Lewis Structures and Octet Rule

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LEWIS SYMBOLS AND THE OCTET RULE

Valence electrons - the outermost electrons of an atom; those that occupy orbitals not occupied in the nearest nobel-gas element of lower atomic number. Used in bonding.

Lewis Symbol - the element's chemical symbol plus a dot for each valence electron. Each side can accommodate 2 electrons (dots), spreading out dots as much as possible.

Drawing Lewis Structures

1. Sum the valence electrons from all atoms.

2. Write the symbols for the atoms, show which atoms are attached to which, and connect them with a single bond.

3. Complete the octets around all the atoms bonded to the central atom.

4. Place any leftover electrons on the central atom, even if it results in more than an octet of electrons around the atoms.

5. If there are not enough electrons to give the central atom an octet, try multiple bonds.

Resonance Structures

*Individual Lewis structures in cases where 2 or more Lewis structures are equally good descriptions of a single molecule. The resonance structures in such an instance are "averaged" to give a more accurate description of the real molecule.

The Octet Rule

*Atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons.

Exceptions:

1. Odd number of electrons (rarest; ClO2, NO, NO2, O2-) - completing pairing of electrons occur impossible

2. Less than an octet (relatively rare; Boron, Beryllium, Hydrogen, Helium) - 1 atom/polyatomic ion does not have a complete outer shell

3. More than an octet (largest/most common; hypervalent - molecules/ions with more than an octet of electrons around the central atom (SF4, AsF6-, ICl4-))