#Molecular Structures: Lewis Diagrams

Hey there, future AP Chem pro! 👋 Let's dive into Lewis diagrams – your key to visualizing molecules and predicting their behavior. This is a major topic, so let's get it down!

#What are Lewis Diagrams?

Lewis diagrams (or Lewis structures) are visual tools that show how atoms bond together. They display valence electrons and bonds, helping us understand molecular shapes and reactivity. Think of them as the blueprints of molecules! ⚛️

• Based on the octet rule: Atoms want a full valence shell (usually 8 electrons).
• Localized electron model: Electrons stay put (no free roaming).
• Electrons are either:
  • Lone pairs: Electrons around a single atom, not shared.
  • Bonding pairs: Electrons shared between atoms (represented by a dash).

#Drawing Lewis Dot Structures (LDS)

#Ionic LDSs

Ionic LDSs are straightforward because electrons are transferred from one atom to another. 🎉

1. Write the empirical formula to identify elements and their quantities.
2. Draw each atom with its valence electrons.
3. Transfer electrons from the metal to the nonmetal to achieve full octets.
4. Use brackets around each ion with charges outside.

#Example: NaBr

1. Formula: NaBr (one Na and one Br).
2. Na has 1 valence electron; Br has 7. 3. Transfer 1 electron from Na to Br.
3. Show charges: [Na]⁺ [Br]^-

Try it: Draw the LDS for magnesium chloride (MgCl₂). Answer at the end!

#Covalent LDSs

Covalent bonds involve sharing electrons, making their LDSs a bit more involved.

1. Count the total valence electrons in the molecule.
2. Draw the central atom (usually C or the least electronegative atom, not H).
3. Connect all atoms with single bonds.
4. Add lone pairs to outer atoms to complete octets.
5. Count the electrons used so far.
6. If too many electrons, form double/triple bonds by erasing lone pairs.
7. If too few electrons, add to the central atom (see exceptions).

#⭐Exceptions to the Octet Rule

• Incomplete octets: H (max 2 e-), Be (max 4 e-), B (max 6 e-).
• Expanded octets: Central atoms with atomic number ≥ 14 can have more than 8 electrons.
• Odd numbers of electrons: Some molecules can't have all paired electrons.
• Multiple bonds: Double or triple bonds form when single bonds aren't enough.

#Examples of Lewis Structures

Let's walk through some examples step-by-step.

#1) Draw a Lewis dot structure for O₂.

• O is in group 16, so each O has 6 valence electrons. Total: 12. - Draw two O atoms side by side.
• Connect with a single bond (2 electrons).
• Add lone pairs to complete octets.

• Too many electrons! Form a double bond.

• Final: 12 valence electrons, octets are satisfied.

Try it: Draw the LDS for N₂. Answer at the end!

#2) Draw a Lewis diagram for CS₂.

• C has 4 v.e., S has 6 v.e. Total: 16. - C is the central atom.
• Connect with single bonds.
• Add lone pairs.

• Too many electrons! Form double bonds.

#3) Draw the Lewis diagram for XeF₂.

• Xe has 8 v.e., F has 7 v.e. Total: 22. - Xe is the central atom.
• Connect with single bonds.
• Add lone pairs.

• Too few electrons! Xe can break the octet rule.

Try it: Draw the LDS for the polyatomic ion NH₄⁺.

#Answers to Practice Questions

#1) Draw the LDS for Magnesium chloride

• MgCl₂: Mg²⁺ and two Cl^- ions.

#2) Draw the LDS for N₂.

• Triple bond needed to satisfy the octet rule.

#3) Draw the LDS for the polyatomic ion NH₄⁺.

• Total valence electrons: 8 (9 - 1 for the +1 charge).

Keep practicing, and you'll be a Lewis diagram master in no time! 😄

Practice Question

Multiple Choice Questions

1. Which of the following molecules has a central atom with an expanded octet? (A) BF₃ (B) SF₄ (C) CO₂ (D) NH₃

2. How many bonding pairs of electrons are in the Lewis structure of carbon dioxide (CO₂)? (A) 2 (B) 4 (C) 6 (D) 8

3. What is the correct Lewis structure for the nitrate ion (NO₃^-)? (A) A structure with one single bond and two double bonds (B) A structure with three single bonds (C) A structure with three resonance structures, each with one double bond and two single bonds (D) A structure with a central nitrogen atom that has 10 valence electrons

Free Response Question

Draw the Lewis structures for the following molecules and ions. Clearly show all bonding and nonbonding valence electrons. For each structure, indicate the number of bonding pairs, lone pairs, and the total number of valence electrons.

(a) Hydrogen cyanide (HCN) (b) The carbonate ion (CO₃^2-) (c) Sulfur hexafluoride (SF₆)

Scoring Rubric:

(a) (3 points) - 1 point for correct skeletal structure (H-C-N). - 1 point for correct placement of bonding and nonbonding electrons. - 1 point for indicating 1 triple bond and 1 single bond and correct lone pairs.

(b) (3 points) - 1 point for correct skeletal structure (C in the center, three O's around it). - 1 point for correct placement of bonding and nonbonding electrons (including double bond). - 1 point for showing the correct charge on the ion and correct resonance structures.

(c) (3 points) - 1 point for correct skeletal structure (S in the center, six F's around it). - 1 point for correct placement of bonding and nonbonding electrons. - 1 point for showing an expanded octet on the sulfur atom.