# Lewis Structures, Formal Charge and Resonance

Chemistry I · Atoms, Bonds and Reaction Rates · https://tryals.app/learn/chemistry-i/lewis-structures-formal-charge-and-resonance

## Bookkeeping for Electrons

A **Lewis structure** accounts for every valence electron as either a bonding pair or a lone pair. The procedure is mechanical:

1. Count all valence electrons, adding one per negative charge and subtracting one per positive charge.
2. Join the atoms with single bonds, putting the least electronegative atom (never hydrogen) at the centre.
3. Complete octets on the outer atoms with lone pairs.
4. Any electrons left go on the central atom.
5. If the centre is short of an octet, convert lone pairs from neighbours into double or triple bonds.

**Formal charge** then tests whether the drawing is sensible:

$$FC = V - N - \tfrac{1}{2}B$$

where $V$ is the free atom's valence electrons, $N$ the nonbonding electrons on it, and $B$ the bonding electrons around it. The best structure keeps formal charges as close to zero as possible and puts any negative formal charge on the most electronegative atom.

Some molecules cannot be drawn correctly at all with one structure. Ozone, $\mathrm{O_3}$, appears to have one single and one double bond, yet both bonds are measurably identical at 128 pm, between a single (148 pm) and a double (121 pm). The truth is a **resonance hybrid**: the real molecule is a single averaged structure, not a rapid flicker between drawings.

Three families break the octet rule. **Incomplete octets** occur in $\mathrm{BF_3}$ (boron with 6) and $\mathrm{BeCl_2}$. **Odd-electron species** such as NO cannot pair everything. **Expanded octets** appear from period 3 downward, where $\mathrm{PCl_5}$ and $\mathrm{SF_6}$ hold 10 and 12 electrons.

> **Common pitfall:** reading resonance as oscillation. The molecule does not spend half its time in each drawing. There is one structure, and the separate drawings are a limitation of a notation that insists bonds be whole numbers.

## Practice questions

7 of this lesson's 12 practice questions, with answers. The full set is in the app.

### 1. How many valence electrons must a Lewis structure of the carbonate ion, $\mathrm{CO_3^{2-}}$, account for in total?

**Answer:** 24

**Why:** $4 + 3 \times 6 = 22$ from the atoms, plus 2 for the $2-$ charge, giving **24 valence electrons** to place.

Page: https://tryals.app/practice/chemistry-i/lewis-structures-formal-charge-and-resonance/how-many-valence-electrons-must-a-lewis-structure-of-the-carbonate

### 2. Resonance structures are an artifact of chemical notation rather than physical transitions. What does this distinction imply when evaluating the electronic structure of the ozone molecule?

A. The central atom alternates between single and double bondings
B. The true geometry oscillates between two discrete forms rapidly
C. The two terminal oxygen atoms exchange their charges over time
D. The molecule exists permanently as an intermediate hybrid state

**Answer:** D. The molecule exists permanently as an intermediate hybrid state

**Why:** Treating resonance as a dynamic shift misinterprets Lewis drawings as distinct physical states. In reality, delocalised electron density is static across the framework, meaning fractional bond orders are genuine, invariant physical properties rather than time-averaged transitions.

Page: https://tryals.app/practice/chemistry-i/lewis-structures-formal-charge-and-resonance/resonance-structures-are-an-artifact-of-chemical-notation-rather-than

### 3. Complete the account of what the ozone bond lengths tell us.

**Answer:** Both bonds in ozone measure 128 pm, which lies **between** a single bond (148 pm) and a double bond (121 pm). Because the two bonds are **identical**, no drawing with one of each can be literally true: the real molecule is a single **resonance hybrid** rather than a structure that **alternates** between two forms.

**Why:** The measured length sits between the single and double values, and crucially the two bonds are identical. A molecule alternating between drawings would still have one long and one short bond at every instant, so the truth is one averaged hybrid that whole-number bond notation simply cannot express.

Page: https://tryals.app/practice/chemistry-i/lewis-structures-formal-charge-and-resonance/complete-the-account-of-what-the-ozone-bond-lengths-tell-us

### 4. Which species genuinely break the octet rule, rather than merely looking unusual?

A. $\mathrm{SF_6}$, with twelve electrons on sulfur
B. $\mathrm{BF_3}$, with six electrons on boron
C. $\mathrm{CH_4}$, with eight electrons on carbon
D. NO, with an odd number of electrons

**Answer:** A. $\mathrm{SF_6}$, with twelve electrons on sulfur; B. $\mathrm{BF_3}$, with six electrons on boron; D. NO, with an odd number of electrons

**Why:** BF3 is an incomplete octet, SF6 an expanded one, and NO has an unpairable odd electron. Methane obeys the octet rule perfectly and is not an exception at all.

Page: https://tryals.app/practice/chemistry-i/lewis-structures-formal-charge-and-resonance/which-species-genuinely-break-the-octet-rule-rather-than-merely

### 5. Put the steps of drawing a Lewis structure into their correct order.

**Answer:**

1. Count all valence electrons, adjusting for charge
2. Connect the atoms with single bonds
3. Complete the octets of the outer atoms
4. Place any remaining electrons on the central atom
5. Form multiple bonds if the central atom lacks an octet

**Why:** You must know the electron budget first, then build a skeleton, satisfy the outer atoms, park the remainder centrally, and only then convert lone pairs into multiple bonds to fix a short-changed centre.

Page: https://tryals.app/practice/chemistry-i/lewis-structures-formal-charge-and-resonance/put-the-steps-of-drawing-a-lewis-structure-into-their-correct-order

### 6. Match each molecule to the way it departs from a simple octet structure.

**Answer:**

- $\mathrm{BF_3}$ → Incomplete octet on the central atom
- $\mathrm{PCl_5}$ → Expanded octet on the central atom
- NO → Odd total electron count
- $\mathrm{O_3}$ → Needs resonance rather than one structure

**Why:** Boron falls short at six, phosphorus exceeds at ten, NO has an unpairable odd electron, and ozone satisfies the octet rule but requires a resonance hybrid to describe honestly.

Page: https://tryals.app/practice/chemistry-i/lewis-structures-formal-charge-and-resonance/match-each-molecule-to-the-way-it-departs-from-a-simple-octet

### 7. Sort each central atom by whether it can hold more than eight valence electrons.

**Answer:**

- Capped at eight: Nitrogen, Oxygen, Carbon
- Can expand beyond eight: Phosphorus, Sulfur, Chlorine

**Why:** Period 2 elements have only $2s$ and $2p$ available and are capped at eight, which is why NF5 does not exist. From period 3 the atoms are larger and can accommodate more, giving real compounds such as $\mathrm{PCl_5}$ and $\mathrm{SF_6}$.

Page: https://tryals.app/practice/chemistry-i/lewis-structures-formal-charge-and-resonance/sort-each-central-atom-by-whether-it-can-hold-more-than-eight-valence