Basic chemistry

What are the molecular representation methods?

What are the rules for writing developed formulas?

Molecular representation methods

  • The gross formula
  • The developed formula
  • Lewis’s formula
  • The semi-developed formula

The objective of the course will be to see in detail what the different methods exist to represent the molecules in the plan, and how to use each method.

The gross formula

The raw formula is the simplest representation of a molecule, known since college. It is called “raw” because it provides only the minimum information on the molecule represented:

  • The chemical nature of the atoms that make up the molecule. Their atomic symbol is then used (for example: O for Oxygen, C for Carbon, H for Hydrogen etc.)
  • The number of each atom, which is specified as an index of each atomic symbol.

Note: when there is no index number of an atom, it implies that only one atom is present.

Below are some examples of rough formulas:

  • Water: H 2 O, two Hydrogen atoms and one Oxygen atom
  • Ethane: C 2 H 6 , two carbon atoms and six hydrogen atoms
  • Acetone: C 3 H 6 O, ie three carbon atoms, six hydrogen atoms and one oxygen atom

The developed formula

From the raw formula of the molecule, it is possible to go further in the development and write the structural formula of this molecule.

This will then give indications on the distribution of atoms in the molecule as well as on the covalent bonds that exist between the different atoms.

Symbolize the links

Single bonds are covalent bonds symbolized by a dash between two atoms.

It is also possible to write multiple bonds between atoms. In particular, the  double bonds  (or double bonds ) which represent the establishment of two bonds between two atoms, are symbolized by two parallel dashes.

Finally, there are also triple bonds (or triple bonds ), which are symbolized by three parallel dashes, corresponding to the establishment of three bonds between two atoms.

Links established by the main atoms

In order to easily write the developed formulas, it is important to know the number of covalent bonds that can be established by the main chemical elements of the periodic table.

Indeed, each atom will only be able to form a limited number of covalent bonds, this number being defined thanks to the rules of the duet and the byte.

The table below summarizes the number of covalent bonds that can be formed by the main chemical elements, as well as the form that these connections can take (single, double or triple):

Rules for writing developed formulas

To write the structural formula of a molecule, it will be necessary to apply the following rules:

  • Represent each atom by its chemical symbol, as many times as it appears in the molecule
  • Represent covalent bonds with dashes
  • Associate with each atom the right number of covalent bonds

Some examples of formulas developed

Example of water: H 2 O

What is this molecule made of? Representation of the molecule of water: in the center the Oxygen and on each side the atoms of Hydrogen

Water contains two Hydrogen atoms that can each form a bond, and one Oxygen atom that can form two bonds.

The most obvious configuration is to put Oxygen in the center and a Hydrogen atom on each side:

Structural formula of a water molecule

Example of ethane: C 2 H 6


What does this factory produce? An ethane manufacturing plant

Ethane contains two carbon atoms, which can each form four bonds, and six hydrogen atoms, which can each form a bond.

The goal is to connect all these atoms together.

Given this information, it is best to start by placing the carbon atoms, which will create the most bonds.

The carbon atoms will therefore be bonded together, and the hydrogen atoms will come around, thus respecting the good number of covalent bonds for the carbon and hydrogen atoms:

Lewis's representation of ethaneNote: Molecules consisting mainly of carbon and hydrogen atoms are organic molecules.

Their carbon atoms will always be placed first in the center of the molecule, bound together and the other atoms of the molecule will then be added around respecting the right number of bonds.

Example of Acetone: C 3 H 6 O


What is this solvent commonly used in the laboratory?Acetone is commonly used in the laboratory

Acetone contains three carbon atoms, each of which can form four bonds, six hydrogen atoms, which can each form a bond and an oxygen atom, which can form two bonds.

The three carbon atoms will bond together in the center of the molecule.

Developed formula of an acetone molecule

Lewis’s formula

The Lewis formula is the writing of a molecule on which we will reveal all the electron pairs: the doublets that form the single and multiple covalent bonds (= doubling bonds) but also the doublets that do not participate with covalent bonds (= non-binding doublets).

With respect to the structural formula, Lewis’s formula involves the writing of non-binding doublets.

Thus, to write Lewis’s formula, one must follow the same rules as for the writing of the structural formula and add the non-binding doublets. Let’s take the previous examples of water, ethane and acetone:

Water Ethane Acetone
Lewis's depiction of water Lewis's representation of ethane Lewis representation of acetone

Conversely, starting from the formula of Lewis, it will be possible to return to the developed formula by removing all the non-binding doublets of the representation.

Lewis’s formula is the richest representation, providing the most information about the molecule represented.

Note: If the atoms of a molecule do not have non-binding doublets then the Lewis formula and the structural formula are identical.

The semi-developed formula

Finally, the last possible flat representation is the semi-developed formula.

This form of representation of a molecule is an intermediate between the raw formula and the developed formula.

A little more compact than the formula developed but a little more detailed than the formula, it consists of starting from the structural formula, and to hide all the bonds that the atoms of Hydrogen do with other atoms. As for the other links, they remain represented.

Below are examples of semi-developed formulas for water, ethane and acetone molecules.

  • On the water molecule, the bonds between Hydrogen and Oxygen have been masked
  • On the Ethane molecule, the covalent bond between the two carbon atoms remains apparent while all the bonds between the carbon atoms and the hydrogen atoms are masked
  • Finally, on the Acetone molecule, the bonds between the carbon atoms as well as the double bond between the carbon atom and the oxygen atom are conserved, whereas the bonds between the carbon atoms and the
  • carbon atoms are conserved. Hydrogen were masked according to the writing rule of the semi-developed formula.
Water Ethane Acetone
semi developed water formula semi developed formula of ethane semi developed formula of acetone

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