Covalent Bonds

Creating the Lewis Structure:

Step 1: Write down the amount of valence electrons each element has.

Ex: CH4 4 + 1(4) = 8

Step 2: Decide the center element by choosing the one with the most unpaired electrons.

Ex: CH4 Carbon has 4 unpaired electrons, while hydrogen has only 1 so carbon will be in the center.

Step 3: Draw the Lewis structure, pairing off all the unpaired electrons, ensuring each atom has achieved the octet rule.

Ex: You can represent the bonds with either the lines or the dots. Notice that in both examples, each atom has achieved octet rule(each line is worth 2 electrons

Step 4: Make sure you didn’t add or subtract any electrons

Ex: This Lewis structure has 8 electrons which is the same amount we started with.

Finding the Molecular Geometry + bond angle:

Step 1: Add up the amount of bonds and lone pairs (we only consider lone pairs of the CENTER atom)

Ex: This structure for NH3 has 3 bonds and 1 lone pair of electrons.

Step 2: Find the associated geometry with the bond angle. Use this table to help you memorize.

# of bonds

# of lone pairs

Molecular Geometry

Bond Angle (degrees)

1

any

Linear

180

2

0

Linear

180

3

0

Trigonal planar

120

4

0

Tetrahedral

109.5

3

1

Trigonal pyramidal

107

2

2

Bent

105

Ex: Using the table, we can see that the shape of this molecule is

Trigonal Pyramidal and the bond angle is 107 degrees.

Finding Polarity METHOD 1: method 2 is a little easier

Step 1: Look at the electronegativity of each element from the periodic table, following the trend below

Ex: CH2Cl2

Carbon is more electronegative than Hydrogen. Chlorine is more electronegative than both of them

Step 2: Create the dipoles by going from the less electronegative element to the more electronegative element.

Ex: CH2Cl2

Step 3: Notice where most of the arrows point and create a new arrow to represent the entire molecule’s polarity.

Ex: this is the overall trend

of this dipole.

The molecule is POLAR

Finding Polarity: Method 2

Step 1: Look at the structure and determine if it is symmetrical or not

Ex: CH2Cl2 is NOT symmetrical. CH4 is symmetrical

Step 2: Determine whether it is polar or nonpolar

Ex: Since CH2Cl2 is not symmetrical, it is polar Since CH4 is symmetrical, it is nonpolar