AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/2/2-5)
Unit 2 · Topic 2.5
2.5 Lewis Diagrams
A Lewis diagram shows every valence electron in a molecule or ion as bonding pairs (shared between atoms) and lone pairs (on one atom). You build one by counting valence electrons, connecting atoms with single bonds and placing the rest so atoms usually reach an octet.
Key terms
- Lewis diagram
- valence electrons
- bonding pair
- lone pair
- octet rule
- double bond
What a Lewis diagram shows
Each line between atoms is a bonding pair: 2 shared electrons. A double bond is two lines (4 electrons) and a triple bond is three (6 electrons). Dots on a single atom are lone pairs: nonbonding electrons that belong to that atom alone.
Lewis diagrams show which atoms are connected and where electrons are, but on their own they don't show 3-D shape. You'll add shape with VSEPR in topic 2.7.
Step by step
- Count the total valence electrons: use the group number for main-group atoms. Add one electron for each negative charge; subtract one for each positive charge.
- Choose the central atom: usually the least electronegative atom that isn't H. Hydrogen is always on the outside with one bond.
- Connect the central atom to each outer atom with a single bond (2 electrons each).
- Give outer atoms (except H) lone pairs until each has 8 electrons around it.
- Put any leftover electrons on the central atom as lone pairs.
- If the central atom has fewer than 8 electrons, move a lone pair from an outer atom into a double or triple bond.
- Check: the total electrons drawn must equal your count. Put brackets and the charge around an ion.
Typical bonding patterns
In neutral molecules, the most common elements usually form the same number of bonds. Use this table to spot mistakes quickly.
| Atom | Usual bonds | Usual lone pairs |
|---|---|---|
| H | 1 | 0 |
| C | 4 | 0 |
| N | 3 | 1 |
| O | 2 | 2 |
| F, Cl, Br, I (as outer atoms) | 1 | 3 |
Exceptions to the octet rule
The octet rule is a guide, not a law. Know three kinds of exceptions.
| Exception | What happens | Examples |
|---|---|---|
| Fewer than 8 | Boron and beryllium are often stable with 6 or 4 electrons around them | BF₃, BeCl₂ |
| More than 8 | Atoms in period 3 and below can hold more than 8 electrons (an expanded octet) | PCl₅ (10 around P), SF₆ (12 around S), XeF₄ |
| Odd number of electrons | One atom must have an unpaired electron | NO, NO₂ |
Why it matters
Almost everything later in Unit 2 and Unit 3 starts with a correct Lewis diagram: formal charge, resonance, shape, polarity, hybridization and even which intermolecular forces a substance has. A wrong count of valence electrons at the start breaks everything after it, so always do the count first and check it at the end.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
A molecule with a double bond
Draw the Lewis diagram for formaldehyde, CH₂O.
Show the solutionHide the solution
- Step 1: Count: C = 4, each H = 1 (×2 = 2), O = 6. Total = 12 valence electrons.
- Step 2: Carbon is the central atom (H must be on the outside, and C is less electronegative than O). Connect C to O and to each H with single bonds: 3 bonds use 6 electrons, leaving 6.
- Step 3: Give O three lone pairs (6 electrons). Now all 12 are used, but carbon has only 6 electrons around it.
- Step 4: Move one lone pair from O into a second C–O bond. Now C has 8 (two single bonds and one double bond), O has 8 (a double bond and two lone pairs), and each H has 2.
Answer: H₂C=O, with two lone pairs on oxygen and no lone pairs on carbon (12 electrons total).
- Example 2
Counting electrons in an ion (classic trap)
How many valence electrons are in the Lewis diagram of the carbonate ion, CO₃²⁻? Describe one valid diagram.
Show the solutionHide the solution
- Step 1: Count: C = 4, three O = 18, plus 2 for the 2− charge. Total = 24. Forgetting the charge gives 22, which can't make a valid structure.
- Step 2: Connect C to each O with a single bond (6 electrons, 18 left). Give each O three lone pairs (18 electrons, 0 left).
- Step 3: Carbon has only 6 electrons, so turn one O lone pair into a C=O double bond.
- Step 4: Result: one C=O and two C–O, with the singly bonded O atoms carrying three lone pairs each, the doubly bonded O carrying two, and brackets with a 2− charge.
Answer: 24 valence electrons; one C=O and two C–O bonds in brackets with a 2− charge (topic 2.6 shows this is one of three resonance structures).
- Example 3
An expanded octet
Draw the Lewis diagram for XeF₄.
Show the solutionHide the solution
- Step 1: Count: Xe = 8, four F = 28. Total = 36.
- Step 2: Xe is central. Four Xe–F single bonds use 8 electrons, leaving 28.
- Step 3: Each F gets three lone pairs: 4 × 6 = 24 electrons, leaving 4.
- Step 4: Put the last 4 electrons on Xe as two lone pairs. Xe now has 12 electrons around it, which is allowed because Xe is in period 5.
Answer: Xe bonded to four F atoms with two lone pairs on Xe; each F has three lone pairs.
Common mistakes
- Forgetting to add or subtract electrons for an ion's charge.
- Putting hydrogen in the middle or giving it two bonds. Hydrogen makes only one bond.
- Giving the central atom an expanded octet when it's in period 2. C, N, O and F never have more than 8 electrons around them.
- Drawing the right number of bonds but losing track of the total electron count. Count every dot and line at the end.
On the exam
- Free-response questions often ask you to complete a Lewis diagram for a molecule or ion, then use it to answer something else, such as shape, bond angle or polarity. Show all lone pairs; a missing lone pair can change the shape.
- Draw lone pairs on outer atoms too. A diagram that leaves them off is usually marked incomplete.
Connected topics
Videos
Check yourself
4 questions on 2.5 Lewis Diagrams. Pick an answer to see if you got it, and why.
Formaldehyde, CH₂O, has a central carbon atom bonded to two hydrogen atoms and one oxygen atom. In the Lewis diagram in which every atom except hydrogen has an octet, how many lone pairs of electrons are there in the molecule?
The Lewis diagram of which of the following molecules contains a triple bond?
In which of the following molecules does the central atom have fewer than eight valence electrons around it in its Lewis diagram?
Hydroxylamine, NH₂OH, has a nitrogen atom bonded to two hydrogen atoms and to an oxygen atom, which is bonded to the remaining hydrogen atom. In its Lewis diagram, how many bonding pairs and lone pairs are there in all?
0 of 4 answered