Wikipedia:Molecular structure diagram
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Chemical structures are presented to help readers understand the nature of the titled material. One can subdivide chemical compounds into two main groups: molecules, which includes most organic, polyatomic gases, and organometallic compounds, and nonmolecular species, which includes most purely inorganic compounds. The structures of many reagents are often misunderstood because simplified formulas are presented in reaction schemes whereas the actual structures are more complex. Examples are methyl lithium and lithium diisopropylamide. Readers of Wikipedia often comment (complain) that structures shown are incorrect for this reason.
Depictions of molecular compounds is well accomplished using ChemDraw and related software. Cations and anions are also typically discrete and can be depicted unambiguously. For simple structures, say <10 atoms, it is helpful to depict all atoms explicitly. For more complex molecules, most hydrogen atoms attached to carbon are omitted, and carbon atoms are represented by vertices. For ease of readability, sans-serif fonts are preferred. Many artists employ color to highlight parts of the molecules.
Stereochemistry demands special attention because three-dimensionality is the most difficult part of a structure to visualize. Techniques for presenting 3-dimensional structures reflect the tastes of the artist. Three dimensionality is best highlighted by the depictions of bonds, using wedges, bolding, and hashed formats. Some artists highlight three-dimensionality by varying fonts sizes, e.g. slightly larger fonts for the "front" atoms. In organic chemistry, double bonds and C-H bonds are shorter than most single bonds.
Most molecules have lone pairs of electrons, which are sometimes stereochemically active. It is a matter of taste whether one includes the lone pair in a drawing. Lone pairs of electrons are more common for depictions that emphasize bonding, as in simple gaseous molecules, such as ammonia and nitric oxide.
Nonmolecular compounds, e.g. sodium hydride, are best represented with colour-coded spheres that emphasise packing. Many solids, e.g. graphite, adopt low-dimensional structures, in which case the layers (2-D) or chains (1-D) should be shown. Some inorganic solids dissociate - or crack - into molecular species heating or upon dissolving, e.g. Aluminium chloride. In such cases it is helpful to depict both the molecular and the nonmolecular forms.
Some important chemical species cannot be easily represented with simple pictures, e.g. hydrochloric acid and non-stoichiometric compounds.