2.5: Lewis Diagrams

Learning Targets/ Content Objectives

SAP-4.A Represent a molecule with a Lewis diagram.

Essential Knowledge

SAP-4.A.1 Lewis diagrams can be constructed according to an established set of principles.

What is Lewis dot Symbol?

Lewis dot symbol is a structural representation that shows the valence electrons around an atom using dots. For example Oxygen atom has 6 valence electrons so the Lewis dot symbol for Oxygen is as follows:

What is a Lewis Diagram or a Lewis structure of a Molecule?

A Lewis structure is a structural representation of a molecule where dots are used to show electron position around the atoms.

To draw Lewis structure/diagram of a given molecule follow the following steps:

Step 1: Find the total number of valence electrons of each atom in the molecule (Using Group A /column number in the periodic table).
Step 2: Draw a skeleton structure. This places a single bond between atoms as a starting point. The atom that needs to make the most bonds to fill its octet (8 shared electrons) is usually placed in the center; this is usually the least electronegative element. Hydrogens and halogen atoms are almost always terminal (on the ends). Elements in the third period are capable of expanding their octet to share to have more than eight electrons when they are a central atom.
Step 3: Place the remaining electrons as lone pairs so that you can fulfill as many octets as possible. Starting with the most electronegative atoms and/or the outside elements ,and work inwards.
Step 4: If you run out of electrons before fulfilling the octets add double bonds or triple bonds as needed

See examples in the table below:

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