Chemistry Unit 3: Lewis Structures

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Questions and Answers

What is a Lewis structure?

Distribution of the valence electrons on the atoms being bonded together.

What are the steps to draw a Lewis structure?

Count the total number of valence electrons. 2. Join atoms symmetrically using single bonds. 3. Subtract electrons used. 4. Complete the octet on exterior atoms. 5. Subtract electrons used. 6. Place leftover electrons on the central atom. 7. If not enough electrons for the central atom, try using multiple bonds.

What is the Lewis structure of NH4+?

N=5e, H=1e x 4=4e, Total 8e. The structure must be placed in square brackets since it is an ion.

How many total electrons are present in PCl4-?

34 electrons. Signup and view all the answers

Why does CO2 have two double bonds?

To achieve a stable electronic configuration. Signup and view all the answers

What defines resonance in a molecule?

Resonance involves different structures that can represent the same molecule, where the true structure is a hybrid of these forms. Signup and view all the answers

What is formal charge?

The charge an atom would have in a molecule if bonding electrons were equally shared. Signup and view all the answers

What is the formula to calculate formal charge?

Formal Charge = Group Number - (Dashes + Dots) Signup and view all the answers

Why is formal charge important?

It helps decide between different resonance forms, preferring the one with the most zero formal charges. Signup and view all the answers

How does the stability of resonance forms affect the overall structure?

The form with the most non-zero formal charges will be the least stable and contribute less to the resonance hybrid. Signup and view all the answers

What does VSEPR theory state?

Electron pairs should stay as far away from each other as possible to determine the molecular geometry. Signup and view all the answers

What is the molecular geometry for two electron pairs?

Linear (180 degrees). Signup and view all the answers

What describes non-polar compounds in terms of geometry?

Linear AB2, Trigonal planar AB3, Square planar AB4, Tetrahedral AB4, Trigonal bipyramidal AB5, Octahedral AB6. Signup and view all the answers

Study Notes

Lewis Structure Definition

Represents the distribution of valence electrons among atoms in a molecule.

Lewis Structure Examples

Various compounds such as NH4+, PCl4-, CO2, and CO32- can be illustrated using Lewis structures.

Steps to Draw Lewis Structures

Count total valence electrons in the molecule.
Connect atoms using single bonds symmetrically.
Subtract the number of electrons used for bonds.
Complete the octet for outer atoms.
Allocate any remaining electrons to the central atom.
If the central atom lacks an octet, consider forming multiple bonds.

NH4+ Lewis Structure

Nitrogen (N) contributes 5 electrons; each of the four Hydrogens (H) contributes 1 electron.
Total valence electrons = 5 (N) + 4 (H) - 1 (positive charge) = 8 electrons.
Represent the ion within square brackets.

PCl4- Lewis Structure

Phosphorus (P) provides 5 electrons; four Chlorines (Cl) provide 28 electrons.
Adding 1 electron for the negative charge gives a total of 34 electrons.

CO2 Lewis Structure

Carbon (C) contributes 4 electrons; two Oxygens (O) contribute 12 electrons total.
Requires two double bonds instead of a single and a triple bond for stability.

Resonance Forms of CO2

CO2 shows three resonance forms, with the middle one being the most stable and important.

Resonance Concept

CO32- anion has a total of 24 electrons.
All CO bonds exhibit equal lengths due to resonance; the actual structure is a resonance hybrid.

Actual Structure of CO3-2

Represents an average of the three resonance forms.
Negative charges are delocalized over the three oxygen atoms.

Contributions from Resonance Forms

Stability of each resonance form is assessed by calculating formal charges.

Formal Charge Definition

Represents the hypothetical charge of an atom in a molecule with equally shared bonding electrons.

Calculating Formal Charge

Formula: Formal Charge = Group Number - (Bonds + Lone Pairs).
The total of all formal charges must equal the overall charge of the molecule.

Importance of Formal Charge

Used to determine the most favorable resonance structures.
Preferred resonance has the fewest non-zero formal charges; negative charges favor the most electronegative atoms.

Stability of Resonance Forms

The most stable resonance form has lower non-zero formal charges; it greatly influences the resonance hybrid representation.

Influence of Electronegativity on Resonance Stability

In scenarios where resonance forms exist, the configuration placing negative charges on the most electronegative atom is favored.

VSEPR Theory

Valence Shell Electron Pair Repulsion theory explains molecular shape based on electron pair repulsion.
Initiates with a valid Lewis structure to determine electron density regions around the central atom.

Electron Pair Geometry

2 pairs: Linear (180°).
3 pairs: Trigonal planar (120°).
4 pairs: Tetrahedral (109.5°).
5 pairs: Trigonal bipyramidal (90° and 120°).
6 pairs: Octahedral (90°).

Molecular Geometry Definition

Refers to the arrangement of atoms within a molecule.

Non-Polar Compounds Characteristics

Identified as linear (AB2), trigonal planar (AB3), tetrahedral (AB4), trigonal bipyramidal (AB5), or octahedral (AB6).
Any compound not fitting these categories is typically polar.

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Description

This quiz focuses on the basics of Lewis Structures, including definitions and steps for constructing them. It covers essential concepts and examples that will help you understand this fundamental topic in chemistry. Perfect for students looking to reinforce their knowledge of molecular bonding.