Skeletal formula
The skeletal formula, or 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 piece 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 organic chemist August Kekulé, while the modern form is closely related to and influenced by the Lewis structure of molecules and their valence electrons. Hence they are sometimes termed Kekulé structures or Lewis–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 mechanisms and electron delocalization can be readily superimposed. Several other ways of depicting chemical structures are also commonly used in organic chemistry (though less frequently than skeletal formulae). For example , conformational structures look similar to skeletal formulae and are used to depict the approximate positions of atoms in 3D space, as a perspective drawing. Other types of representation, such as Newman projection, Haworth projection or Fischer projection, also look somewhat similar to skeletal formulae. However, there are slight differences in the conventions used, and the reader needs to be aware of them in order to understand the structural details encoded in the depiction. While skeletal and conformational structures are also used in organometallic and inorganic chemistry, the conventions employed also differ somewhat. The skeleton. Terminology. The skeletal structure of an  organic compound is the series of atoms bonded together that form the essential structure of the compound. The skeleton can consist of chains, branches and/or rings of bonded atoms. Skeletal atoms other than carbon or hydrogen are called heteroatoms. The skeleton has hydrogen and/or various substituents bonded to its atoms. Hydrogen is the most common non-carbon atom that is bonded to carbon and, for simplicity, is not explicitly drawn. In addition, carbon atoms are not generally labelled as such directly (i.e. with "C"), whereas heteroatoms are always explicitly noted as such ("N" for nitrogen, "O" for oxygen, etc.) Heteroatoms  and other groups of atoms that give rise to relatively high rates of chemical reactivity, or introduce specific and interesting characteristics in the spectra of compounds are called functional groups, as they give the molecule a function. Heteroatoms and functional groups are collectively called "substituents", as they are considered to be a substitute for the hydrogen atom that would be present in the parent hydrocarbon of the organic compound. Basic structure. 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 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 simplifications: In the standard depiction of a molecule, the canonical form (resonance structure) with the greatest contribution is drawn. However, the skeletal formula is understood to represent the "real molecule" that is, the weighted average of all contributing canonical forms. Thus, in cases where two or more canonical forms contribute with equal weight (e.g., in benzene, or a carboxylate anion) and one of the canonical forms is selected arbitrarily, the skeletal  formula is understood to depict the true structure, containing equivalent bonds of fractional order, even though the delocalized bonds are depicted as nonequivalent single and double bonds. Contemporary graphical conventions. Since skeletal structures were introduced in the latter half of the 19th century, their appearance has undergone considerable evolution. The graphical conventions in use today date to the 1980s. Thanks to the adoption of the ChemDraw software package as a "de facto" industry standard (by American Chemical Society, Royal Society of Chemistry, and Gesellschaft Deutscher Chemiker publications, for instance), these conventions have been nearly universal in the chemical literature since  the late 1990s. A few minor conventional variations, especially with respect to the use of stereobonds, continue to exist as a result of differing US, UK and European practice, or as a matter of personal preference. As another minor variation between authors, formal charges can be shown with the plus or minus sign in a circle (⊕, ⊖) or without the circle. The set of conventions that are followed by most authors is given below, along with illustrative examples. Implicit carbon and hydrogen atoms. For example, in the image below, the skeletal formula of hexane is shown. The carbon atom  labeled C appears to have only one bond, so there must also be three hydrogens bonded to it, in order to make its total number of bonds four. The carbon atom labelled C has two bonds to other carbons and is therefore bonded to two hydrogen atoms as well. A ball-and-stick model of the actual molecular structure of hexane, as determined by X-ray crystallography, is shown for comparison, in which carbon atoms are depicted as black balls and hydrogen atoms as white ones. It does not matter which end of the chain one starts numbering from, as  long as consistency is maintained when drawing diagrams. The condensed formula or the IUPAC name will