January 4, 2011Objectives

1. Define molecule2. Discuss the relationship of the physical and

chemical properties of the molecules to the properties of the atoms from which they are made (They’re different!)

3. Explain what covalent bonds are and why they are formed.

4. Draw a Lewis Dot diagram for any covelent molecule, indicating the number of valence electrons on each atom in the diagram.

What holds molecules together?

Molecule: collection of atoms that are bound together

What is a covalent bond?

• What holds a molecule together? Covalent bond

• Force of attraction that results from valence electrons being attracted to two nuclei.

• Valence electrons being shared between two nuclei

• Core electrons are not involved in bonding• Valence electrons are shared between the

two nuclei

Ways to represent shared electrons

H:H H..H H-H

Covalent Bonds are very strong!

Bond formation always releases energy

How many valence electrons does each molecule have?

Why won’t two Helium atoms form a molecule?

What do all elements want to be like? Why?

The Octet Rule

• Remember for the representative elements the Roman Numeral group number tells you the number of valence electrons.

• He is the only exception. Why?

• Formula for Water and Methane.

Practice

• What is the formula for ammonia?

• What is the formula for carbon and chlorine?

The Lewis Dot Diagram

Lewis Dot diagrams

• Show an atom’s valence electrons• The number of dots is equal to the group

number• Pairs of dots(electrons) are electron pairs• Single electrons are unpaired electrons

Metal Metalloids Nonmetals

I II III IV V VI VII VIII

Transition metals

Non Metals and The Dots

Electron Pairs vs. Unpaired Electron

Bonding Pairs vs. Lone Pairs

Steps to drawing dot diagrams

• Count up how many valence electrons will be in the final diagram (PCl3) (CO)

• Connect the atoms with single bonds. Assume first non hydrogen atom is the central atom (PCl3)

• Put the remaining dots in as lone pairs. First on the terminal atoms then on the central atom. Don’t use more electrons then you have.

Steps to drawing dot diagrams

• (CO ) Remember every atom needs an octet. Lone pairs to the rescue. Only from adjacent atoms.

Multiple Covalent Bonds

O2

N2

Multiple Covalent Bonds

CO2

C2H2