Chemical Bonding – The Big Picture
A chemical bond is the force of attraction that holds two or more atoms together to form a molecule. Why do atoms form chemical bonds? Because of the OCTET
RULE.
All atoms want to have either _____ or _____ outer electrons. They want to have
1.
2.
3. a full valence shell (octet).
How are atoms able to get a full valence shell?
There are three ways that atoms are able to do this:
1.
A complete transfer of electrons
Ex. lithium fluoride
We see that Li wants to lose one e- and F wants to gain one e-. This means that an e- could be transferred from the Li atom to the F atom.
What ions will be formed when this occurs?
What will happen to these ions if they are near each other?
The force that holds these ions together is the attractive force between oppositely charged ions. This type of bond, where there is a complete transfer of e-, is called an IONIC BOND.
The groups that are likely to form ionic bonds are those from family ____ and family ____. Why is this?
2.
An Equal Sharing of A Pair of Electrons
Consider the hydrogen molecule. How many valence e- does H want?

Why can’t an e- be transferred in this case? Like identical twins in a tug of war, neither of the atoms is able to pull the e- to itself. What about sharing the e-?
By sharing a pair of electrons each atom is able to obtain a full outer shell. When a pair of electrons is equally shared between two atoms, this is called a covalent
bond.
Notice that there are NO IONS formed when a covalent bond is formed.
What holds the atoms together in a covalent molecule like H
2
?
Each e- is attracted to both nuclei at the same time. It is the simultaneous attraction of both e- to both nuclei that holds the atoms together in a covalent molecule.
Try drawing the following molecules:
Cl
2
, F
2
, O
2
, N
2
We can use a Lewis diagram to simplify the drawing of molecules that contain covalent bonds. In Lewis diagrams, each shared pair of electrons is represented by a single line.
For Example: Cl
2
:
O
2
:
N
2
:
A covalent bond is formed when a pair of electrons is shared equally between two atoms. When will a covalent bond be formed?
What are diatomic molecules? Explain why they are formed.

3.
Unequal Sharing of Electrons
What about FCl? Fluorine and Chlorine have different electronegativity values.
Which element is better at pulling electrons to itself (electron affinity)?
Compare the size of a fluorine atom to a chlorine atom:
Fluorine is better at pulling electrons to itself because it is a small atom and its outer shell is closer to the nucleus. BUT, it is not able to totally pull the electrons away from chlorine as it would if it were an ionic bond.
In this case, there is a sharing of electrons, but an unequal sharing of the electron pair. This is called a POLAR COVALENT bond.
How do we know which atoms are better at pulling electrons towards themselves in a molecule? We could figure it out based on the size of the atoms (where the smaller atom is better at pulling electrons towards itself), but it is already figured out for us.
The ability of an atom to attract electrons in a molecule is called the
electronegativity of the atom. See the periodic table for electronegativity values.
Describe the electronegativity pattern.
Electronegativity values are related to the size of the atom and the ionization potential.