Chapter 16 Notes, part Chapter 16 Notes, part IVIVPolarity and IMFsPolarity and IMFs

Types of BondsTypes of Bonds

Up until now, we have Up until now, we have assumed that there are two assumed that there are two types of bonds: Covalent and types of bonds: Covalent and Ionic.Ionic.

This is true, but covalent This is true, but covalent bonds can be broken into two bonds can be broken into two categories:categories:

Nonpolar CovalentNonpolar CovalentIn nonpolar covalent bonding In nonpolar covalent bonding

electrons are shared electrons are shared equallyequally..

Electrons spend an equal Electrons spend an equal amount of time with both amount of time with both elements in the bond.elements in the bond.

Typical in diatomic elements:Typical in diatomic elements:

HH22, N, N22, O, O22, F, F22, Cl, Cl22, Br, Br22, I, I22

Polar CovalentPolar CovalentIn polar covalent bonding, In polar covalent bonding,

electrons are still shared, but electrons are still shared, but they are shared they are shared unequallyunequally..

This is due to one nucleus This is due to one nucleus pulling the shared pair harder pulling the shared pair harder than the other.than the other.

This creates a This creates a dipoledipole—a bond —a bond where one side is slightly where one side is slightly positive and the other is positive and the other is slightly negative.slightly negative.

A dipole is caused because A dipole is caused because the electron spends more the electron spends more time on one side than the time on one side than the other.other.

The polarity of the bond is The polarity of the bond is shown like:shown like:

Polar CovalentPolar Covalent

H—Cl H—Cl + -

OR

Ionic BondsIn an ionic bond, electrons In an ionic bond, electrons

are are transferredtransferred..

The nucleus of one element The nucleus of one element pulls hard enough to take pulls hard enough to take electrons away completely.electrons away completely.

How can you tell what kind of bond there is?

By looking at the difference in By looking at the difference in electronegativity!electronegativity!

Remember, electronegativity Remember, electronegativity is the tendency of an atom is the tendency of an atom to attract an electron when to attract an electron when bonding.bonding.

The more electronegative, the The more electronegative, the more it will pull electrons.more it will pull electrons.

Nonpolar Bonds:

EN Diff ≤ 0.4

Polar Bonds:

0.4 < EN Diff ≤ 1.7

Ionic Bonds:

EN Diff > 1.7

What type of bond is between:What type of bond is between:

H and ClH and Cl

Li and ClLi and Cl

C and SC and S

F and OF and O

What type of bond is between:What type of bond is between:

N and BrN and Br

Na and FNa and F

C and OC and O

Polar Polar MoleculesMolecules

If the slightly positive and If the slightly positive and slightly negative ends of slightly negative ends of polar bonds can collect on polar bonds can collect on two different sides of a two different sides of a molecule, it can make an molecule, it can make an entire molecule polar.entire molecule polar.

If bonds are nonpolar, a If bonds are nonpolar, a molecule will always be molecule will always be nonpolar.nonpolar.

If bonds are polar and the shape If bonds are polar and the shape of a molecule is symmetrical, of a molecule is symmetrical, the molecule will be nonpolar the molecule will be nonpolar because the charges cancel out.because the charges cancel out.

(Linear, trigonal planar, (Linear, trigonal planar, tetrahedral and trigonal tetrahedral and trigonal bipyramidal are the symmetrical bipyramidal are the symmetrical shapes we talked about.)shapes we talked about.)

If bonds are polar and the If bonds are polar and the molecule is asymmetrical, molecule is asymmetrical, the molecule will be polar!the molecule will be polar!

(Bent and pyramidal (Bent and pyramidal molecules are molecules are asymmetrical)asymmetrical)

Practice Problem

Are the following polar or nonpolar molecules?

CO2

H2O

Attractions between Attractions between moleculesmolecules

They are what make solid They are what make solid and liquid molecular and liquid molecular compounds possible.compounds possible.

The weakest are called The weakest are called van der Waal’svan der Waal’s forcesforces - - there are two kinds:there are two kinds:

1. Dispersion forces1. Dispersion forcesweakest of all, caused by weakest of all, caused by motion of emotion of e--

increases as # eincreases as # e-- increases increases

halogens start as gases; halogens start as gases; bromine is liquid; iodine is bromine is liquid; iodine is solid solid

2. Dipole interactions2. Dipole interactions Occur when polar molecules are Occur when polar molecules are

attracted to each other.attracted to each other. Slightly stronger than dispersion Slightly stronger than dispersion

forces.forces. Opposites attract but not Opposites attract but not

completely hooked like in ionic completely hooked like in ionic solids.solids.

H F

H F

Dipole InteractionsDipole Interactions

Hydrogen bondingHydrogen bonding The strongest of the intermolecular The strongest of the intermolecular

forces.forces. When a hydrogen is covalently When a hydrogen is covalently

bonded to a highly electronegative bonded to a highly electronegative atom, AND is also weakly attracted atom, AND is also weakly attracted to an unshared electron pair of to an unshared electron pair of another EN atom.another EN atom.

The hydrogen is left very electron The hydrogen is left very electron deficient, thus positive, so it attracts deficient, thus positive, so it attracts with any nearby negative chargewith any nearby negative charge

Hydrogen bondingHydrogen bonding

F, O, or N, are F, O, or N, are veryvery electronegative. electronegative.

A strong dipole forms when A strong dipole forms when Hydrogen is bonded with one Hydrogen is bonded with one these, and the hydrogen these, and the hydrogen partially share with the lone partially share with the lone pair in the molecule next to it.pair in the molecule next to it.

Hydrogen bondingHydrogen bonding

HH

O H HO

HH

O

H

H

OH

HO

H

HO HH

O