CHAPTER 11Chemical Bonding

TYPES OF CHEMICAL BONDS 11.1

Bond – a force that holds groups of atoms of two or more atoms together and makes them function as a unit

Bond Energy – the amount of energy required to break the bond

Types of Bonds: 4 TYPES

Cations packed in “a sea of electrons”Metals

Metals consist of closely packed cations floating in a “sea of electrons”.

All of the atoms are able to share the electrons.

The electrons are not bound to individual atoms.

Metallic Artists rendering of a metallic bond

Type 1: Metallic

Properties of Metals Good conductors Ductile Malleable

Electrons act as a lubricant, allowing cations to move past each other

Metals have a Crystalline Structure

Packed spheres of the same size and shape: Body Centered

Cubic Face Centered

Cubic Hexagonal Close

Packed

Metals Example: Body Centered Cubic (Chromium)

More examples

Face-Centered Cubic (gold) picture

Last example

Hexagonal Close-Packed (zinc)

picture

Type 2: IONIC

Bond between closely packed, oppositely charged ions

Bond between a metal and a nonmetal

hard solid @ 22oC high mp temperatures nonconductors of electricity

in solid phase good conductors in liquid

phase or dissolved in water

(aq)

IONIC picture

Covalent Bonding (2 Bonding (2 types)types)

Instead of gaining or losing electrons atoms can get stable by sharing electrons

This is always between two non-metals. Two fluorine atoms, for example, can form

a stable F2 molecule in which each atom has 8 valence electrons by sharing a pair of electrons.

In covalent bonds they can share more than two electrons

Type 3&4: COVALENT

Electrons are shared Have low melting, boiling

points Do not conduct

electricity when melted or dissolved in water

relatively soft solids as compared to ionic compounds at room temp

COVALENT picture

Covalent bond –subtype #1

When two of the same elements bond they are called diatomic molecules, some examples of this are Hydrogen H2, Oxygen O2 and Nitrogen N2.

The atoms in these bonds would have the same electronegativities. This means that both atoms attract the shared electrons to that same extent.

Non-polar Covalent picture

Covalent Bonds – subtype #2

POLAR COVALENT picture

Unequal sharing of electrons

Dipole Moment 11.3

A molecule that has a center of positive charge and a center of negative charge

Dipole often represented by an arrow Points towards

negative charge center and its tail indicates the positive charge center

Review: 3 types of bonds thus far

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LEWIS STRUCTURES

Section 11.6

Lewis Dot Structures

Show valence electrons Use group number to figure it out

The Octet Rule

The octet rule says that atoms tend to gain, lose or share electrons so they have eight electrons in their outer shell.

There are some exceptions to the octet rule (imagine that) BF3

BCl3 PF5

SF6

Ionic Bonding: (this should be review)

http://www.youtube.com/watch?v=T40sM8-SXso

Covalent Bonding:

http://www.wisc-online.com/objects/ViewObject.aspx?ID=GCH6404

Follow the interactive website!

Drawing Lewis Structures

Arrange the element symbols. Central atoms are generally those with the highest bonding

capacity. Carbon atoms are always central atoms Hydrogen atoms are always peripheral atoms

Add up the number of valence electrons from all atoms.

For polyatomic ions, add one electron for each negative charge and subtract one for each positive charge.

Draw a skeleton structure with atoms attached by single bonds.

Complete the octets of peripheral atoms. Place extra electrons on the central atom. If the central atom doesn’t have an octet, try forming

multiple bonds by moving lone pairs.

