Worked example 1
Draw the Lewis structure of CO₂.
Try it first: Count the budget before drawing anything.
0 of 4 steps revealed.
Molecular StructureLewis structuresContent level: Core 22 min
Drawing Lewis Structures Step by Step
What you'll be able to do: Follow a reliable five-step procedure to draw a correct Lewis structure for a small molecule.
Best after: Valence Electrons and the Octet Rule
Introduction
Lewis structures are not guesswork. The same five steps work for water, for sulfate and for almost everything in between.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Count total valence electrons including any ionic charge
- Select a sensible central atom
- Distribute electrons to satisfy outer-atom octets first
- Convert lone pairs into multiple bonds when the centre is short
- Verify that the electron total in the finished structure is correct
Lesson
Step 1: count the electrons
Add the valence electrons of every atom, then adjust for charge: add one for each negative charge, subtract one for each positive charge. This number is a budget you must spend exactly, no more and no less.
Every later step spends from this total. If the finished drawing does not match it, the structure is wrong.
Step 2: choose the central atom
The central atom is normally the least electronegative atom that is not hydrogen. Carbon is central in CO₂ and CH₄, sulfur in SO₂ and sulfate. Hydrogen and fluorine are always terminal because they can form only one bond.
Step 3: connect with single bonds
Draw one single bond from the centre to each outer atom. Each bond uses two electrons, so subtract two per bond from the budget before moving on.
Step 4: complete the outer atoms
Place the remaining electrons as lone pairs on the outer atoms until each has an octet, hydrogen excepted. If electrons are still left after that, put them on the central atom, which is how expanded octets appear.
Step 5: form multiple bonds if needed
If the central atom still lacks an octet, pull a lone pair from an outer atom into the space between the two atoms to make a double or triple bond. The total electron count never changes when you do this, the electrons simply move.
Carbon, nitrogen and oxygen are the atoms most likely to require multiple bonds.
Key ideas
Rule
Central atom choice
Least electronegative non-hydrogen atom goes in the middle.
Rule
Outer atoms first
Satisfy terminal octets before adding electrons to the centre.
Rule
Multiple bonds fix shortages
Move an outer lone pair into a bond rather than inventing new electrons.
Key concept
CO₂
16 electrons, two C=O double bonds, four lone pairs on the oxygens.
Equation
Electron budget check
total = 2(number of bonds) + 2(number of lone pairs)
Worked examples
Worked example 2
Draw the Lewis structure of the hydronium ion H₃O⁺.
Try it first: Remember to subtract for the positive charge.
0 of 3 steps revealed.
Common mistakes
Putting hydrogen in the centre of a structure.
Why it's wrong: Hydrogen forms only one bond so it can never bridge two atoms.
Check instead: Place hydrogen on the outside, usually attached to oxygen or nitrogen.
Adding extra electrons to complete an octet.
Why it's wrong: The valence total is fixed and cannot grow during the drawing.
Check instead: Move existing lone pairs into multiple bonds instead.
Ignoring the charge when counting.
Why it's wrong: A missing or extra electron changes the whole structure.
Check instead: Write the adjusted total down before drawing.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
The procedure is fixed: count total valence electrons, choose the least electronegative atom as the centre, connect atoms with single bonds, distribute remaining electrons as lone pairs on the outer atoms first, then move lone pairs into multiple bonds if the central atom is short of an octet.
- Count the valence total and adjust for charge before drawing
- The least electronegative non-hydrogen atom is central
- Single bonds first, then lone pairs on outer atoms
- Move lone pairs into multiple bonds when the centre is short
- Check the finished drawing against the original electron budget
Sources and further reading
- Chemistry 2e, Section 7.3: Lewis Symbols and StructuresOpenStax · Rice University · 7.3View source
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