Covalent Molecules
• Lewis Structures only show us the 2-Dimensional shapes. Not the
3-D molecules we see in the real world
• This 3-D shape is important in determining the chemical and
physical properties of a molecule.
• Sugar, as an example, fits certain receptors on the tongue that
sends signals to the brain. The brain interprets these signals as
“sweet” tasting (a physical property).
• It is this particular shape that fits nicely into the proper receptors.
• A Lewis Structure can help predict molecular shapes
• Using the Valence Shell Electron Pair Repulsion (VSEPR)
Theory we can determine the shapes.
• The unshared pairs of electrons repel one another and migrate
to as fair away as they can.
• Consider and draw the Lewis Structures for CO2 and SnCl2
• Carbon Dioxide is a “linear” molecule
• The double bond takes up two bonding sites. The other two migrate to
opposite ends where the two oxygens bond to the central carbon.
• Tin (II) Chloride is a “bent” molecule
• This occurs because the two loan pairs of electrons on the Tin push the two
oxygens away to form the four bonding sites we are use to.
• According to the VSEPR Theory, the shape of the molecule is
determined by the valence electrons surrounding the central
atoms.
• Think about what would happen when the central atom is
surrounding by three atoms.
• How will they arrange themselves around the central atom?
• Now, what if there were three atoms with the central atoms having an
addition unbonded pair?
• HINT: Keep in mind we are always considering three-dimensions!
• Draw out the Lewis structure for BF3
• Draw out the Lewis structure for BF3
• This formation is known as Trigonal Planar.
• So not we have linear, bent, and trigonal planar. Great test questions!!
HINT, HINT!!
• Think about what happens when the central atom is surrounded
by four pairs of electrons.
• Draw out the Lewis Structure for methane, CH4
• Think about what happens when the central atom is surrounded
by four pairs of electrons.
• Draw out the Lewis Structure for methane, CH4
• Notice that on a FLAT PLANE these atoms are not as fair apart
as they could possible be in a 3-D world.
• This is known as a Tetrahedron Shape
• Unshared electrons influence the shape of a molecule but are
not visible in the space-filling molcule.
• For example, the shared and unshared pairs of electrons in
SnCl2 form a trigonal planar geometry, but the molecule has a
bent shape.
• SO2 has a bent shape also, but has two unshared pairs of
electrons. This is due to the “not visible in the space-filling
molecule”
• Page 211: Predict the shapes of the following molecules and
polyatomic ions.
NH2Cl
H2O
NOCl
NH4+
• A molecule’s shape affects both the physical and chemical
properties of the substance.
• One specific property that shape determines is the polarity of
the molecule.
• Recall: What is polarity??
• CO2 is a linear molecule with two double bonds.
• The C is lower in electronegativity than the O’s, so each O
attracts the electrons more, pulling them closer to their side.
O=C=O
The O’s are partially negative and the C is partially positive.
So, we say these double bonds between C and O are Polar.
• Notice also that CO2 has a linear shape.
• The shape determines overall polarity of the molecule.
• The polarities of the double bonds extend outward from the
central carbon atom in opposite directions.
• As a result, they cancel each other out and the CO2 is nonpolar
even though the individual covalent bonds are polar.
• Draw out the Lewis structure of water
• The O is pulling the electrons closer to the central atom, giving
the H’s a partially positive charge and the O a partially
negative charge.
• The “bent” shape will NOT cancel out the charges as in a linear
molecule. This makes water a POLAR molecule.
• Think about the molecule’s overall polarity like this:
• You and a friend pull on a cart in equal and opposite directions. The cart
does not move anywhere. The pull forces cancel one another out in the
same way as the polarities do in carbon dioxide.
• Now, think about what happens when you guys pull will equal
forces in nonopposite directions. If you pull it north and your
friend pulls it west.
• The cart has a net force now and it will move.
• The water molecule has a net partial positive charge on the H side and a
net negative charge on the O side. As a result, the molecule has an
overall charge and is therefore polar.
• Polarity can affect the strength of the molecule.
• It can affect how it interacts with other ions
• Charged particles will be attracted to the poles in a polar molecule.
• This is why water dissolves many substances.
• The Universal Solvent