Chemical Bonding and Shapes of Molecules – Complete Notes

Question: How do you represent atoms and ions using the Lewis dot method? Provide examples.

Answer: The Lewis dot method (electron dot structure) represents the valence shell electrons of an atom as dots surrounding the chemical symbol.

Atoms: The group number (1-8 for main groups) usually tells you the number of valence electrons.

Ions: Cations (positive ions) lose valence electrons (often showing no dots or the next shell down, usually written with brackets and charge). Anions (negative ions) gain electrons to complete their octet (usually showing 8 dots, brackets, and charge).

Examples:

• Sodium Atom (Na): Group 1, has 1 valence electron. Symbol: Na⋅
• Chlorine Atom (Cl): Group 17 (7A), has 7 valence electrons. Symbol: :Cl¨⋅
• Sodium Ion (Na+): Lost its 1 valence electron. Symbol: [Na]+
• Chloride Ion (Cl−): Gained 1 electron to have 8. Symbol: [:Cl¨:]−

Question: What is an ionic bond and what are the characteristic properties of ionic compounds?

Answer: Ionic Bond: An ionic bond is a type of chemical bond formed through the electrostatic attraction between oppositely charged ions. This typically occurs when a metal atom completely transfers one or more electrons to a non-metal atom.

Properties of Ionic Compounds:

• Physical State: Usually crystalline solids at room temperature
• Melting/Boiling Points: High melting and boiling points
• Solubility: Generally soluble in polar solvents (like water)
• Conductivity: Conduct electricity when molten or dissolved
• Hardness and Brittleness: Hard but brittle crystals

Question: Define covalent and co-ordinate bonds, and list the properties of covalent compounds.

Answer: Covalent Bond: A bond formed by the mutual sharing of electron pairs between two atoms (usually non-metals).

Co-ordinate Bond (Dative Bond): A special type of covalent bond where both electrons in the shared pair come from the same atom.

Properties of Covalent Compounds:

• Physical State: Can be gases, liquids, or soft solids
• Melting/Boiling Points: Generally low
• Conductivity: Poor conductors of electricity
• Solubility: Generally insoluble in water but soluble in organic solvents

Question: What are the distinguishing features of Sigma (σ) and Pi (π) bonds?

Answer:

Sigma (σ) Bond:

• Formed by head-on overlap along internuclear axis
• Stronger bond due to maximum overlap
• Free rotation possible
• Exists independently (every single bond is sigma)

Pi (π) Bond:

• Formed by sideways overlap of parallel p-orbitals
• Weaker than sigma bonds
• Restricts rotation
• Always formed after a sigma bond (in double/triple bonds)

Question: Describe co-ordinate covalent compounds and provide examples.

Answer: Co-ordinate covalent compounds contain at least one dative bond where a lone pair from one atom is shared with an electron-deficient atom.

Examples:

1. Ammonium Ion (NH₄⁺): NH₃ donates lone pair to H⁺ → [NH₄]⁺
2. BF₃·NH₃ Adduct: NH₃ donates lone pair to electron-deficient BF₃ → F₃B←NH₃

Question: Draw/Describe the Lewis dot diagrams for typical ionic and covalent compounds.

Answer:

1. Sodium Chloride (Ionic – NaCl): [Na]⁺ [:Cl¨:]⁻

2. Methane (Covalent – CH₄): Carbon shares 1 electron with each of 4 Hydrogens

3. Carbon Dioxide (CO₂): O¨::C::O¨ (or O¨=C=O¨)

Question: What is resonance? Write resonance structures for Ozone and Carbonate ion.

Answer: Resonance occurs when a single Lewis structure cannot fully describe the bonding; the actual structure is a hybrid of multiple structures.

1. Ozone (O₃): Two resonance structures: O=O−O and O−O=O

2. Carbonate Ion (CO₃²⁻): Three resonance structures with double bond shifting between C and O atoms

Question: Explain the properties of molecular solids and metallic solids.

Answer:

Molecular Solids:

• Soft and brittle
• Low melting points
• Electrical insulators
• Examples: Ice, Dry Ice, Iodine

Metallic Solids:

• High electrical/thermal conductivity
• Malleable and ductile
• Lustrous
• Variable melting points (usually high)
• Examples: Copper, Iron

Question: Using VSEPR theory, describe the shapes of simple molecules.

Question: Describe hybridization and how it applies to sp, sp², and sp³ molecules.

Answer: Hybridization is the intermixing of atomic orbitals to form new equivalent “hybrid” orbitals.

sp Hybridization: Mixing one s and one p orbital → 2 linear orbitals (180°) → Example: BeCl₂

sp² Hybridization: Mixing one s and two p orbitals → 3 trigonal planar orbitals (120°) → Example: BF₃

sp³ Hybridization: Mixing one s and three p orbitals → 4 tetrahedral orbitals (109.5°) → Example: CH₄

Question: Explain bond characteristics: dipole moment, ionic character, and bond length.

Answer:

1. Bond Length: Average distance between nuclei. Shorter with higher bond multiplicity and smaller atoms.

2. Dipole Moment (μ): Measure of bond polarity. μ = Q × r. Zero in symmetrical molecules.

3. Ionic Character: Higher with greater electronegativity difference. >1.7 = largely ionic, <1.7 = largely covalent.

Question: What is hydrogen bonding and why is it important?

Answer: Hydrogen Bonding: Strong dipole-dipole attraction when H bonded to N, O, or F attracts lone pair of neighboring molecule.

Importance:

• High boiling point of water
• Ice floats (less dense structure)
• DNA structure (base pairing)
• Protein secondary structure
• Solubility of compounds like ethanol