Snow Flakes

Naming Binary Covalent Compounds• The first element in the formula is named first,

followed by the second with the suffix -ide

• Prefixes are added to the elements name for both the first and the second

Never use mono- for the first element

mono- 1di- 2tri- 3tetra- 4penta- 5hexa- 6hepta- 7octa- 8nona- 9deca- 10

Polar and Non-polar Covalent Bonds

• Reminder: The ability of an atom within a molecule to draw electrons toward itself is called electronegativity.

• An electron pair shared between two atoms of the same element is called a non-polar covalent bond– N2, H2, F2 all have non-polar covalent bonds

– Each element has the same electronegativity so both atoms attract electrons equally.

Polar and Non-polar Covalent Bonds• What if two different atoms with different

electronegativities?– HF

– H = 2.2, F = 4.0 so F will attract electron better

• An electron pair is more likely to reside closer to the nucleus of the atom with the greater electronegativity and forms a polar covalent bond. The greater the difference between the electronegativites, the more polar the bond.

H F

Polar and Nonpolar Covalent Bonds• What if the difference between the

electronegativities is really large?– Na and F

• Na = 0.93, F = 4.0 Difference is 3.07!

– Called an ionic bond.

Polarity of Molecules• Just as bonds can be polar

(electronegativity of atoms) so can molecules

• If the atoms attract electrons around the central atom symmetrically, the molecule is non-polar

B

F F

F

CO O

O

H H

If the atoms pull electrons around the central atom asymmetrically, the molecule is polar

Oil and Water

• Oil is non-polar

• Water is polar

• That is why oil and water don’t mix

VSEPR Theory• Valence Shell Electron Pair Repulsion Theory

• Use VSEPR to predict the shape of molecules based on the Lewis structure

• Shape of the molecule is affected by each of the valence shell electron pairs surrounding a central atom

• The regions of electron space around the atom repel each other and strive to get as far away from each other a possible

• Region of electron space are: – a non-bonding lone pair

– bonding electrons

Possible VSEPR Shapes

• 5 VSEPR shapes corresponding to number of regions of electron space– 6 regions: Octahedron– 5 regions: Trigonal bipyramid– 4 regions: Tetrahedron - 109.5° apart– 3 regions: Trigonal Planar - 120° apart– 2 regions: Linear - 180° apart

Predicting Shape - CH4

CH

H

H

H1. Draw Lewis Structure of Molecule

2. Count regions of electron space around central atom

- Each bonding region (single, double or triple bond) counts as one- Each lone pair counts as one

3. Assign VSEPR shape

4. Place lone pairs (if applicable) on shape

5. Assign structure by positions of the bonded atoms

4 Regions

Tetrahedron

No Lone Pairs

Tetrahedron

Predicting Shape - H2O

O

H

H

1. Draw Lewis Structure of Molecule

2. Count regions of electron space around central atom

- Each bonding region (single, double or triple bond) counts as one- Each lone pair counts as one

3. Assign VSEPR shape

4. Place lone pairs (if applicable) on shape

5. Assign structure by positions of the bonded atoms

4 Regions

Tetrahedron

2 Lone Pairs

Bent

• •

••

Shapes of Molecules

# lone pairs Shape0 Tetrahedron1 Pyramidal2 Bent3 Linear

Tetrahedron

# lone pairs Shape0 Trigonal Planar1 Bent2 Linear

Trigonal Planar

Linear# lone pairs Shape

0 Linear1 Linear

Group Work

• Predict the shape and polarity (polar or non-polar) of the following molecules

• NH3

• BCl3

• HF

Interactions between molecules• Three phases of matter

– Solid • ordered structure• fixed volume and shape independent of container• Strong interaction between molecules

– Liquid• loosely ordered• fixed volume, but not fixed shape - dependent on container• Moderate interaction between molecules

– Gas• No order• Volume dependent on container• Always occupies all of its container• Almost no interaction between molecules