Chapter 2: Chemical Bonds

What we are going to learn

Quick reminder on ionic and covalent bonding.

Lewis dot symbols.

More Lewis dot symbols.

And more lewis dot symbols.

Bond strength and length.

Bonding: Finding that special element(s) that completes you

Share or take electrons to be isoelectronic with noble gas. aka have a full octet.

How about me and you go back to my place and form a covalent bond or two?

I have 6 valence electrons.

Me too!

Types of Bonds Ionic:

Covalent:

Metallic:

•“trades” electrons • Metal and a non-metal

•“shares” electrons •non-metals

•delocalized electrons •metals •electrically conductive •collective description of many bonds

NaCl

CO

Ag

Energy of Ionic Bond Formation

All chemical processes consume (___________) or release (______________) energy.

We use this for many many things that you’ll see in Chem 1B

Here we will look at the energy of forming an ionic bond.

Energy of Ionic Bond Formation Break it into three parts.

Na ________ an electron (_______________)

Cl _______ an electron (________________)

+ and – attract and form a bond.

Now add all the energies

Energy is lower, so it is ____________.

Ionization energy must be low enough to make this ___________ (aka __________________), typically only happens with metallic elements.

e-

Lewis Dot Symbols

Represents Elements with their valence electrons.

Element symbol goes in middle

Valence electrons placed around

Bonds formed by line representing two electrons

Examples:

Ionic Bonds K I K I

Mg F F Mg F

K + I

Mg + 2F

F

+

+ +

+ -

- - 2+

Making Ionic Compounds: Criss-Cross Trick

Al3+ O2-

Al O

Al O

Combine to make neutral compound

Chapter 9

Making Ionic Compounds

Mg2+ O2-

Mg O

Mg O

MgO

Chapter 9

Combine to make neutral compound

Covalent bonding: Molecular Compounds

In a covalent bond electrons are shared

Cl Cl +

Lewis Structure

Covalent bonding: Molecular Compounds

In a covalent bond electrons are shared

S 2 O + O S O

General Lewis Structure Guidelines

Step 1: Add up all valence electrons.

Step 2: Make a skeletal structure by connecting each element with a bond (which is 1 pair of electrons)

Step 3: Distribute the remaining electrons to satisfy octet rule

Check for common exceptions to the octet rule S, P, Xe, Be, B, Al

Step 4: Check formal charges, if you can minimize them by moving electrons, do so.

Non-Octet Breaking Examples

Here are the ones we will do in class: N2

CH2O

BH3NH3

XeF44-

N2O

Breaking the Octet Rule

Second Period Elements can’t have more than 8 electrons.

Seriously- don’t do it!!!!!!

Third Period elements can

This is because they have _____________that can be used for bonding.

Some elements also commonly have less than 8 atoms.

Examples that Break the Octet Rule

Here are the ones we will do in class: XeF4

SF6

BH3

AlH3

H2SO4

POCl3

ClF4-

Resonance Structures

Molecule has resonance if there are are more than one allowed arrangements of electrons.

Can’t move _________________________

In reality structure is a mix of the resonance structures.

Examples we’ll do in class NO3

[HCO2]-

Back through N2O

Delocalized Electrons Look a the structures we drew.

How are the electrons really placed around the molecule, which one is “right”?

Equivalent structures, or structures with the same energy ___________________________.

Low energy structure contribute more than high energy structures.

Electrons in these are called “_________________________”

Line Structures Short-hand notation for molecules

Typically used for organic molecules

Rules Each Carbon is represented by a corner or end of a line

All hydrogens attached to carbons are not drawn

Other atoms, all heteroatoms, and hydrogens attached to heteroatoms need to be drawn

Examples

What is the formula for the following:

Benzene

6 membered ring

Shown in many forms

p orbitals become __________ over entire molecule

Each bond is about the length of __________________

H

H

H

H

H

H

Kekule Structure.

Electronegativity Ability of an atom to __________________________ __________________________________________

Same trend as electron affinity and ionization energy.

Not as many important exceptions, mostly involving the D block. We won’t worry about them.

Electronegativity and Polarity A difference in electronegativity between two elements of >2 is ionic

If there is little or no difference in electronegativity it is non-polar.

If there is a difference between the two that is <2, it is a polar covalent bond.

I3- HLi HF

An odd example O3

O

O O

O

O+

O-

O-

O+

O

Microwaves: Quantum Mechanics at work.

http://phet.colorado.edu/en/simulation/microwaves

Polar water molecules are excited rotationally by microwaves

Friction heats up food

Why can’t you put metal in a microwave?

Going Back to Lewis Structures we did previously:

Which bonds are polar?

Polarizing Power and Covalent Character

More electrons are further from the _________

The more electrons the more “______________”

The more polarizable a molecule, the more covalent character it has.

Which has more ionic character, MgI2 or NaCl?

NaCl MgI2

Bond Strengths and Lengths Bond Strength is Measured in dissociation energy (D)

The stronger the bond the ________________

E.G. A C-C single bond is always ____________ and ___________ than a C-C triple bond.

Resonance structure bonds are ________________________ structures.

Bonds are ________________ SSbetween larger atoms