Molecules

Compounds are formed through the combination of two or more elements held together by chemical bonds. The number and identity of each atom present in the compound are given by the chemical formula. Symbols from the periodic table are used to identify atoms, and the relative number of each atom present is indicated using a subscript attached to the symbol. Subscripts are used only when two or more atoms of the same element appear in the formula. The symbol without a subscript is used to represent the presence of a single atom in the formula. For example, H2O is the chemical formula for water, a compound containing one atom of oxygen bound to two atoms of hydrogen. Compounds are electrically neutral and divided into two broad classes based on the type of chemical bond present: ionic bonds form ionic compounds and covalent bonds form covalent compounds. It is important to note that pure ionic and pure covalent represent the extremes of chemical bonding and rarely exist. The vast majority of chemical bonds contain both ionic and covalent character and classification is based on the type present in the highest percentage. For example, a bond that contains a higher percentage of ionic character is termed ionic; however, this does not mean that the bond contains no covalent character. Characterizing chemical bonds as ionic or covalent is a common, reliable practice that is universally accepted. This convenient language will be used to study bonding and structural properties in this chapter.