Classification of compounds

Chemical compounds may be classified according to several different criteria. One common method is based on the specific elements present. For example, oxides contain one or more oxygen atoms, hydrides contain one or more hydrogen atoms, and halides contain one or more halogen (Group 17) atoms. Organic compounds are characterized as those compounds with a backbone of carbon atoms, and all the remaining compounds are classified as inorganic. As the name suggests, organometallic compounds are organic compounds bonded to metal atoms.

Another classification scheme for chemical compounds is based on the types of bonds that the compound contains. Ionic compounds contain ions and are held together by the attractive forces among the oppositely charged ions. Common salt (sodium chloride) is one of the best-known ionic compounds. Molecular compounds contain discrete molecules, which are held together by sharing electrons (covalent bonding). Examples are water, which contains H₂O molecules; methane, which contains CH₄ molecules; and hydrogen fluoride, which contains HF molecules.

A third classification scheme is based on reactivity—specifically, the types of chemical reactions that the compounds are likely to undergo. For example, acids are compounds that produce H⁺ ions (protons) when dissolved in water to produce aqueous solutions. Thus, acids are defined as proton donors. The most common acids are aqueous solutions of HCl (hydrochloric acid), H₂SO₄ (sulfuric acid), HNO₃ (nitric acid), and H₃PO₄ (phosphoric acid). Bases, on the other hand, are proton acceptors. The most common base is the hydroxide ion (OH⁻), which reacts with an H⁺ ion to form a water molecule. H⁺ + OH⁻ → HOH (usually written H₂O)

Oxidation-reduction reactions constitute another important class of chemical reactions. Oxidation involves a loss of electrons, whereas reduction involves a gain of electrons. For example, in the reaction between sodium metal and chlorine gas to form sodium chloride, 2Na + Cl₂ → 2NaCl, electrons (e⁻) are transferred from sodium atoms to chlorine atoms to form Na⁺ and Cl⁻ ions in the reaction product, sodium chloride.

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In this process, each sodium atom loses an electron and is thus oxidized, and each chlorine atom gains an electron and is thus reduced. In this reaction, sodium is called the reducing agent (it furnishes electrons), and chlorine is called the oxidizing agent (it consumes electrons). The most common reducing agents are metals, for they tend to lose electrons in their reactions with nonmetals. The most common oxidizing agents are halogens—such as fluorine (F₂), chlorine (Cl₂), and bromine (Br₂)—and certain oxy anions, such as the permanganate ion (MnO₄⁻) and the dichromate ion (Cr₂O₇²⁻).

Inorganic compounds

Inorganic compounds include compounds that are made up of two or more elements other than carbon, as well as certain carbon-containing compounds that lack carbon-carbon bonds, such as cyanides and carbonates. Inorganic compounds are most often classified in terms of the elements or groups of elements that they contain. Oxides, for example, can be either ionic or molecular. Ionic oxides contain O²⁻ (oxide) ions and metal cations, whereas molecular oxides contain molecules in which oxygen (O) is covalently bonded to other nonmetals such as sulfur (S) or nitrogen (N). When ionic oxides are dissolved in water, the O²⁻ ions react with water molecules to form hydroxide ions (OH⁻), and a basic solution results. Molecular oxides react with water to produce oxyacids, such as sulfuric acid (H₂SO₄) and nitric acid (HNO₃). In addition, inorganic compounds include hydrides (containing hydrogen atoms or H⁻ ions), nitrides (containing N³⁻ ions), phosphides (containing P³⁻ ions), and sulfides (containing S²⁻ ions).

Transition metals form a great variety of inorganic compounds. The most important of these are coordination compounds in which the metal atom or ion is surrounded by two to six ligands. Ligands are ions or neutral molecules with electron pairs that they can donate to the metal atom to form a coordinate-covalent bond.

The resulting covalent bond is given a special name because one entity (the ligand) furnishes both of the electrons that are subsequently shared in the bond. An example of a coordination compound is [Co(NH₃)₆]Cl₃, which contains the Co(NH₃)₆³⁺ ion, a cobalt ion (Co³⁺) with six ammonia molecules (NH₃) attached to it, acting as ligands.

In the early days of the science of chemistry, there was no systematic approach to naming compounds. Chemists coined names such as sugar of lead, quicklime, milk of magnesia, Epsom salts (see magnesium), and laughing gas to describe familiar compounds. Such names are called common or trivial names. As chemistry advanced, it became evident that, if common names were used for all known compounds, which number in the millions, great confusion would result. It clearly would be impossible to memorize trivial names for such a large number of compounds. Therefore a systematic nomenclature (naming process) has been developed. There are, however, certain familiar compounds that are always referred to by their common names. The systematic names for H₂O and NH₃, for example, are never used; these vital compounds are known only as water and ammonia, respectively.

The simplest chemical compounds are binary compounds—those consisting of two elements. Different rules apply for the nomenclature of binary ionic compounds and binary molecular (covalent) compounds, and so they will be considered separately.