Simple Rules

1. Figure out number of electrons by counting the TOTAL valence electrons in whole compound

2. Place the central element in the middle and surround it with the other elements

3. Place single bonds between elements 4. Place lone pairs around each element until

there are a total of eight (Hydrogen only wants 2) 5. Count total electrons surrounding the compound

(don’t forget the bonds count as 2 electrons) If electrons from #1 and #5 don’t match…. Erase

electrons and put in double bond and recount

Single, Double and Triple Bonds

With Covalent bonds the elements can share two or more electrons

A Single Bond is when 2 electrons are shared they are represented by a single line in bond diagrams

A Double bond is when 4 electrons are shared they are represented by two lines in bond diagrams

A Triple bond is when 6 electrons are shared they are represented by three lines in bond diagrams

Lewis Dot Structures: Lewis Dot Structures:

H2CO

H

H

C O C O

H

H

Isomers – multiple correct structures for a single compound (requires breaking

bond to make new compound)

CH2Cl2

H

Cl C Cl

H Cl

Cl C H

H

ELECTRONEGATIVITY AND POLARITY

Section 11.2

Electronegativity Values The electronegativity values can be

found in the periodic table The higher the value the higher the

electronegativity The Pauling scale is used to measure

electronegativity. It is a relative scale running from 0.7 to 4.0 (hydrogen = 2.2).

The units for electronegativity are Pauling units.

Electronegativity

The ability of an atom to attract electrons when bonded

1. Nonmetals have high electronegativity2. Metals have low electronegativity3. Electronegativity increases across a period and

decreases down a group. WHY???

Electronegativity Chart

Why would the metals have low electronegativity numbers?Why don’t the noble gases have electronegativity numbers?

Nonpolar Covalent Bond

When electrons are shared between 2 atoms, a covalent bond is formed.

If the atoms are identical, e.g. Cl2, the electrons are shared equally (nonpolar)

Cl = 3.0 therefore the ∆EN = 3.0-3.0 = 0 ∆EN = electronegativity Difference 0 = nonpolar

Polar Covalent Bond

If the electrons are shared between 2 different atoms, e.g. HBr, the sharing is unequal

The bonding electrons spend more time near the more electronegative atom

H = 2.1 and Br = 2.8 THEREFORE 2.8-2.1 = 0.7 0.7 = a polar covalent bond

H Br

Bond Type by Electronegativity Value Remember the higher

the atom’s electronegativity value, the closer the shared electrons tend to be to that atom when it forms a bond

Therefore, the polarity of a bond depends on the difference between the electronegativity values of the atoms forming the bond

The greater the difference, the more polar the bond.

Electronegativity Difference

Type of Bond Formed

0.0 to 0.2 nonpolar covalent

0.21 to 1.7 polar covalent

≥ 2.0 ionic

Electronegativity Differences Why is there a gap between 1.7 and

2.0???? If the two atoms are nonmetals =polar

covalent bond If nonmetal & metal = ionic bond

Electronegativity Difference

0 to 0.2

Nonpolar covalent

0.21 to 1.7

Polar covalent

≥ 2.0

Ionic

Sample Problems

Choose the bond that will be more polar H-P or H-C O – F or O – I N – O or S – O N – H or Si - H

Sample Problems

Choose the bond that will be more polar H-P or H-C O – F or O – I N – O or S – O N – H or Si - H

Polar Molecules (overall polarity of the molecule)

Note: Not all molecules with polar bonds are polar molecules

The dipoles in symmetrical molecules cancels out The bond is polar but the molecule is nonpolar

How to determine polar moleculesThere are two important factors1. The polarity of the individual bonds in the

molecule; 2. The shape or geometry of the molecule. Steps to takea) Determine if a given individual bond is polar,

Look at the difference between electronegativity of the atoms in the perioidc table. If the difference is:

0.2 < non polar 0.2 - greater = polar

b) Determine the shape of molecule. For now I will give them to you. Later you will figure out the shape yourself.

i) if all bonds are non-polar, then the whole molecule is non-polar regardless of its shape.

ii) If there is symmetry in the molecule so that the polarity of the bonds cancels out, then the molecule is non-polar. (symmetry arround the central atom)

iii) If there are polar bonds but there is no symmetry the overall molecule is polar.

Which molecules are polar?

Which molecules are polar?

For these two molecules, even though there are polar bonds the overall molecule is nonpolar because the molecule is symmetrical