Molecular ShapesCovalent Molecules

Determining Molecular Shapes• 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.

Lewis Structures Help Us With Shapes• 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

CO2 Vs. 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.

VSEPR Theory• 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!

Practice With VSEPR Theory

• Draw out the Lewis structure for BF3

Practice With VSEPR Theory

• 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!!

Surrounded Central Atoms• Think about what happens when the central atom

is surrounded by four pairs of electrons.

• Draw out the Lewis Structure for methane, CH4

Surrounded Central Atoms• 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 Pairs of Electrons• 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”

Predict The Following Shapes • Page 211: Predict the shapes of the following

molecules and polyatomic ions.

NH2Cl H2O

NOCl NH4+

Molecular Shapes and Properties• 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??

Shape Affects 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.

Shapes and Polarity

• 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.

Now, Think About Water• 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.

In Other Words…• 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 Affects Many Things• 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