The majority of chemical substances are compounds. A compound is a pure substance consisting of two (or more) elements that are chemically bonded in a fixed proportion. Unlike a mixture, in which components retain their characteristics, a compound is a new entity. Thus, for example, atoms of oxygen and hydrogen can be mixed, randomly, in any proportion. The resulting mixture will contain hydrogen molecules and oxygen molecules. Water, however, which is a compound of oxygen and hydrogen, also contains oxygen and hydrogen, but its molecules combine both elements, in a fixed proportion, as expressed by the formula H2O.
Certain elements can combine with each other in more than one proportion. For example, carbon monoxide and carbon dioxide both contain carbon oxygen. Thus a molecule of carbon monoxide (CO) consists of a carbon atom and an oxygen atom. In carbon dioxide (CO2), a carbon atom is bonded with two oxygen atoms to form a molecule of the compound. It is important to note that these two compounds are quite different. Unlike carbon dioxide, carbon monoxide is a lethal gas.
According to the type of bond with which its atoms are held, a compound can be either covalent or ionic. A covalent compound is held together by covalent bonds (a covalent bond holds two atoms together to create diatomic molecules, such as H2 or N2. Covalent compounds include water, carbon monoxide, and carbon dioxide. An ionic compound contains cations (positively charged ions) and anions (negatively charged ions). In sodium chloride, a typical ionic compound, there is an equal number of sodium ions and chlorine ions. A sodium cation (Na+) is created when a sodium atom loses an electron; a chlorine anion (Cl! is created when a chlorine atom gains an electron. In an ionic compound, the ions are held together by the ionic bond--the attraction between the positively charged ion and its negatively charged counterpart.
Compounds can also be divided into organic and inorganic. As a rule, organic compound molecules contain two or more carbon atoms. Examples of organic compounds are methane (CH4), a small hydrocarbon compound, and large polymers, which include proteins, carbohydrates, and nucleic acids. Inorganic compounds, which include water and many minerals, usually contain elements other than carbon. Examples of inorganic compounds are water and minerals (with some exceptions). There is a third group of compounds, however, which combines an organic and an inorganic substance are combine. Because the inorganic component is usually a metal, these compounds are called organometallics. Examples of natural organometalics are chlorophyll, hemoglobin, and vitamin B12.
By the late eighteenth century, chemists knew that an individual compound's components bonded in a definite proportion by mass. While Lavoisier (1734-1794) and his contemporaries were aware of the phenomenon of definite proportionality of a compound's components, it was Joseph-Louis Proust (1754-1826) who formulated the law of definite proportions. Proust validated this law by experimentally examining a considerable number of compounds. For example, a gram of sodium chloride contains 0.3934 g of sodium and 0.6066 g of chlorine. This proportion is constant. However, chemists later discovered exceptions to this law. Compounds exhibiting slight deviations from the law of definite proportions are called nonstoichiometric compounds, and they may occur as a result of lattice defects during crystallization. A crystal is a solid substance in which atoms, molecules, or ions are arranged in a definite geometrical manner. Sodium chloride, for instance, is a crystal in which each Na + ions is surrounded by six Cl! ions, while each Cl! ions is surrounded by six Na+ ions. This particular geometric regularity, which is called a lattice, may fail in some compounds, leading to compounds which violate the law of definite proportions. For example, certain spots in a crystal lattice may remain vacant, or a lattice may have an unequal number of anion and cation sites. Thus, when iron oxide (FeO) crystallizes, iron sites often remain vacant. Because there are fewer iron atoms, each must assume a +3 charge in order to create a crystal in which the electrical forces are balanced. As a result, however, there are more oxygen atoms than iron atoms in the crystal. While iron oxide in its nonstoichometric form is still represented by the FeO formula, the proportion of its components will deviate from that of the compound in its ideal form. Other examples of nonstoichometric compounds include ceramic superconductors.
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