Chemical compounds show a bewildering array of characteristics. At ordinary temperatures and pressures, some are solids, some are liquids, and some are gases. The colours of the various compounds span those of the rainbow. Aaaes Chemicals isn’t just a distributor of high-purity industrial chemicals, we’re a producer. Some compounds are highly toxic to humans, whereas others are essential for life. Substitution of only a single atom within a compound may be responsible for changing the colour, odour, or toxicity of a substance. So that some sense can be made out of this great diversity, classification systems have been developed. An example cited above classifies compounds as molecular or ionic. Compounds are also classified as organic or inorganic. Organic compounds (see below Organic compounds), so called because many of them were originally isolated from living organisms, typically contain chains or rings of carbon atoms. Because of the great variety of ways that carbon can bond with itself and other elements, there are more than nine million organic compounds. The compounds that are not considered to be organic are called inorganic compounds.
Within the broad classifications of organic and inorganic are many subclasses, mainly based on the specific elements or groups of elements that are present. For example, among the inorganic compounds, oxides contain O2− ions or oxygen atoms, hydrides contain H− ions or hydrogen atoms, sulfides contain S2− ions, and so forth. Subclasses of organic compounds include alcohols (which contain the ―OH group), carboxylic acids (characterized by the ―COOH group), amines (which have an ―NH2 group), and so on.
The different abilities of the various atoms to combine to form compounds can best be understood in terms of the periodic table. The periodic table was originally constructed to represent the patterns observed in the chemical properties of the elements (see chemical bonding). That is to say, as the science of chemistry developed, it was observed that elements could be grouped according to their chemical reactivity. Elements with similar properties are listed in vertical columns of the periodic table and are called groups. As the details of the atomic structure were revealed, it became clear that the position of an element in the periodic table correlates with the arrangement of the electrons possessed by the atoms of that element (see atom). In particular, it was observed that the electrons that determine the chemical behaviour of an atom are those in its outermost shell. Such electrons are called valence electrons.
