Relative mass, 45 Dalton

Once Dalton s hypotheses had been proposed, the next logical step was to determine the relative masses of the atoms of the elements. Since there was no way at that time to determine the mass of an individual atom, the relative masses were the best information available. That is, one could tell that an atom of one element had a mass twice as great as an atom of a different element (or times as much, or 17.3 times as much, etc.). How could even these relative masses be determined They could be determined by taking equal (large) numbers of atoms of two elements and by determining the ratio of masses of these collections of atoms. 

As a result of his work on relative weights, Dalton formulated the Law of Multiple Proportions, which states that when elements combine to form more than one compound, then the ratio of the masses of elements in the compounds are small whole number ratios of each other. For example, the elements carbon and oxygen form the two compounds carbon monoxide (CO) and carbon dioxide (CO ). The ratio of... 

Avogadro, A. 1811 Essay on a manner of determining the relative masses of the elementary molecules of bodies, and the proportions in which they enter into their compounds. J. de Physique 73, 58-76. (English edn Dalton, J. and others Foundations of the molecular theory. Edinburgh William Clay, Alembic Club Reprint no. 4 (1893).)... 

Dalton determined and published the known relative masses of a number of different atoms. He also formulated the law of partial pressures. Dalton s work focused on the ability of atoms to arrange themselves into molecules and to rearrange themselves via chemical reactions, but he did not investigate the composition of atoms themselves. Dalton s model of the atom was a tiny, indivisible, indestructible particle of a certain mass, size, and chemical behavior, but Dalton did not deny the possibility that atoms might have a substructure. 

The early pioneers of chemistry, trying to verify Dalton s atomic theory, could not measure the mass of individual atoms. The best they could do was to measure the masses of equal numbers of atoms (or other known ratios of atoms) of two (or more) elements at a time, to determine their relative masses. They established one element as a standard, gave it an arbitrary value of atomic mass, and used that value to establish the atomic mass scale. The last naturally occurring mixture of isotopes that was used as a standard was oxygen, defined as having an atomic mass of exactly 16 atomic mass units (amu). That standard has been replaced see the next subsection. The atomic mass unit is tiny it takes... 

A great deal of difficulty was encountered at first, because Dalton s fifth postulate gave an incorrect ratio of numbers of atoms in many cases. Such a large number of incorrect results were obtained that it soon became apparent that the fifth postulate was not correct. It was not until some 50 years later than an experimental method was devised to determine the atomic ratios in compounds, at which time the scale of relative atomic masses was determined in almost the present form. These relative masses are called atomic masses, or sometimes atomic weights. 

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 fact, an infinite number of other possibilities exists. Dalton could not deduce absolute formulas from the available data on relative masses. However, the data on the composition of compounds in terms of the relative masses of the elements supported his hypothesis that each element consisted of a certain type of atom and that compounds were formed from specific combinations of atoms. 

You have gone back in time and are working with Dalton on a table of relative masses. Following are his data. 

Lorenzo Romano Amadeo Carlo Avogadro, Essay on a Manner of Dermining the Relative Masses of the Elementary Molecules of Bodies, and the Proportions in Which They Enter Into These Compounds, Journal de Physique, de Chimie et d Histoire naturelle, vol. 73, (1811) 58. Translation from Alembic Club Reprints, No. 4, Foundations of the Molecular Theory Comprising Papers and Extracts by John Dalton, Joseph Louis Gay-Lussac, and Amadeo Avogadro, (1808-1811), available at dbhs.wvusd.kl2.ca.us/ webdocs/Chem-History/Avogadro.html. 

The masses for the elements listed in the table inside the back cover of this text are relative masses in terms of atomic mass units (amu) or daltons. The atomic mass unit is based on a relative scale in which the reference is the C carbon isotope, which is assigned a mass of exactly 12 amu. Thus, the amu is by definition 1/12 of the mass of one neutral c atom. The molar mass of is then... 

In the form of the atomic theory proposed by John Dalton in 1805, all atoms of a given element were thought to be identical. Chemists in the following decades set themselves the task of finding the relative masses of the atoms of the different elements by precise quantitative chemical analysis. Over a hundred years after Dalton s proposal was made, investigations with radioactive substances showed that not all atoms of a given element are identical. 

Finally, in the 17 century, the great English scientist Robert Boyle argued that an element is composed of simple Bodies,. .. of which all mixed Bodies are compounded. Boyle s hypothesis is remarkably similar to today s idea of an element, in which the simple Bodies are atoms. Further studies in the 18 century gave rise to laws concerning the relative masses of substances that react with each other. Then, at the beginning of the 19 century, John Dalton proposed an atomic model that explained these mass laws. By that century s close, however, further observation exposed the need to revise Dalton s model. A burst of creativity in the early 20 century gave rise to a picture of the atom with a complex internal structure, which led to our current model. 

Dalton s atomic model was crucial to the idea that masses of reacting elements could be explained in terms of atoms, and it led to experiments to learn the relative masses of atoms in compounds. However, the model did not explain why atoms bond as they do for example, why do two, and not three, hydrogen atoms bond with one oxygen atom in water Also, Dalton s model did not account for the charged particles that were being observed in experiments. Clearly, a more complex atomic model was needed. 

The mass of an atom is measured relative to the mass of an atomic standard. The modern atomic mass standard is the carbon-12 atom. Its mass is defined as exactly 12 atomic mass units. Thus, the atomic mass unit (amu) is -j the mass of a carbon-12 atom. Based on this standard, the H atom has a mass of 1.008 amu in other words, a C atom has almost 12 times the mass of an H atom. We will continue to use the term atomic mass unit in the text, although the name of the unit has been changed to the dalton (Da) thus, one C atom has a mass of 12 daltons (12 Da, or 12 amu). The atomic mass unit, which is a unit of relative mass, has an absolute mass of 1.66054 X10 " g. 

The assumed formulas are presented in line 1. The percent composition of each compovmd, calculated in the usual way, is presented in line 3, showing that these two compovmds, indeed, have different compositions, as required by the law of multiple proportions. Line 4 contains the ratio of the mass of mercury to the mass of oxygen, for each compound. Those ratios can be expressed as the ratio of simple whole numbers (2.25 4.5 = 1 2), fulfilling a condition required by the law of multiple proportions. Notice that Dalton s ideas do not depend upon the values assigned to the elements or the formulas for the compounds involved. Indeed, the question as to which compound, red or black, is associated with which formula cannot be answered from the data available. Thus, although Dalton was unable to establish an atomic mass scale, his general theory did provide an rmderstanding of the three mass-related laws conservation, constant composition, and multiple proportion. Other information was required to establish the relative masses of atoms. 

A compound is a distinct substance that is composed of the atoms of two or more elements and always contains exactly the same relative masses of those elements, in light of Dalton s atomic theory, this statement simply means that a compound always contains the same relative numbers of atoms of each element. For example, water always contains two hydrogen atoms for each oxygen atom. 

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