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week 2, term 3

posted 24 Jul 2014, 16:51 by Maree Pullar   [ updated 24 Jul 2014, 19:27 ]

Hopefully people have worked out the skill of drawing Lewis structures. Eventually it will become intuitive. Practice makes perfect!

It is important to be able to draw the Lewis structures because a L

A Lewis structure is required for you to be able to determine the shape of a molecule. That is the topic for this week.

The  links below were in last weeks’ work but just in-case you haven’t watched them or you want to refresh yourself, I have included them again (the 2nd one is better than the 1st)

·         https://www.youtube.com/watch?v=-pq2wum1uDc

·         https://www.youtube.com/watch?v=nxebQZUVvTg

 

To determine the shape of a molecule follow the following steps:

1.       Draw a Lewis diagram.

2.       Find the total number of regions of negative charge (note: a double or a triple bond counts as only one region).

3.       Determine how many of the regions are bonding and how many are non- bonding (lone pairs).

4.       Use the table below to determine the shape.

Bonding regions

Non-bonding regions

Shape of molecule

example

Bond angle(0)

2

0

linear

CO2

180

2

2

Bent or V shaped

H2O

109

2

1

Bent or V shaped

SO2

109

3

0

Trigonal planar

BF3

120

3

1

Trigonal pyramid

NH3

109

4

0

tetrahedral

CH4

109

 

VSEPR  ( valence shell electron pair repulsion) theory: Each region of electron density around a central atom repels other regions of electron density.

Molecules are most stable when the repulsive forces are at a minimum. This occurs when the electron regions are as far apart as possible.

See page 106 black for the symbols of how each bond is drawn to make your molecule look 3 dimensional.

·         Do Act 9C black

·         Complete p48-50 in ABA manual

·         Do Act 4D in learning workbook.

 

Determining the polarity of a molecule:

https://www.youtube.com/watch?v=uYtwU0uRK7o

 

To determine the polarity of a molecule follow these steps:

1.       Draw the Lewis structure

2.       Determine the shape of the molecule

3.       Does the molecule contain polar bonds? (do the atoms in a bond have a difference in electronegativity of greater than 0.4).If the answer is ‘no’ the molecule is non-polar. If the answer is ‘yes’ go to step 6.

4.       Are the polar bonds arranged symmetrically around the central atom?

If ‘yes’ the molecule is non-polar.

If ‘no’, the bonds are asymmetrically arranged the molecule is polar.

Things to do :

·         Act 9D study guide

·         Act 4E learning workbook

·         p51-53 in ABA manual.

 

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