Composition of Compounds

Dalton s atomic model stands as a landmark in the development of chemistry. The major premises of his model are still valid, but some of his statements must be modified or qualified because later investigations have shown that (1) atoms are composed of subatomic particles (2) not all the atoms of a specific element have the same mass and (3) atoms, under special circumstances, can be decomposed. 

In chemistry we use models (theories) such as Dalton s atomic model to explain the behavior of atoms, molecules, and compounds. Models are modified to explain new information. We frequently learn the most about a system when our models (theories) fail. That is the time when we must rethink our explanation and determine whether we need to modify our model or propose a new or different model to explain the behavior. 

We often summarize our general observations regarding nature into a statement called a natural law. In the case of the composition of a compound, we use the law of definite composition, which states that a compound always contains two or more elements chemically combined in a definite proportion by mass. 

Some examples of the law of multiple proportions are given in Table 5.1. The reliability of this law and the law of definite composition is the cornerstone of the science of chemistry. In essence, these laws state that (1) the composition of a particular substance will always be the same no matter what its origin or how it is formed, and (2) the composition of different compounds formed from the same elements will always be unique. 

You need to recognize the difference between a law and a model theory). A law is a summary of observed behavior. A model (theory) is an attempt to explain the 

A chemical formula, in combination with the molar masses of its constituent elements, indicates the relative quantities of each element in a compound, which is extremely useful information. For example, about 30 years ago, scientists began to suspect that synthetic compounds known as chlorofluorocarbons (or CFCs) were destroying ozone (O3) in Earth s upper atmosphere. Upper atmospheric ozone is important because it acts as a shield, protecting life on Earth from the sun s harmful ultraviolet light. 

CECs are chemically inert compounds that were used primarily as refrigerants and industrial solvents. Over time, CFCs accumulated in the atmosphere. In the upper atmosphere, sunlight breaks bonds within CFCs, releasing chlorine atoms. The chlorine atoms then react with ozone, converting it into O2. So the harmful part of CFCs is the chlorine atoms that they carry. How can we determine the mass of chlorine in a given mass of a CFC  

One way to express how much of an element is in a given compound is to use the element s mass percent composition for that compound. The mass percent composition or mass percent of an element is that element s percentage of the compound s total mass. We can calculate the mass percent of element X in a compound from the chemical formula as follows  

Suppose, for example, that we want to calculate the mass percent composition of Cl in the chlorofluorocarbon CCI2F2. The mass percent Cl is given by  

A The chlorine in chlorofluorocarbons caused the ozone hole over Antarctica. The dark blue color indicates depressed ozone levels. 

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Analysis of the composition of compounds formed in systems containing Nb02F and alkali metal carbonates reveals several common peculiarities. The first stage of the interaction takes place according to Equation (11) and can be formulated in general as follows ... 

Let us begin by looking again at the kinds of evidence we already have for the existence of atoms—the evidence from chemistry. We shall consider, in turn, the definite composition of compounds, the simple weight relations among compounds, and the reacting volumes of gases. Each behavior provides experimental support for the atomic theory. 

The atomic theory provides a ready explanation for the definite composition of chemical compounds. It says that compounds are composed of atoms, and every sample of a given compound must contain the same relative number of atoms of each of its elements. Since the atoms of each element have a characteristic weight, the weight composition of a compound is always the same. Thus, the definite composition of compounds provides experimental support for the atomic theory. 

Dalton argued that these laws are entirely reasonable if the elements are composed of atoms. For example, the reason that mass is neither gained nor lost in a chemical reaction is that the atoms merely change partners with each other they do not appear or disappear. The constant composition of compounds stems from the fact that the compounds consist of a definite ratio of atoms, each with a definite mass. The law of multiple proportions is due to the fact that different numbers of atoms of... 

The concept of percentage is often used to describe the composition of compounds. If the formula of a compound is known, the percent by mass of an element in the compound is determined by computing the fraction of the formula weight which is made up of that element, and multiplying that fraction by 100%. Thus, an element X with atomic weight 40.0 amu in a compound XY of formula weight 99.0 amu will be present in... 

The concept of valence has been subject to revision over the years. Initially, valence was regarded as the combining power of an element and was derived from the composition of compounds. At the end of the period before the age of quantum chemistry, valence was generally formulated in relation to the octet rule [1—3), a simple relation which still finds useful application in modem chemistry. 

The differences in sizes of carpal glands between sexes and between females of different reproductive status are reflected in our results on the chemical composition of their secretions. The extracts from boars and reproductive females contained more compounds than those from non-reproductive females. Furthermore, the composition of compounds is more similar between reproductive females and boars than between the two types of females. 

The Ecovio patent portfolio covers the composition of compounds based on all aliphatic-aromatic polyesters and different compound partners (e.g., PLA) and branching agents, including different applications (EP-B 1656423, EP-B 1838784, EP-B 1204700, EP-A 2121838). 

Dalton s application of the Greek atomic theory to the facts established by the analyses of compounds enabled him to attach to each element a number which he called the atomic weight of the element, and to summarise all the facts concerning the compositions of compounds in the statement, that the elements combine in the ratios of their atomic weights, or in the ratios of whole multiples of their atomic weights. All the investigations which have been made into the compositions of compounds, since Dalton s time, have confirmed the generalisation which followed from Dalton s application of the atomic theory. 

The study of the properties of the elements shows that these substances fall into groups, the members of each of which are like one another, and form compounds which are similar. The examination of the properties and compositions of compounds has shown that similarity of properties is always accompanied by similarity of composition. Hence, the fact that certain elements are very closely allied in their properties suggests that these elements may also be allied in their composition. Now, to speak of the composition of an element is to think of the element as formed by the union of at least two different substances it implies the supposition that some elements at any rate are really compounds. 

Therefore the composition of compounds with this relation is Ba2j+i(Fe2S4)2j j. Figure 2.45 shows EDPs for the compounds with j = 8,9,10,11. 

According to Lebeau,2 arsine does not enter into the composition of compounds analogous to the metal-ammines, and the latter in ammoniacal solution react with arsine to produce arsenides of the metals. 

This means that with our present system of representing the composition of compounds, the hydroxyl groups in hyponitrous acid may be disposed on the same side of the molecule—the syn or cis (cis, on this side of) type or on opposite sides of the molecule—the anti or trans (tram, across) type. Evidence of the possible existence of organic derivatives of three types are known ... 

Carbon dioxide and carbon monoxide contain the same elements but have different proportions of these elements. In other words, they are composed differently. Chemists express the composition of compounds in various ways. One way is to describe how many moles of each element make up a mole of a compound. For example, one mole of carbon dioxide contains one mole of carbon and two moles of oxygen. Another way is to describe the percent mass of each element in a compound. 

The main problem of chemistry, the investigation of the true composition of compounds, necessarily carries along with itself the constant endeavour to elaborate and perfect the means employed for arriving at this end. 

STRUCTURAL FORMULAE Using the concept of valency the composition of compounds can be expressed with structural formulae. Each valency of an element can be regarded as an arm or hook, through which chemical bonds are formed. Each valency can be represented by a single line drawn outwards from the symbol of the element, like... 

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