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