Chapter 9 Sections 3-5
Lewis Structures
Molecular Geometry
Polarity
Lewis Structures
• Used to show the position of atoms in a
molecule
• Can help predict the shape of a molecule
• Hydrogen will always be on the end (only
bonds once)
• The most electronegative atom should be
placed in the center (farthest up and to the
left on table)
• 1 line or 2 dots represent a pair of electrons
Steps for drawing Lewis Structures
1.
2.
3.
4.
5.
6.
7.
Draw a stick figure
Give atoms what they want:
1. Group 1= 2 electrons
2. Group 2
= 4 electrons
3. Group 3A
= 6 electrons
4. Group 4A-8A
= 8 electrons
Count up the total number of electrons you USED
Determine the total number of electrons AVAILABLE – get this from the
periodic table (valence electrons = group numbers). Add any charge
electrons also.
Compare the USED with the AVAILABLE – if they match you are done
If you USED too many – remove a lone pair from 2 bonded atoms and place
another bond between them – continue until USED matches AVAILABLE
If you have more AVAILABLE than used – add them as lone pairs to the
central atom
Practice Lewis Structures
1. CH4
2. NH3
3. CO2
4. PO435. PCl5
Lone pairs of electrons take up more space than a
bonded pair of electrons. So the more lone pairs you
have the smaller the angles become.
Normally an atom with 4 attached groups has an angle of 109.5o
Determining the type of Bond
• Look at the differences in electronegativies
Generally:
– 0-0.5 difference = nonpolar covalent (equal sharing)
– 0.5-1.7 = polar covalent (unequal sharing)
– 1.7 and higher = ionic (transfer of electrons)
• What type of bond exist between H and Cl if the
electronegativity of H is 2.20 and Cl is 3.16?
• 3.16 – 2.20 = 0.96 so it is a polar bond
A molecule can have polar bonds and still be overall
nonpolar if the polar bonds cancel out by going in
opposite directions.
Polarity
• Polarity determines solubility
• Remember “like dissolves like”
• This means that polar substances dissolve in
polar solvents
• Nonpolar dissolves in nonpolar
• Water is a POLAR molecule!
• Why will oil not dissolve in water?
Molecular Structures Lab Activity
Draw the Lewis Structures for the following, then build the
molecule and determine the shape and if it is polar or not.
Molecule
BeCl2
BH3
CH4
PH3
H2O
PCl5
SF6
Lewis
Structure
Molecular Polar?
Shape