-cut products. The difference is mainly due to the weakness of the forces but it is also due to... 

While Lavoisier undoubtedly laid the foundations for the rapid rise of chemistry in the nineteenth century, his work was supported by that of other French chemists both before and after him.247 Without the Lavoisierian notion of the chemical element, it is unlikely that the chemical atomic theory and the laws of chemical combination would have been so widely accepted. However, a new perspective on the history of the concept of the chemical compound as the basis of modem chemistry suggests that it began long before Lavoisier and belongs to a different line of development involving the notion of chemical affinity .248-250... 

In the early years of the nineteenth century, the new chemistry began to bear fruit on both sides of the Channel,270 as well as in other countries, notably in Sweden. The chemical atomic theory proposed by Dalton and developed by Berzelius led to the formulation of the stoichiometric laws of chemical combination and the diligent search for accurate atomic weights. The important link between atoms and electrical charges in the early years of the nineteenth century enabled a new interpretation of chemical combination and the theory of valency. Significant improvements in... 

To interpret the classical laws of chemical combination using Dalton s atomic theory... 

In 1803, John Dalton (1766-1844) (Figure 3.3) proposed his atomic theory, including the following postulates, to explain the laws of chemical combination discussed in Section 3.1 ... 

The first three postulates have had to be amended, and the fifth was qnickly abandoned altogether. But the postulates explained the laws of chemical combination known at the time, and they cansed great activity among chemists, which led to more generalizations and further advances in chemistry. 

The postulates of Dalton s atomic theory explained the laws of chemical combination very readily. 

Dalton suggested that the elements are composed of indivisible atoms and that the atoms of each element have a characteristic mass, different from the mass of any other element. He stated that the atoms combine to form molecules when the elements combine to form compounds. These postulates explained the laws of chemical combination known at that time, but most of them have been... 

Indirect Evidence for the Existence of Atoms Laws of Chemical Combination... 

More than 2000 years passed before a group of European chemists demonstrated experimentally that elements combine only in masses with definite ratios when forming compounds, and that compounds react with each other only in masses with definite ratios. These results could be interpreted only by inferring that smallest indivisible units of the elements (atoms) combined to form smallest indivisible units of the compounds (molecules). The definite mass ratios involved in reactions were interpreted as a convenient means for counting the number of atoms of each element participating in the reaction. These results, summarized as the laws of chemical combination, provided overwhelming, if indirect, evidence for the existence of atoms and molecnles. 

The laws of chemical combination provided indirect evidence for the existence of atoms. The experiments of Thomson, Wien, and Rutherford provided direct physical evidence for the existence of the elementary particles that make up the atom. We conclude this chapter by describing an experimental method that allows us not only to image individual atoms and molecules but also to observe and control a chemical reaction at the single molecule level—a feat only dreamed of as recently as the mid-1980s. 

The laws of chemical combination assert that chemical reactions occur in such a way that the number of atoms of a given type are conserved in every chemical reaction, except nuclear reactions. How do we weigh out a sample containing exactly the number of atoms or molecules needed for a particular chemical reaction What is the mass of an atom or a molecule These questions must be answered indirectly, because atoms and molecules are far too small to be weighed individually. 

Which law of chemical combination is illustrated by these data ... 

It is not surprising that the application of such a powerful method of investigation should have led, on the experimental side, to a vast extension of our knowledge of the properties of alloy systems. Even more important, however, is the fact that it has also laid the foundations of the modem theory of the metallic state, for, as we have seen in chapter 5, the basic concept on which this theory is based is that of the periodic field in a crystal structure. The development of metallurgy in the past has been hampered by attempts to make metal systems conform to the laws of chemical combination established by observations on bodies in which forces of an entirely different character are operative. Alloys differ profoundly in many of their properties from... 

One must not, however, be led astray by this a eement between picture and reality, and confound the two. So far as we have treated them, the chemical processes occurred in such a way as f the substances were composed of atoms in the sense explained. At best there follows from this the possibility that they are in reality so not, however, the certainly. For it is impossible to prove that the laws of chemical combination cannot be deduced with the same completeness by means of quite a different assumption, (emphasis added)... 

In chemical reactions, however spectacular, mass is found to be conserved within the limits of the sensitivity of chemical balances, and this is as it should be if the transformations are mere regroupings of the units of a material substratum. The atoms of ancient speculation fill the roles reqxiired. The quantitative laws of chemical combination then follow and permit the development of chemistry in the form in which it is known today. 

The quantitative laws of chemical combination provide clear pointers to the molecular theory of matter, which increases progressively in vividness and realism with the application of Newton s laws to the motions of the particles. The interpretation of phenomena such as the pressure and viscosity of gases and the Brownian motion, and the assignment of definite magnitudes to molecular speeds, masses, and diameters render it clear that a continual interchange of energies must occur between the molecules of a material system, a circumstance which lies at the basis of temperature equilibrium and determines what in ordinary experience is called the flow of heat. It is responsible indeed for far more than this, and a large part of physical chemistry follows from the conception of the chaotic motion of the molecules. This matter must now be examined more deeply. 

Dalton s theory explains several laws of chemical combination that were known during his time, including the law of constant composition — (Section 1.2), based on postulate 4 ... 

Information from the WebElements site laws of chemical combination See... 

Lavoisier s emphasis on the quantification of chemistry also paved the way for the laws of chemical combination, which soon prompted John Dalton to develop his atomic theory. 

Meanwhile, others had made observations concerning the combination of masses of any two elements in more than one compound. It had been realized that if A reacts with B to form more than one compound, the various amounts of B that react with a fixed amount of A bear a simple whole number ratio to each other. Dalton carried out further experiments on this relationship, and as a result of his work, this, too, became regarded as a law of chemical combination (the law of multiple proportions), and one that his atomic hypothesis could readily explain. On the atomic hypothesis, the law of multiple proportions results from the fact that, for example, one oxygen atom can combine with one atom of carbon to form a compound, and in addition, two atoms of oxygen can combine with one atom of carbon to form a different compound. The ratio of amounts of oxygen combining with a fixed amount of carbon is therefore the simple ratio of 2 to 1. These two compounds are carbon monoxide and carbon dioxide, respectively. 

Dalton also suggested symbols for the elements. The theory was used to explain the law of CONSERVATION OF MASS and the laws of CHEMICAL COMBINATION. 

These statements, which contain the whole of Dalton s chemical atomic theory, were arrived at completely in September to October 1803, and remained unchanged in all Dalton s later publications. The symbolic representation of the compositions of compounds used by Dalton (his symbol law ) contained implicitly the laws of definite, multiple, and reciprocal proportions, and Dalton does not give verbal statements of these laws. It was asserted that the law of constant proportions would be a better name than the law of definite proportions and that a further law of compound proportions (vi) is necessary, stating that the combining weight of a compound is the sum of the combining weights of its components, which does not follow from the other laws of chemical combination. 

---