Structure and Composition of Compounds

A dividing and discarding process like that just described for the element silver may also be performed on compounds, and the result would be similar—an extremely small fundamental structural unit (visible only through our mythical microscope) that, when divided, would no longer be that compound. The situation is more complex, however, because the nature of this fundamental unit is 

The vast majority of the 13 million compounds that exist are molecular. That fact certainly does not diminish the importance of those that are ionic, however, since there are some very common compounds in our daily lives that fall into the ionic category. 

Chemical formulas, which we defined in Section 1.3.2 as symbolic designations for compounds, are used for both molecular compounds and ionic compounds. Thus we can tell what elements are chemically combined to make up a compound by looking at its formula. Examples we presented in Section 1.3.2 included H2O (water), a combination of hydrogen (H) and oxygen (O), and NaCl (sodium chloride, or salt), a combination of sodium (Na) and chlorine (Cl). Water is an exanple of a molecular compound, while sodium chloride is an example of an ionic compound. Since compounds are composed of elements in chemical combination, it follows that molecules and formula units are composed of the atoms of the elements in chemical combination. Thus, molecules of water are composed of atoms of hydrogen in chemical combination with atoms of oxygen, and formula units of salt are composed of sodium atoms (actually ions) and chlorine atoms (ions) in chemical combination. It is the nature of this chemical combination that defines whether a compound is molecular or ionic. This will be detailed further as our study proceeds. 

The number of atoms of each element that make up a molecule or formula unit can be seen in the formulas. The numbers written as subscripts in the formulas, such as the 2 in the formula for water, H2O, and the 2 and the 4 in the formula for potassium chromate, K2Cr04, represent the number of atoms of the elements immediately to the left of each subscript in the formula. If there is no subscript immediately to the right of an element symbol, then there is only one atom of that element in the molecule or formula unit. Thus, in a molecule of water, H2O, there are 2 atoms of hydrogen and one atom of oxygen that are chemically combined. In a formula unit of salt, NaCl, there is one atom (ion) of each element, sodium and chlorine. In a formula unit of potassium chromate (also an ionic compound), K2Cr04, there are two atoms of potassium, one atom of chromium, and four atoms of oxygen. Thus chemical formulas are reasonable representations of the molecules and formula units of which compounds are composed. 

A few elements consist of two atoms that form a molecule of the element. Because of this, the symbol of these elements is actually a formula that represents this diatomic situation. These elements are hydrogen (Hj), fluorine (Fj), chlorine (CI2), oxygen (O2), nitrogen (N2), bromine (Brj), and iodine (I2). 

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