Skeletal atom
The skeletal formula, also called line-angle formula or shorthand formula, of an organic compound is a type of molecular structural formula that serves as a shorthand representation of a molecule's bonding and some details of its molecular geometry. A skeletal formula shows the skeletal structure or skeleton of a molecule, which is composed of the skeletal atoms that make up the molecule. It is represented in two dimensions, as on a page of paper. It employs certain conventions to represent carbon and hydrogen atoms, which are the most common in organic chemistry.
An early form of this representation was first developed by the organic chemist Friedrich August Kekulé von Stradonitz. For this reason, they are sometimes termed Kekulé structures. Skeletal formulae have become ubiquitous in organic chemistry, partly because they are relatively quick and simple to draw, and also because the curved arrow notation used for discussions of reaction mechanism and/or delocalization can be readily superimposed. As in Lewis structures, covalent bonds are indicated by line segments, with a doubled or tripled line segment indicating double or triple bonding, respectively. Likewise, skeletal formulae generally indicate formal charges associated with each atom (although lone pairs are usually optional, see below). In fact, skeletal formulae can be thought of as abbreviated Lewis structures that observe the following conventions: (1) Carbon atoms are generally represented by the vertices (intersections and termini) of line segments. As exceptions, methyl groups are often explicitly written out as Me or CH3, while cumulene carbons are usually represented by a heavy center dot. (2) Hydrogen atoms attached to carbon are implied: an unlabeled vertex is understood to be a carbon atom attached to the number of hydrogens required to satisfy the octet rule, while a vertex labeled with a formal charge and/or nonbonding electrons is understood to have the number of hydrogen atoms required for consistency with these properties. Frequently, acetylenic and formyl hydrogens are also explicitly shown for the sake of clarity. (3) Hydrogen atoms attached to a heteroatom must be explicitly shown; the heteroatom and hydrogen atoms attached thereto are usually shown as a single group (e.g., OH, NH2) without explicitly showing the hydrogen–heteroatom bond; (4) Nonbonding electron pairs on heteroatoms are optionally shown, while ones on carbon (e.g., carbenes) must be indicated explicitly.
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