LEWIS DIAGRAMS

 A Lewis diagram shows how the valence electrons are arranged in a molecule. 

The lone pair of electrons are drawn as dots and the two electrons in a bond are usually drawn as a straight line joining the two atoms in the bond (although they can be drawn as two dots between the atoms).

What is a molecule?

A molecule is made of two or more non-metal atoms sharing their electrons. However, there are a couple of exceptions. Beryllium and boron are both metals but form covalent bonds with a lot of nonmetals. (We will look at why in the next lesson when we cover electronegativity). 

If you need to recap covalent bonding and molecules vs ionic bonding and ionic compounds, you can watch the video below. Otherwise, you can skip the video and read on to get the steps for drawing Lewis diagrams. 

STEPS FOR DRAWING LEWIS DIAGRAMS

• Use the Periodic Table to find which atom needs to gain the most electrons to become stable. Add up the number of valence electrons in each atom to find the total number of valence electrons. Watch the video at the bottom of this lesson if you need to revise how to use The Periodic Table.
• Place the atom that needs to gain the most electrons in the centre.
• Imagine 4 areas around the central atom. Now you can place the central atom’s valence electrons around it. Add one electron to each of the four areas working clockwise. Once all four areas have one electron you can continue clockwise around the atom adding a second electron to each area until all of the atom’s valence electrons have been added. 
• Add the other atoms around the central atom. Place these atoms near any single, bonding electrons around the central atom. Or, if there are only single, bonding electrons around the central atom, spread the other atoms evenly around the central atom. 
• Add the outer atoms’ valence electrons using the same method as for the central atom. It is best if you add the first electron to the area clockwise from the area facing the central atom. Continue to work clockwise, adding one electron to each of the four areas as you move around the atom.
• Join any single bonding electrons by replacing the two dots with a line.
• Count up the valence electrons in your Lewis diagram (remember each line represents two electrons) and make sure the total matches the total number you worked out at the start.
• Count the electrons around each atom to make sure there are 8.

Remember there are a few exceptions to this octet rule:

• Hydrogen needs 2
• Beryllium needs 4
• Boron needs 6

In the exam:

Drawing Lewis diagrams correctly will give you an Achieved point in your exam.

Periodic Table Basics

Learning Activities

• Lewis Diagrams Worksheet

Lesson Activity

Download and complete the worksheet on drawing Lewis diagrams. Remember that hydrogen needs two electrons while beryllium needs 4 and boron needs 6.

Once you have attempted all six Lewis diagrams, watch the videos below to check if your Lewis diagrams are correct. 

PLEASE draw the Lewis diagrams before watching the videos.

Don't worry if you think your Lewis diagram is incorrect. Mistakes are a part of learning!

ANSWERS:

The Lewis diagram for CH4

Lewis diagram for H2O

Lewis diagram for NH3

Lewis Diagram for NOCl

Lewis diagram for BCl3

Lewis diagram for BeI2