How to Draw Lewis Structures

The Problem: The Molecular Blueprint

In the last chapter, we learned that atoms share electrons to form covalent bonds and achieve stability. But how do we know *how* they connect? How many bonds do they form? Where do the extra electrons go?

To build a molecule correctly, we need a blueprint. In chemistry, that blueprint is a Lewis Structure (also called a Lewis dot diagram). It's a simple 2D drawing that shows the arrangement of atoms, bonds, and lone pairs of electrons.

Mastering Lewis structures is the key to predicting molecular geometry, polarity, and reactivity. Let's learn how to draw them, step by step.

Step 1: Count the Valence Electrons

The very first step is to take inventory of all the valence electrons available. These are the outermost electrons that will participate in bonding. You need to find the total number of valence electrons from all atoms in the molecule.

Use the interactive calculator below to practice. We'll use CO₂ as our running example. See how its total is calculated.

Step 2: Find the Central Atom

Now that we have our electron count, we need to arrange the atoms. Every molecule (except for simple diatomic ones like O₂) has a central atom that all the other atoms bond to. How do we pick it?

The rules are simple:

• The central atom is usually the least electronegative atom (the one that is "least greedy" for electrons).
• Hydrogen (H) is never the central atom, as it can only form one bond.
• Carbon (C) is almost always a central atom.

For our CO₂ example, Carbon is less electronegative than Oxygen, so it will be our central atom. Use the module below to confirm this.

Step 3: Draw the Skeleton Structure

With our electron count and central atom identified, it's time to draw the basic "skeleton" of the molecule. This is simple: connect every terminal (outer) atom to the central atom with a single bond.

Remember, each single bond is made of 2 electrons. This is where we start spending our electron budget. For CO₂, we'll connect each Oxygen to the central Carbon. Use the module below to see how this affects our electron count.

Step 4: Place Remaining Electrons on Terminal Atoms

Now we use the electrons left over from making the skeleton. The goal is to give each terminal (outer) atom a full octet (8 electrons), with the exception of Hydrogen, which only wants 2.

For CO₂, each Oxygen atom already has 2 electrons from its single bond, so it needs 6 more (3 lone pairs). Use the module below to see how many electrons this step uses up.

Step 5: The Final Touches

This is the last and most important step. We have two rules to follow:

1. Place any electrons that are still left over from Step 4 onto the central atom as lone pairs.
2. Check the central atom. If it does not have a full octet (8 electrons), you must form double or triple bonds. Do this by taking a lone pair from a terminal atom and turning it into another bond with the central atom. Repeat until the central atom has its octet.

For CO₂, after Step 4 there are no electrons left for the central Carbon, which only has 4 electrons from its two single bonds. It needs 4 more! To fix this, we'll take one lone pair from each Oxygen and turn them into two new bonds, creating two double bonds. Use the final module to see this happen.