Chemistry · General chemistry II · Concept
One molecule, five representations
Chemists write the same molecule in several ways. A skeletal structure shows the carbon framework, a Lewis structure shows every valence electron, a molecular formula counts the atoms, a SMILES string writes the connections as one line of text, and a systematic name spells out the parent chain and the groups on it. Each keeps some information and leaves the rest out, so no single drawing is the whole molecule.
One molecule, several representations
Ethanol is one molecule, but it can be shown as a zigzag line with an OH at the end, as a Lewis structure with every bond and lone pair, as the formula C₂H₆O, as the text CCO or as the name ethanol. They all describe the same atoms joined the same way. What changes is how much each one tells you.
Skeletal structures
In a skeletal structure every corner and line end is a carbon atom, and the hydrogens on carbon are left out: each carbon has as many as it needs to make four bonds. Other atoms, such as oxygen, nitrogen and chlorine, are written as their symbols with the hydrogens attached to them. A skeletal drawing shows connections clearly, but its angles are a convention, not the molecule’s real shape.
Lewis structures
A Lewis structure shows every atom, every bond as a line and every lone pair as a pair of dots, so you can count all the valence electrons. It still doesn’t show the molecule’s three-dimensional shape; VSEPR predicts that from the electron groups around each atom.
Molecular formulas
A molecular formula counts the atoms of each element and nothing more. Ethanol and dimethyl ether are both C₂H₆O: in ethanol the oxygen is at the end of the chain, and in dimethyl ether it sits between the two carbons. Molecules with the same formula and different connections are constitutional isomers, so a formula alone can’t name a compound.
SMILES strings
SMILES writes a structure as text: atoms as their symbols, a branch in parentheses and a ring closure as a pair of matching digits. Ethanol is CCO, and OCC is ethanol too, read from the other end, so one molecule can have several valid SMILES. A SMILES string records connections, not where the atoms were drawn.
Systematic names
A systematic name is built from the structure: a root for the parent chain, prefixes for the groups on it with numbers for where they sit, and a suffix for the main functional group. Ethanol is a two-carbon chain (eth-) with an –OH group (-ol). A name tells you the connections, but you have to draw it to see them.
Common mistakes
- Assuming two compounds with the same molecular formula are the same compound.
- Reading a line end in a skeletal structure as a hydrogen atom: it is a carbon.
- Treating one SMILES string as the only correct one for a molecule.
- Taking the angles in a drawing as the molecule’s real three-dimensional shape.
Key terms
- Skeletal structure
- An organic drawing in which unlabeled ends and corners usually mean carbon. Carbon hydrogens follow stated valence conventions; heteroatoms and relevant charges are shown.
- SMILES
- A way to write a structure as one line of text: atoms as their symbols, branches in parentheses, ring closures as matching digits, as in CCO for ethanol. It records connections, not the drawing’s positions, and one molecule can have several valid SMILES.
- Isomer
- Isomers have the same molecular formula but are different compounds. Constitutional (structural) isomers join their atoms differently, like butane and 2-methylpropane. Stereoisomers share the same connections but differ in three-dimensional arrangement, like cis- and trans-2-butene.
- Functional group
- A characteristic atom or group of atoms that strongly influences an organic compound’s reactions and properties.
Work through an example
Ethanol and dimethyl ether both have the formula C₂H₆O. Which representations tell them apart?
Compare ethanol and dimethyl ether →Sources and scope
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Open the example inputs, change a value and keep a useful result on your board.
See ethanol in every representation See dimethyl ether in every representation Draw ethanol in the Skeletal tool Open worked example on a board Organic nomenclature in Chemistry ReferenceYour existing work stays on this device. Examples open as editable copies.