Lecture05222

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Information about Lecture05222

Published on July 23, 2008

Author: uladzimir

Source: slideshare.net

Description

a supplemental resource for students

The Shapes of Molecules: Lewis Structures Lecture 5

Six valence electrons

Eight valence electrons

A sample problem on writing Lewis structures for molecules with one central atom.

How to deploy a Lewis formula from a molecular formula: Place the atoms relative to each other. Determine the total number of valence electrons available. Draw a single bond from each surrounding atom to the central atom. Subtract 2e - for each bond. Distribute the remaining electrons in pairs so that each atom ends up with 8e - (2e - for H) .

Place the atoms relative to each other.

Determine the total number of valence electrons available.

Draw a single bond from each surrounding atom to the central atom. Subtract 2e - for each bond.

Distribute the remaining electrons in pairs so that each atom ends up with 8e - (2e - for H) .

Remember: Hydrogen atoms form one bond. Carbon atoms form four bonds. Nitrogen atoms form three bonds. Oxygen atoms form two bonds. Halogen atoms form one bond when they are surrounding atoms; fluorine is always a surrounding atom.

Hydrogen atoms form one bond.

Carbon atoms form four bonds.

Nitrogen atoms form three bonds.

Oxygen atoms form two bonds.

Halogen atoms form one bond when they are surrounding atoms; fluorine is always a surrounding atom.

A sample problem on writing Lewis structures for molecules with more than one central atom.

A sample problem on writing Lewis structures for molecules with multiple bonds.

Sometimes more than one Lewis formula can be written for the same molecule. These are resonance structures (or resonance forms).

Resonance structures and delocalization in ozone O=O–O

Resonance structures and delocalization in ozone O–O=O

Resonance structures are not real bonding depictions. The actual molecule is a resonance hybrid.

Resonance structures

The more resonance structures can be written, the higher is electron density delocalization in the molecule.

The higher electron density delocalization in the molecule, the more stable is this molecule.

A sample problem on writing resonance structures.

 

Which resonance structure is the most important? Smaller formal charges (positive or negative) are preferable to larger ones. Like formal charges on adjacent atoms are not desirable. A more negative formal charge should reside on a more electronegative atom.

Smaller formal charges (positive or negative) are preferable to larger ones.

Like formal charges on adjacent atoms are not desirable.

A more negative formal charge should reside on a more electronegative atom.

A sample problem on writing Lewis structures with octet rule exceptions.

THE END

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