Lewis Structures.

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Presentation transcript:

Lewis Structures

How to draw Lewis structures for Neutral molecules Determine the total valence electrons for the molecule. Find the # of valence electrons for each individual atoms and add them together. Figure out how many octet electrons the molecule should have, using the octet rule. Remember most want 8 valence electrons Exception  Hydrogen and Helium want 2. Boron is a weird exception! It wants 6.

How to draw Lewis structures for Neutral molecules Determine the number of Bonding Electrons. Subtract the number you found in #1 from the number you found in #2. Determine the number of bonds in the molecule. Divide the number you found in #3 by 2 since every bond has two electrons.

How to draw Lewis structures for Neutral molecules 5. Determine the number of lone pairs (nonbonding) electrons by subtracting the bonding electrons (step #3) from the valence electrons (step #1). Arrange these around the atoms until all of the atoms have a full valence shell.

How to draw Lewis structures for Neutral molecules 6. Determine the central atom Here are some rules to remember: Carbon is always in the middle. Hydrogen is never in the middle. If carbon is not present, the least electronegative atom goes in the middle.

How to draw Lewis structures for Neutral molecules 7. Arrange the other atoms in the molecule around the central atom with single bonds. Rules to remember: Hydrogen and group 17 bond once. Group 16 bonds twice. Group 15 bonds three times. So does Boron. Group 14 bonds four times.

How to draw Lewis structures for Neutral molecules 8. Arrange electrons around all of the atoms in the molecule so each atom is following the octet or duet rule.

How to draw Lewis structures for Neutral molecules 9. Double Check your work. Count to see that the total number of electrons matches the number calculated in Step #1. If you have too many electrons, add a bond and erase a pair of electrons from each of the elements that were bonded. Count to make sure that each atom has 8 valence electrons or two for hydrogen. A bond counts as 2 electrons for each atom that is bonded. Check to see that you have the correct number of bonds (step # 4) and lone pairs (step #6).

How to draw Lewis structures for Ions Follow all of the same rules Except step #1 you will: Add the number of electrons equal to the negative charge of an anion. Subtract the number of electrons equal to the positive charge of a cation.

How to draw Lewis structures for Ions Draw brackets around the entire structure. Outside of the bracket in the upper right corner, write the charge of the ion.

Resonance Structures This is when there are more than one correct way to draw a structure.