confirm the orientation. Some molecules will become familiar regardless of the orientation. Explicit heteroatoms and hydrogen atoms. All atoms that are not carbon or hydrogen are signified by their chemical symbol, for instance Cl for chlorine, O for oxygen, Na for sodium, and so forth. In the context of organic chemistry, these atoms are commonly known as heteroatoms (the prefix "hetero-" comes from Greek "ἕτερος" héteros, meaning "other"). Any hydrogen atoms bonded to heteroatoms "are" drawn explicitly. In ethanol, CHOH, for instance, the hydrogen  atom bonded to oxygen is denoted by the symbol H, whereas the hydrogen atoms which are bonded to carbon atoms are not shown directly. Lines representing heteroatom-hydrogen bonds are usually omitted for clarity and compactness, so a functional group like the hydroxyl group is most often written −OH instead of −O−H. These bonds are sometimes drawn out in full in order to accentuate their presence when they participate in reaction mechanisms. Shown below for comparison are a ball-and-stick model (top) of the actual 3D structure of the ethanol molecule in the gas phase, as determined by microwave  spectroscopy, its Lewis structure (centre) and its skeletal formula (bottom). Pseudoelement symbols. There are also symbols that appear to be chemical element symbols, but represent certain very common substituents or indicate an unspecified member of a group of elements. These are called pseudoelement symbols or organic elements and are treated like univalent "elements" in skeletal formulae. A list of common pseudoelement symbols: Sulfonyl/sulfonate groups. Sulfonate esters are often leaving groups in nucleophilic substitution reactions. "See the articles on sulfonyl and sulfonate groups for further information." Protecting groups. A protecting group or protective group is introduced into a molecule by  chemical modification of a functional group to obtain chemoselectivity in a subsequent chemical reaction, facilitating multistep organic synthesis. Multiple bonds. Two atoms can be bonded by sharing more than one pair of electrons. The common bonds to carbon are single, double and triple bonds. Single bonds are most common and are represented by a single, solid line between two atoms in a skeletal formula. Double bonds are denoted by two parallel lines, and triple bonds are shown by three parallel lines. In more advanced theories of bonding, non-integer values of bond order exist. In these cases, a combination of  solid and dashed lines indicate the integer and non-integer parts of the bond order, respectively. Benzene rings. In recent years, benzene is generally depicted as a hexagon with alternating single and double bonds, much like the structure Kekulé originally proposed in 1872. As mentioned above, the alternating single and double bonds of "1,3,5-cyclohexatriene" are understood to be a drawing of one of the two equivalent canonical forms of benzene (the 1,3,5- and 2,4,6-isomers), in which all carbon–carbon bonds are of equivalent length and have a bond order of exactly 1.5. For aryl rings in general, the  two analogous canonical forms are almost always the primary contributors to the structure, but they are nonequivalent, so one structure may make a slightly greater contribution than the other, and bond orders may differ somewhat from 1.5. An alternate representation that emphasizes this delocalization uses a circle, drawn inside the hexagon of single bonds, to represent the delocalized pi orbital. This style, based on one proposed by Johannes Thiele, used to be very common in introductory organic chemistry textbooks and is still frequently used in informal settings. However, because this depiction does not keep track of electron pairs and is  unable to show the precise movement of electrons, it has largely been superseded by the Kekuléan depiction in pedagogical and formal academic contexts. Stereochemistry. Stereochemistry is conveniently denoted in skeletal formulae: The relevant chemical bonds can be depicted in several ways: An early use of this notation can be traced back to Richard Kuhn who in 1932 used solid thick lines and dotted lines in a publication. The modern solid and hashed wedges were introduced in the 1940s by Giulio Natta to represent the structure of high polymers, and extensively popularised in the 1959 textbook "Organic Chemistry" by Donald J.  Cram and George S. Hammond. Skeletal formulae can depict "cis" and "trans" isomers of alkenes. Wavy single bonds are the standard way to represent unknown or unspecified stereochemistry or a mixture of isomers (as with tetrahedral stereocenters). A crossed double-bond has been used sometimes; is no longer considered an acceptable style for general use, but may still be required by computer software. Hydrogen bonds. Hydrogen bonds are generally denoted by dotted or dashed lines. In other contexts, dashed lines may also represent partially formed or broken bonds in a transition state.