Bonding Forces
Unit 5 – Bonding IVMrs. Callender
Lesson Essential Question
How do molecules interact with each other?
Basic Units of Ionic vs. Covalent
Ionic Compounds Form Repeating Units. NaCl: Atoms
of Cl and Na can add individually forming a compound with million of atoms.
Covalent Compounds Form Distinct
Molecules. H2O: O and H
cannot add individually, instead molecules of H2O form the basic unit.
HO
H
HO
H H
O H
Then how do water molecules stay together?
There must be some kind of attractive force keeping them “stuck” together.
INTRAMOLECULAR forces occur
between atoms.
INTERMOLECULAR forces occur
between molecules.
Imagine a glass or water.
H H
H
HH
HO
OO
Intramolecular forces are much stronger than Intermolecular
forces.
Intermolecular forces are not considered in ionic bonding because
there are no molecules.
Intramolecular bonds determine the type of intermolecular force. This will be looked at later in the power point.
Electrons Are Not All Shared Equally in a
Bond.H2 H H
0 0
Covalent (non-polar)
HCl H Cl
Polar Covalent
+ –LiCl [Li]+[ Cl ]–
+ –
Ionic
To figure out which type of bond each compound has you need to look at
electronegativity.
Recall that electronegativity is “a number that describes the relative ability of an
atom, when bonded, to attract electrons”
You can calculate the nature of the bond by subtracting (ΔEN) the two
electronegativities.
H has an electronegativity of 2.1. Since it is connected to another H the ΔEN is 2.1 – 2.1
= 0. This makes it a non-polar covalent bond.
H has an electronegativity of 2.1 and Cl has an electronegativity of 3.0. The ΔEN is 3.0 –
2.1 = 0.9. This makes it a polar covalent bond.
Li has an electronegativity of 1.0 and Cl has an electronegativity of 3.0. The ΔEN is 3.0 –
1.0 = 2.0. This makes it an ionic bond.
Last part to add – Dipoles. These occur with the uneven sharing of the electrons in
covalent bonds. Dipoles are shown by the δ symbol. δ- where electrons are more concentrated and δ+ where they are
moving away from.
Ionic Compounds do not have dipoles.
Overview
Basically: a EN
below 0.5 = covalent (non-polar)
0.5 - 1.7 = polar covalent above 1.7 = ionic
Try These
Determine the EN and bond type for these:
1. HCl 4. H2O2. CrO 5. CH4
3. Br2 6. KCl
Answer’s
HCl: 3.0 – 2.1 = 0.9 polar
covalent
CrO:3.5 – 1.6 = 1.9 ionic
Br2: 2.8 – 2.8 = 0 covalent
H2O:3.5 – 2.1 = 1.4 polar
covalent
CH4: 2.5 – 2.1 = 0.4 covalent
KCl: 3.0 – 0.8 = 2.2 ionic
Lets look at HCl. Partial charges keep molecules together.
+ –
+–
+ – +
–
– +The situation is similar in NaCl but the attraction is even greater. ΔEN = 2.1 vs. 0.9 for HCl.
+ –Which would have a higher melting and boiling point?NaCl because it has a higher electronegativity difference - ΔEN
Electronegativity and Physical Properties
Which has the higher
Boiling Point?1.CaCl2, CaF2
2.KCl, LiBr3.H2O, H2S
H20 would have a higher melting/boiling point:H2O= 3.5 – 2.1 = 1.4H2S = 2.5 – 2.1 = 0.4
CaF2 would have a higher melting/boiling point:
CaCl2 = 3.0 – 1.0 = 2.0CaF2 = 4.0 – 1.0 = 3.0
KCl would have a higher melting/boiling point:KCl = 3.0 – 0.8 = 2.2LiBr = 2.8 – 1.0 = 1.8
ONE IMPORTANT NOTE:
There are other factors such as atomic size within the molecules
that also affects melting and boiling points. EN is an important factor
but not the only factor. It is most useful when comparing atoms and molecules of similar
size.
Oil and Water
Why do oil and water never mix?
Oil is polar and water in non-polar (covalent)+ – + + – +
+ – +
oil
oiloil
Opposite dipoles of water are attracted to each other. They are trying to get closer and closer pushing the oil molecules out of the way.
+ – + + – +
+ – +
oil oil
oil
+ – +
+ – +
+ – +
oil oil oil
Cleansing Action of Soap