What if I run out of electrons before all octets are full?

This is your cue to create double or triple bonds. Take a lone pair from an atom that already has a full octet and share it with the atom that is short on electrons.

How do I know which atom goes in the center?

The central atom is usually the least electronegative element in the formula. A good rule of thumb is that the atom there is only one of (like the C in $CO_2$) goes in the middle. Remember, hydrogen and fluorine are almost always on the outside.

Do all atoms follow the octet rule?

No, there are exceptions. Hydrogen only needs two electrons (a duet). Boron is often stable with just six electrons. Elements in the third row of the periodic table and below (like phosphorus and sulfur) can have expanded octets holding 10 or 12 electrons.

How do you draw a Lewis structure?

Drawing a Lewis structure is like putting together a puzzle where the pieces are atoms and the connectors are valence electrons. To draw one, you count the total valence electrons, connect the atoms with single bonds, and then distribute the remaining electrons to give every atom a full outer shell (usually eight electrons).<br><br>These structures are simple diagrams that show how atoms bond in a molecule and where the unshared (lone) pairs of electrons sit. They are essential tools in chemistry because they help us predict a molecule's shape, reactivity, and physical properties.

The Step-by-Step Method

First, find the total number of valence electrons by adding up the group numbers of each atom. If the molecule is an ion, add an electron for every negative charge or subtract one for every positive charge. Next, pick a central atom (usually the least electronegative one) and draw single bonds to the surrounding atoms. Finally, distribute the remaining electrons as lone pairs to satisfy the octet rule for each atom. If you run out of electrons before everyone is happy, you will need to share more by creating double or triple bonds.

Why it Works: The Octet Rule

Think of atoms as wanting to feel like noble gases, which have completely full outer electron shells. For most common elements like carbon, nitrogen, and oxygen, a full shell means having eight valence electrons. The Lewis structure is just a bookkeeping tool to make sure every atom gets to 'claim' eight electrons through sharing (bonds) or keeping them to themselves (lone pairs).

Where Students Slip Up

A very common mistake is forgetting to adjust the total electron count for polyatomic ions. For example, $OH^-$ has one extra electron compared to a neutral oxygen and hydrogen atom. Another trap is putting hydrogen in the center of the molecule. Hydrogen can only hold two electrons, meaning it can only form one single bond, so it must always be on the outside (terminal) position of your drawing.

Worked through

Draw the Lewis structure for the water molecule, $H_2O$ .

Step 1: Count valence electrons. Oxygen is in Group 16, so it has 6. Hydrogen is in Group 1, so it has 1, and we have two of them. Total = 6 + 2(1) = 8 valence electrons. Step 2: Choose the central atom. Hydrogen can never be in the center, so oxygen goes in the middle. Connect each H to O with a single bond. That uses 4 electrons (2 per bond). Step 3: We have 4 electrons left. Hydrogen's shell is full with 2 electrons (which it gets from the bond), so we place the remaining 4 electrons as two lone pairs on the oxygen atom. Step 4: Check. Oxygen now has 8 electrons (4 from bonds, 4 from lone pairs), and each hydrogen has 2. The structure is complete.

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Where this comes from: OpenStax Chemistry 2e, Chapter 7: Chemical Bonding and Molecular Geometry · Khan Academy, Unit: Chemical bonds (Lewis structures)