section 8.4 polar bonds and molecules. objectives after studying this section you should be able to:...
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Objectives
After studying this section you should be able to:1. Differentiate among 4 types of bonds2. Explain the importance of electronegativity in polar
molecules.3. Determine the electronegativity of atoms4. Calculate the difference in electronegativity between
atoms5. Identify and provide examples for three types of
intermolecular forces6. Describe the relationship between bonding and the
physical characteristics of compounds
Bond Polarity
• Non-polar Covalent Bond:
When electrons are shared equally between two atoms.
Formed between diatomic halogen molecules, H2, O2, and N2
Polar Covalent Bond / Polar Bond
• Electrons shared unequally between two atoms.
• The atom with greater electronegativity attracts the electrons, so electrons spend more time with that atom
• That side of the molecule is more negative than the rest.
• The other side of the molecule is positive.
Remember electronegativity?
• Definition: The ability of an atom to attract electrons when the atom is in a compound.
Electrons will “side” with the more electronegative atom
But which atom is more
electronegative?
So, which is more electronegative?
• Hydrogen or Bromine?Bromine!
• Chlorine or Fluorine?Fluorine!
• Oxygen or Lithium?Oxygen!
• Hydrogen or Oxygen?Oxygen!
Types of Bonds
• The type of bond that forms between two atoms depends on their differences of electronegativity.
• Types of bonds are: nonpolar covalent, moderately polar covalent, very polar covalent, and ionic.
Use the table below to determine the electronegativity differences:
H 2.1
Li 1.0
Be 1.5
B 2.0
C 2.5
N 3.0
O 3.5
F 4.0
Na 0.9
Mg 1.2
Al 1.5
Si 1.8
P 2.1
S 2.5
Cl 3.0
K 0.8
Ca 1.0
Ga 1.6
Ge 1.8
As 2.0
Se 2.4
Br 2.8
Rb 0.8
Sr 1.0
In 1.7
Sn 1.8
Sb 1.9
Te 2.1
I 2.5
Cs 0.7
Ba 0.9
Tl 1.8
Pb 1.9
Bi 1.9
Calculate the electronegativity differences between:
• Hydrogen & Fluorine:
4 - 2.1 = 1.9
• Potassium & Chlorine:
3.0 – 0.8 = 2.2
• To figure out the type of bond that forms, look at the following table!
Electronegativity and Bond Types
Electronegativity difference range
Type of bond Example
0.0 – 0.4 Nonpolar covalent
H – H (0.0)
0.4 – 1.0 Moderately polar covalent
H – Cl (0.9)
1.0 – 2.0 Very polar covalent
H – F (1.9)
> 2.0 Ionic Na+Cl- (2.1)
Place the following bonds in order from least to most polar:
A. H – Cl
B. H – S
C. H – Br
D. H – C
Answer: B and D are tied, C, A
Answers!
• H and Br form a moderately polar covalent bond
• C and O form a moderately to very polar covalent bond
• Cl and F form a moderately to very polar covalent bond
• Li and O form an ionic bond
• Br and Br form a nonpolar covalent bond
Polar Molecules
• A molecule that has two poles is called a dipole.
• Polar bonds don’t always make the whole molecule polar!
Shape Matters!
CO2 is non-polar even though there are two polar bonds: the shape is linear and the bonds pull in equal and opposite directions, so they cancel each other.
O=C=O
Look at H2O:
Water is a bent molecule. The polar bonds do not pull in equal and opposite directions. Therefore, water IS a POLAR molecule
Attractions between molecules
• Called intermolecular forces
• They are Weaker than ionic or covalent bonds.
• Yet, they are still important and, yes, you must know them!
Dipole Interactions
• Occurs when polar molecules are attracted to one another.
• The negative region of one molecule is attracted to the positive region of the other molecule
Dispersion Forces
• Weakest of all intermolecular forces
• Caused by the movement of electrons
• Because of the constant motion of electrons a molecule can develop a temporary dipole when the electrons are distributed asymmetrically around the nucleus.
More on dispersion forces
• They grow stronger as the number of electrons increases
• They have an effect on the state of the compound—solid, liquid, or gas
Example: Chlorine
• Has 17 electrons—a relatively low number
• Thus, the strength of the dispersion force between Chlorine molecules are low.
• Because of the weak
dispersion force,
chlorine is a gas.
Example: Bromine
• Has 35 electrons—a high number
• Dispersion force between molecules is strong
• Bromine is a liquid
What about Iodine?
• How many electrons?53
• Is this a high or low number?High
• Look at the periodic table: What state of matter is iodine found in?Solid
Hydrogen Bonds in Words
• Involves a hydrogen that is covalently bonded to a very electronegative atom (within a compound) and is bonded to an unshared electron pair another electronegative atom.
• Strongest of intermolecular forces• Important in determining the properties of
water and biological molecules such as proteins.
Characteristic Ionic Comp. Covalent Comp.
Rep. Unit Formula Unit Molecule
Bond Formation Transfer of e- Sharing e-
Type of Elements Metallic & non-metallic
Nonmetallic
Physical State Solid S, L, or Gas
Melting Point High > 300º High to Low
Solubility in H2O Usually High High to Low
Electrical Conductivity of aqueous solution
Good conductor Poor to nonconducting