Chemical Symbols

Because the elements are the building blocks of all materials in the universe, we need an easy way to identify and refer to them. For this purpose, each chemical element is identifi ed by an internationally used symbol consisting of one or two letters. Chemists write symbols together in formulas to identify compounds. For example, the letters CO represent a compound of carbon and oxygen. Be careful to distinguish the formula CO from the symbol Co, which represents the element cobalt. The capitalization of letters is very important! Formulas are sometimes written with subscripts to tell the relative proportions of the elements present. For example, H₂O represents water, which has two atoms of hydrogen for every atom of oxygen present.

Polymorphism It is obvious therefore that the different structural units that elements can form (allotropy) affects their chemical and physical properties, while even the packing of a given structural unit within a crystal lattice (polymorphism) can affect the physical properties of a substance. Nowadays we know that sulfur has two forms of simple substanse S₈. First form (α -form) is called rhombic sulfur and melts at 113^oC while the second form (β -form) known as monoclinic sulfur melts at 119^oC. They also have different densities and different thermodynamic properties. They are different polymorphs of sulfur.

Polymorphism is the phenomenon in which different kinds of crystals of a solid-state compound are obtained in which the atomic arrangements are not the same. Changes between different polymorphous phases with variations in temperature and/or pressure, or phase transition, are an interesting and important problem in solid-state chemistry of physics. Polymorphism does affect the physical properties.

Bertolides and Daltonides. We should keep in mind that in solid-state inorganic chemistry the elemental composition of a compound are not necessarily integers. There are extensive groups of compounds, called nonstoichiometric compounds, in which the ratios of elements are non-integers, and these non-stoichiometric compounds characteristically display conductivity, magnetism, catalytic nature, color, and other unique solid-state properties. Therefore, even if an inorganic compound exhibits non-integral stoichiometry, unlike an organic compound, the compound may be a thermodynamically stable, orthodox compound. This kind of compound is called a non-stoichiometric compound or Berthollide compound ( “Berthollides”, “indefinite” ), whereas a stoichiometric compound is referred to as a Daltonide compound ( “Daltonides”, “definite” ). Stoicheiometric compounds are compounds that obey the law of constant composition to very high accuracy, e.g. CH₄, NaCl, and Mg₂Sn. Non-stoicheiometric compounds has a variable composition within certain limits. Examples of non-stoicheiometric compounds are provided by the copper-zinc system. The three compounds referred to above have the following compositions at 300^oC: CuZn_0.85–0.95, CuZn_{1.4− 2.0}, and CuZn_2.5–6.2.