Lewis structure: Difference between revisions

It has been suggested that Lewis dot diagram be merged into this article. (Discuss)

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The Lewis Structure was named after G.N. Lewis. In chemistry, a Lewis structure is a 2D representation of a molecule on paper. It is used primarily to show the approximate relative positions of each atom in relation to the other atoms in the molecule, the topology of the covalent bonds linking them, and the allocation of electrons to the atoms in the molecule.

There are a number of simplified Lewis structures. In organic chemistry, rather than drawing every single atomic symbol, simple lines are used. Essentially, at every "bend" in the line, there is assumed to be a saturated carbon (CH2), and the end of every line is a saturated carbon (CH3). This often saves quite a bit of time. For a few examples: propane, which is a simple three carbon chain, would be a simple zig-zag line, with two line segments; cyclohexane, which is a hexagonal ring of carbons, would be drawn simply as a hexagon.

Lewis structures only deal with the valence electrons of the atom in question. The valence electrons are the ones in the outer orbitals that interact with the valence electrons of other atoms to form molecular bonds. Valence electrons in the s orbital are shown as dots to the right of the element's name. Electrons in each of the three p orbitals are shown above, to the left of, and below the element's name.

When Lewis structures are used in the display of the form of molecules, each molecular bond (not each shared electron, as some may think; each molecular bond contains two shared electrons) is represented by a line. The Noble Gases do not form bonds because they have a full octet.

A number of rules (based mostly on the octet rule and the duet rule) can be applied to work out the distribution of charge across the structure. This does not always produce correct results, but works in enough cases to make Lewis structures a useful tool in chemistry. Limitations of Lewis structural representations are overcome partly by incorporation of other forms of notation, for example resonance structures.

The lewis structure for H₂O,

1.) Count the valence electrons in H₂O.

   1 for each H, 6 for O 
   1 + 1 + 6 = 8

2.) Use a pair of electrons for each bond.

   H--O--H

3.) Use the remaining electrons to fullfil the octet rule and the duet rule for H.

File:Chem lewisstructure.png

4.) Count the formal charges for each atom (=#valance electrons - #bondings/2 - #lone electrons)

5.) A structure with less formal charges is always more stable. Try to make an extra bonding between 2 atoms by using a lone pair of an atom who has a negative formal charge until there are as less formal charges as possible.

Sometimes there are multiple correct lewis structures for the same molecule. This is called resonance. The molecule will exist in a mixture of all resonanceforms.

• Think Quest run by Thinkquest offers more practice and examples with lewis structures.

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