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Structural Formula


Compounds are characterized by its formula. The formula gives us the ratios of the compounds constituent elements and identifies each element by its atomic symbol. The atoms in a molecule are not simply piled together randomly. Rather the atoms are chemically bonded in a definite way. The structural formula shows all the atoms present in a molecule as well as the bonds that connect the atoms to each other.

Two dimensional structural representations for organic molecules are called structural formulas. A skeletal structural formula is a structural formula that shows the arrangements and bonding of carbon atoms present in an organic molecule but does not show the hydrogen atoms attached to the carbon atoms.

What is Structural Formula?

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A structural formula is more informative. It clearly shows the "arrangement of atoms in a molecule." A covalent bond is shown as a line joining the two atoms.

Structural Formula

As the name implies it is a short hand method of representing the structural formula. The convention is to omit most of the covalent bonds that connect the atoms within the molecule.

Condensed Structural Formula

Structural formulas may be condensed even further if a molecule has two or more identical groups of atoms. In these cases, brackets are used to include a repetitive group of atoms.

Structural Formula with Repetitive Group

For the sake of clarity double or triple bonds are usually shown in a condensed structural formula.

Double and Triple Condensed Structural Formula

Lewis Structures

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In Lewis theory we combine the Lewis structures of elements to form Lewis structures for compounds. The ionic bonding is represented by the bonding between a metal and a nonmetal through the transfer of electrons by moving dots from the Lewis structure of the metal to the Lewis structure of the nonmetal. The metal becomes a cation and the nonmetal becomes an anion. For example, sodium and chlorine have the following Lewis structures.

Lewis Structure

In forming the Lewis structure for NaCl, we move sodium valence electron to chlorine. Because sodium loses its valence electron, it has no dots. This corresponds to a stable configuration because the outermost occupied Bohr orbit contains an octet.

Consider an example of magnesium fluoride, MgF2. Magnesium and fluorine have the following Lewis structures.

Magnesium Fluoride

For magnesium to obtain octet, it must lose both of its valence electrons. However, fluorine needs only one electron to fill its octet; therefore two fluorine atoms are needed for every magnesium atom. Magnesium has lost all its valence electrons and appears without any dots, while the fluorine atoms both acquire octets. When magnesium fluoride is in natural state, it is composed of two fluoride ions to every magnesium ion, just follows the Lewis theory.

Structural Isomers

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Structural isomers are compounds that differ in the covalent arrangements of their atoms. Structural isomers are also called constitutional isomers. For example, the compound with molecular formula C2H6O has two structural isomers as illustrated below.

Structural Isomers

Structural isomers are further classified into
  • Ionization isomers - This results from the interchange of ions inside and outside the coordination sphere.
  • Hydrate isomers - This results from crystalline complexes in which water can be either inside or outside the coordination sphere.
  • Coordination isomers - This occurs in compounds containing both complex cations and complex anions. Such isomers involve exchange of ligands between cation and anion that is between coordination spheres.
  • Linkage isomers - This results when certain ligands can bind to metal ions in more than one way. One example of such a ligand is nitro (NO2-) and nitrito(-ONO-).
Structural isomers with identical constitutions but differ in the spatial arrangements of their atoms are called stereoisomers. Structural formula tell us how atoms are connected and so allow us to distinguish different structural isomers.

Structural Geometry

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Isomerism is a rather comprehensive term embracing several types of structural differences between molecules having the same chemical composition. The structural differences between isomers range from those between topological isomers, which represent different ways of connecting together the same set of atoms and are usually regarded as different chemical compounds.

Between a pair of geometrical isomers there is an energy barrier which may be due to $\pi$-bonding or on to the fact that inter conversion takes place through an intermediate with different geometry. For example, the rearrangement of a cis square planar complex into the trans isomer via a tetrahedral intermediate.

For example, the structural geometry of square planar complex having unsymmetrical bidentate ligands are shown below.

Structural Geometry

Structural Problems

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Example problems based on structural formula are given below.

Solved Examples

Question 1: In a compound magnesium fluoride, magnesium (atomic symbol Mg) and fluorine (atomic symbol F) combine in a ratio of 1:2, what is the formula of magnesium fluoride?
The formula gives the atomic symbol of each element combined in the compound, and subscripts give the ratio of its constituent elements.

The formula is MgF2. The subscript 1 should not be written after Mg.

Question 2: Name the following ionic compound from its formula. Mg3N2
Magnesium Group (II) A metal, is expected to form only a 2+ ion (Mg2+, the magnesium ion). Nitrogen (Group VA) is expected to form an anion of charge equal to the group number minus 8 (N3-, the nitride ion). So these ions would give the formula Mg3N2. The name of the compound is magnesium nitride (the name of the cation followed by the name of the anion).

Question 3: The structural formula for acetic acid is CH3COOH

Acetic Acid

Complete the Lewis structure by adding unshared pairs of electrons so that each atom of carbon and oxygen has a complete octet.
Each carbon atom must be surrounded by eight valence electrons to have a complete octet. each oxygen must have two bonds and two unshared pairs of electrons to have a complete octet. If four regions of electron density surround the atom, the predicted bond angles are 109.5o. If three regions surround it, the predicted bond angles are 120o

Lewis structure of Acetic Acid