ELEMENTS CAN COMBINE TO FORM COMPOUNDS

See Chapter g foT a calculation of the number of caTbon atoms in a 1-carat diamond. 

The oxygen we breathe, 02, is converted to ozone, 03, in the presence of an electric spaTk. Is this a physical or chemical change  

UU compound. Sodium atoms and chlorine atoms, for example, bond to make the compound sodium chloride, commonly known as table salt. Nitrogen atoms and hydrogen atoms join to make the compound ammonia, a common household cleaner. 

The compounds sodium chloride and ammonia are represented by their chemical formulas, NaCl and NH3. A chemical formula shows the ratio of atoms used to make the compound. 

I Sodium metal and chlorine gas react together to form sodium chloride. Although the compound sodium chloride is composed of sodium and chlorine, the physical and chemical properties of sodium chloride are very different from the physical and chemical properties of either sodium metal or chlorine gas. 

Was this your answer When atoms regroup, the result is an entirely new substance, and that is what happens here. The oxygen we breathe. O,. is odorless and life-giving. Ozone. O, can be toxic and has a pungent smell commonly associated with electric motors.The conversion of to O, is 

I I I hen arom.s of dijferent elements bond to one another, they make a 

A compound is represented by its chemical formula, in which the symbols for the elements are written together. Hie chemical formula for. sodium chloride is NaCl and that for ammonia is NII3. Numerical subscripts indicate the ratio in which the atoms combine. By convention, the subscript 1 is understood and omitted. So the chemical formula NaCi tells us that in the compound sodium chloride there is one. sodium for every one chlorine, and the chemical formula N113 tells us that in the compound ammonia there is one nitrogen atom for every three hydrogen atoms, as figure 2.12 shows. 

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Elements can combine to form compounds with different properties. Compounds, especially those based on carbon, are the basis for most of the biochanical complexity of living things. See Sections 3.2 and 3.6. 

Elements can combine to form compounds with different properties. 

The atoms of various elements can combine to form new substances. For example, atoms of the elements hydrogen and oxygen combine to form water. This chemical combination of atoms of two elements to form a new product is an example of a chemical reaction. Note that the properties of water, a colorless, tasteless liquid, are very different from those of its constituent elements, which are both gases. A substance, such as water, made from the chemical combination of atoms of two or more elements is called a compound. Whde an element cannot be broken down into a simpler form by normal chemical means, a compound can be broken down into atoms of... 

Atoms of a single element may combine into one molecule, and atoms of different elements may combine to form compounds, which are also molecules. The latter usually happens when elements having incomplete electron shells interact. Atoms of different elements can attain full and stable electron shells by transferring or sharing electrons with each other. When this happens, these atoms are then held closely together by chemical bonds. Elements whose atoms have full electron shells, like helium and neon, tend to be the most stable and least likely to form compounds with other elements. 

If atoms of two elements can combine to form more than one componnd, the most stable compound has the atoms in a 1 1 ratio. (This postulate was quickly shown to be incorrect.)... 

Combination reactions involve the reaction of two (or more) snbstances to form one compound. Perhaps the easiest combination reaction to recognize is one in which two free elements (at least one of which is a nonmetal) react with each other. The elements can do httle except react with each other (or not react at all). For example, if we treat aluminum metal with chlorine gas, the elements can combine to form aluminum chloride ... 

Describe two processes by which elements can combine to form stable compounds. Name the type of bonding that results from each process. 

The Law of Multiple Proportions If two elements can combine to form two or more different compounds, the mass of one element compared to a fixed mass of the other will always be in a ratio of small, whole numbers. This really requires an explanation to make sense. 

Atoms of constituent elements in a particular compound are always combined in the same proportions by mass (law of definite proportions). When two elements can combine to form more than one type of compound, the masses of one element that combine with a fixed mass of the other element are in a ratio of small whole numbers (law of multiple proportions). 

Dalton s third hypothesis supports another important law, the law of multiple proportions. According to the law, if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers. Dalton s theory explains the law of multiple proportions quite simply Different compounds made up of the same elements differ in the number of atoms of each kind that combine. For example, carbon forms two stable compounds with oxygen, namely, carbon monoxide and carbon dioxide. Modem measurement techniques indicate that one atom of carbon combines with one atom of oxygen in carbon monoxide and with two atoms of oxygen in carbon dioxide. Thus, the ratio of oxygen in carbon monoxide to oxygen in carbon dioxide is 1 2. This result is consistent with the law of multiple proportions (Figure 2.2). 

In 1808, an English schoolteacher named John Dalton proposed an explanation that encompassed all these laws. He reasoned that elements were composed of atoms and that only whole numbers of atoms can combine to form compounds. His theory can be summed up by the following statements. 

Dalton explained the law of definite proportions by saying that only whole numbers of atoms can combine to form compounds. In water there are always two hydrogen atoms for every oxygen atom. In carbon dioxide there are always two oxygen atoms for every carbon atom. Since each kind of atom has its own characteristic mass, the ratios of the masses of the elements in any given compound will always be the same. 

Almost all chemical properties can be explained in terms of the properties of atoms, so this material is central to developing an understanding of chemistry. The topics we cover here account for the structure of the periodic table, the great organizing principle of chemistry, and provide a basis for understanding how elements combine to form compounds. The material is also important because it introduces the theory of matter known as quantum mechanics, which is essential for understanding how electrons behave. 

These propagation steps are repeated many times while the organic mercury compound is con-sinned and alcohol and elemental mercury are released. This process is interrupted only by termination steps (Figure 1.14). Thus, for example, two mercury-free radicals can combine to form one dimer, or a mercury-free and a mercury-containing radical can combine to form a dialkylmercury compound. 

Two compounds can combine to form a ternary compound, a compound composed of three elements. One example is the reaction of water and a Group 1 or Group 2 metal oxide to form a metal hydroxide. An example is the formation of slaked lime, or calcium hydroxide. 

The Dalton theory of the atom and related ideas were the basis for our study of composition stoichiometry (Chapter 2) and reaction stoichiometry (Chapter 3), but that level of atomic theory leaves many questions unanswered. Why do atoms combine to form compounds Why do they combine only in simple numerical ratios Why are particular numerical ratios of atoms observed in compounds WToy do different elements have different properties WToy are they gases, liquids, solids, metals, nonmetals, and so on Why do some groups of elements have similar properties and form compounds with similar formulas The answers to these and many other fascinating questions in chemistry are supplied by our modern understanding of the nature of atoms. But how can we study something as small as an atom ... 

Based on your knowledge of the most common bonding patterns for the nonmetallic elements, predict the formulas with the lowest subscripts for the compounds that would form from the following pairs of elements. (For example, hydrogen and oxygen can combine to form H2O and H2O2, but H2O has lower subscripts.)... 

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