General Chemistry Lecture Notes

Introduction to General Chemistry

  • Focus on bonding and chemical interactions.

  • Importance of understanding this chapter for advancing into organic chemistry.

  • Brief mention of recorded lectures and study materials available.

  • The General Chemistry textbook follows a predictable and sequential algorithm to teach concepts, starting from atomic structure and the periodic table, to how atoms react and bond.

Atomic Structure and Chemical Bonds

  • Atoms bond to form molecules held together by chemical bonds.

  • Chemical bonds are founded on the octet rule, which states that atoms prefer to bond in ways that fill their valence shell (achieving stability).

  • Example analogy: Just as living in a comfortable apartment provides security, completing the valence shell gives atoms a sense of stability and satisfaction.

Octet Rule Exceptions

  1. Incomplete Octets

    • Some atoms are stable with fewer than eight valence electrons:

      • Examples: Hydrogen, Helium, Lithium, Beryllium, and Boron (first five elements).

  2. Expanded Octets

    • Atoms like phosphorus, sulfur, and chlorine can have more than eight valence electrons:

      • Max electron counts: Phosphorus (10), Sulfur (12), Chlorine (14).

      • Phosphorus is in the same group as nitrogen and can utilize d orbitals due to being in the third period of the periodic table, allowing it to accommodate additional bonds.

      • Nitrogen's electron configuration can be written as:
        1s^2 \, 2s^2 \, 2p^3

    • VSEPR Theory determines the shapes of molecules based on electron pair repulsion.

    • Example of nitrogen bonding: Ammonia ( NH_3 ) is trigonal pyramidal due to a lone pair pushing down on the bonding pairs.

  3. Odd-Electron Molecules

    • Molecules like Nitrous Oxide (N₂O) that have an odd number of valence electrons do not follow the octet rule as they cannot evenly distribute all valence electrons.

Bond Types and Characteristics

  • Electronegativity Difference: Refers to the difference in electronegativity between bonding atoms. A significant difference (≥ 1.7) indicates ionic bonding.

  • Types of Bonds:

    • Covalent Bonds: Sharing of electrons, can be equal (nonpolar) or unequal (polar).

    • Coordinate Covalent Bonds: Both shared electrons originate from one atom, common in Lewis acid-base reactions.

Types of Ionic Interactions

  • Ion-Ion Attractions: The strongest due to the electrostatic forces between charged ions.

  • Intermolecular forces discussed include:

    • London Dispersion Forces (LDF): Temporary dipoles induced in nonpolar molecules due to electron movement.

    • Dipole-Dipole Interactions: Occur between polar molecules due to their permanent dipoles.

    • Hydrogen Bonds: Strong type of dipole-dipole interaction requiring H atoms bonded to F, O, or N.

Properties of Bonds

  • Bond Order: The number of shared electron pairs between two atoms (i.e., single, double, triple bonds).

  • Incident Formulaic Measures:

    • Electronegativity Difference for Polarity:

      • Nonpolar: 0 < x < 0.5

      • Polar: x \, ≥ 0.5

  • Dipole Moments: Quantified in Debye (D). Represented with arrows pointing towards the more electronegative atom.

pKa and Acid Strength

  • Strong Acids fully dissociate in solution; e.g., Hydrochloric acid (HCl). Acetic acid (CH₃COOH) does not fully dissociate, indicated by its low K_a (1.8 x 10⁻⁵) and a pKa of approx. 4.75.

  • Discussion on the structure of hydrogen bonds in biological systems and how they contribute to molecular folding and function.

Molecular Geometry

Valence Shell Electron Pair Repulsion (VSEPR) Theory

  • Electron pairs repel, resulting in specific geometrical arrangements:

    • Linear Geometry: 180° angle in molecules like BeCl₂.

    • Trigonal Planar Geometry: 120° angles in molecules like BH₃.

    • Tetrahedral Geometry: Has a 109.5° angle with four electron domains, as in CH₄.

    • Trigonal Pyramidal: NH₃ has three bond pairs and one lone pair, influencing its shape.

    • Bent Shape: Water (H₂O) has two bond pairs and two lone pairs, creating a bent molecular structure.

    • Trigonal Bipyramidal and Octahedral: Molecular shapes resulting from five and six electron domains, respectively, with characteristic angles (120° and 90°).

Conclusion

  • Overview of discussed bonding concepts, exceptions to rules, and their real-world applications in fields such as biochemistry and material science.

  • Acknowledgment of complexities involved and encouragement to review and understand all discussed materials in preparation for advanced topics in chemistry.