LEWIS STRUCTURES

An easy way to make them

Lewis structures are necessary to understand the bonding, structure, and ultimately the properties, of compounds of the p-block elements. Here I describe an easy method to draw, as well as evaluate, Lewis structures. There is no need to sum all of the valence electrons, reorganize electron pairs, draw and pair up individual electrons, or remember a complex formal charge formula.

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INTRODUCTION

Knowing the formal charges of atoms is essential to understand chemical structures and molecular properties, and it is the basis of this method. The formal charge is determined by the group number (number of valence electrons) and the number of bonds, with the sum of formal charges then being equal to the total charge. This leads directly to a valid Lewis structure.

This method has been published: Owen J. Curnow, Journal of Chemical Education, 2021, 98, 1454-1457.

https://dx.doi.org/10.1021/acs.jchemed.0c00952

WHAT YOU NEED TO UNDERSTAND BEFORE DRAWING COMPLEX LEWIS STRUCTURES

You need to understand the basics of Lewis dot structures before you can tackle more complex molecules. In particular: valence electrons and the octet rule as it applies to atoms and ions; covalent bonding, multiple bonds, and lone pairs; electronegativity.

The basics of Lewis structures

HOW TO DETERMINE FORMAL CHARGES

The formal charge can be easily determined from the number of bonds that the atom has and which group of the periodic table it is in. You can find the rationalization and procedure here.

Formal charge determination

THE PROCEDURE

Molecules can be constructed in three simple steps because the sum of the formal charges equals the total charge.

Draw the molecular framework with one covalent bond between each atom, and assign the formal charges;
Add bonds until the total charge is correct (ensuring that no atom has more than 4 bonds).
Place lone pairs so that every atom has an octet.

Note that each additional bond will increase the sum of charges by +2.

A worked example

WHICH IS THE MOST STABLE LEWIS STRUCTURE?

After you've drawn a number of possible valid Lewis structures, how do you decide which is the most stable one? Understanding these key principles will also help you draw the right one to start with.

Stable Lewis Structures

RESONANCE STRUCTURES

Some molecules have a number of resonance structures that may be important and help contribute to their stability.

ELECTRON DEFICIENT SPECIES

Some atoms do not have enough electrons to satisfy the octet rule. What are these molecules and how do we deal with them?

RADICALS

Species with an odd number of electrons also cannot satisfy the octet rule. Here's how we determine their Lewis structures and formal charges.

HYPERVALENT MOLECULES

Many of the heavier atoms appear to exceed the octet rule. How do we identify and draw molecules with these atoms?

If you want to learn more about my other research, please don’t hesitate to get in touch or look at my research page:

https://www.canterbury.ac.nz/science/contact-us/people/owen-curnow.html

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