Study Notes for Exam on Chemical Bonding and Molecular Geometry

Covalent Bonds vs. Ionic Bonds

• Electronegativity and Bond Formation

• Elements form covalent or ionic bonds based on electronegativity differences.

  • Covalent Bonds:

  • Occur between nonmetals with similar electronegativities.

  • Electrons are shared, leading to stable molecules.

  • Ionic Bonds:

  • Form between metals and nonmetals with significantly different electronegativities.

  • Electrons are transferred from one atom to another, creating charged ions.

Binary Molecular Compounds

• Naming and Formulas

• Binary molecular compounds are named using prefixes that indicate the number of atoms.

  • Examples:

  • CO = Carbon monoxide

  • CO₂ = Carbon dioxide

• Prefix Rules:

  • "Mono-" is never used for the first element.

Lewis Structures

• Drawing Lewis Structures

• Follow these steps:

  1. Count valence electrons.

  2. Arrange atoms considering the octet rule.

  3. Use lines to represent bonds and dots for lone pairs.

• Examples of issues:

  • Expanded octet for elements in the third period or beyond.

  • Resonance structures for molecules with delocalized electrons.

Molecular Geometry

• Electronic Geometry vs. Molecular Geometry

• Electronic Geometry: Based on the number of electron domains (bonding and lone pairs) around the central atom.

• Molecular Geometry: Defined by the arrangement of atoms after accounting for lone pairs.

  • Examples:

  • Tetrahedral, trigonal bipyramidal, and octahedral shapes.

Polarity of Molecules

• Determining Polar vs. Nonpolar

• Analyze lone pair and bond dipole distributions.

  • If dipoles cancel, the molecule is nonpolar; if not, it is polar.

• Net Dipole Moment: Evaluates the strength and direction of the overall polar character of the molecule.

Intermolecular Forces (IMF)

• Types and Strengths

• London Dispersion Forces (LDF):

  • Present in all molecules; stronger in larger/nonpolar molecules due to induced dipoles.

• Dipole-Dipole Forces: Occur between polar molecules with permanent dipoles.

• Hydrogen Bonding:

  • A strong type of dipole-dipole interaction occurs specifically when hydrogen is bonded to N, O, or F.

Empirical and Molecular Formulas

• Definitions

• Empirical Formula: Simplest integer ratio of elements.

• Molecular Formula: Actual number of atoms in a molecule.

• Calculating Empirical Formula: From mass composition data, convert mass to moles and simplify to lowest whole number ratios.

Exam Preparation Tips

• Types of Questions:

• Multiple choice, sorting, numerical entry, passage-based.

• Written response example: Drawing Lewis structure, naming the compound, predicting geometry and polarity.

• Key Concepts to Review:

• Octet rule, resonance, types of intermolecular forces, properties influenced by intermolecular forces (boiling/melting points), and the relationship between solubility and ion-dipole interactions.

• Important Exceptions and Points:

• Prefixes in naming, non-usage in ionic compounds, and consideration of larger molecules' polar regions (e.g., when comparing alcohol to hydrocarbons).

Dipoles refer to the separation of positive and negative charges in a molecule, resulting in a dipole moment. A dipole moment is a vector quantity that measures the polarity of a molecule, indicating the direction and strength of the charge separation. In polar molecules, dipoles arise when there is an uneven distribution of electrons due to differences in electronegativity between bonded atoms. For example, in a water molecule (H₂O), oxygen is more electronegative than hydrogen, causing the electrons to be pulled closer to the oxygen atom, which creates a dipole with a partial negative charge on oxygen

Practice Test

Questions:

1. Why do some elements form covalent bonds while others form ionic bonds?
   a) Because of their size
   b) Because of their color
   c) Because of their electronegativity differences
   d) Because of their temperature

2. What is the proper use of "mono" in naming compounds?
   a) It's always used for the first element
   b) It's never used for the first element
   c) It's only used for metals
   d) It's only used for nonmetals

3. What is the octet rule in chemistry?
   a) Atoms want 8 protons
   b) Atoms want 8 neutrons
   c) Atoms want 8 electrons in their outer shell
   d) Atoms want 8 total particles

4. What determines the electronic geometry of a molecule?
   a) The number of protons
   b) The number of neutrons
   c) The number of electron domains around the central atom
   d) The temperature of the molecule

5. How can you tell if a molecule is polar or nonpolar?
   a) By its color
   b) By its size
   c) By analyzing the distribution of lone pairs and bond dipoles
   d) By its taste

6. What are London Dispersion Forces?
   a) Forces between planets
   b) Forces in the city of London
   c) Weak forces between nonpolar molecules
   d) Strong forces between polar molecules

7. What is a key difference between dipole-dipole forces and hydrogen bonding?
   a) Hydrogen bonding is generally stronger
   b) Dipole-dipole forces only occur in water
   c) Hydrogen bonding only occurs between nonpolar molecules
   d) Dipole-dipole forces are always stronger than hydrogen bonding

8. What accounts for the solubility of some substances in water?
   a) London dispersion forces
   b) Ion-dipole interactions
   c) Covalent bonds
   d) Nuclear forces

9. How can intermolecular forces affect a substance's properties?
   a) They can change its color
   b) They can affect its boiling and melting points
   c) They can change its atomic number
   d) They have no effect on a substance's properties

10. What is resonance in chemistry?
    a) The sound a molecule makes
    b) The color of a molecule
    c) Multiple valid Lewis structures for the same molecule
    d) The temperature of a molecule

11. Which of the following best explains why HF has a higher boiling point than HCl, despite Cl being more electronegative than F?
    a) HF has stronger London dispersion forces
    b) HF forms stronger hydrogen bonds
    c) HCl is a larger molecule
    d) HF has a lower molecular mass

12. Which of the following molecules would you expect to have the highest dipole moment?
    a) CCl₄
    b) CHCl₃
    c) CH₂Cl₂
    d) CH₃Cl

13. Which of the following best explains why noble gases have such low boiling points?
    a) They have strong covalent bonds
    b) They have weak London dispersion forces
    c) They have strong ionic bonds
    d) They have strong dipole-dipole interactions

14. Which of the following would you expect to have the highest surface tension?
    a) Ethanol
    b) Water
    c) Hexane
    d) Diethyl ether

Answer Key:

• (1) c

• (2) b

• (3) c

• (4) c

• (5) c

• (6) c

• (7) a

• (8) b

• (9) b

• (10) c

• (11) b

• (12) d

• (13) b

• (14) b