LEC 34 (8.3-8.4)

Key Concepts

  • Lewis Structures and Ionic Bonds

    • Lewis structures describe the number of valence electrons in an atom.

    • Atoms strive to have 8 electrons (octet rule) or resemble the nearest noble gas when ionizing.

    • Lattice energy varies with charge difference and atomic radius.

  • Practice Problem: Lattice Energy

    • Determine which has the highest lattice energy: MgO, NaCl, CaO, HCl.

    • HCl is an acid (no lattice energy), Mg and Ca have larger charges. Since Mg has a smaller radius, MgO has the highest lattice energy.

  • Periodic Table Overview

    • Elements & Properties: Hydrogen (H), Lithium (Li), Sodium (Na), Magnesium (Mg), etc.

    • Summary of element groups: Alkali metals, Alkaline earth metals, Transition metals, etc.

  • Electronegativity

    • Different atoms have distinct electronegativities, affecting electron sharing.

    • Electronegativity increases across a period (left to right) and decreases down a group (top to bottom).

    • Relationship between differences in electronegativity:

    • Nonpolar covalent (0 difference)

    • Polar covalent (0 < difference < 1.7)

    • Ionic (difference ≥ 1.7)

  • Covalent Bonds

    • Bonds formed by sharing pairs of electrons fulfill atom requirements, stabilizing them.

    • Atoms form single, double, or triple bonds based on shared electron pairs.

  • Dipole Moments

    • Molecules with partial charges (due to unequal sharing of electrons) are called dipoles.

    • The strength of partial charges and distance between them is quantified as the dipole moment.

  • Molecular Geometry

    • Examples of Lewis structures:

    • H2O: H-O-H

    • NH3: H-N-H, with an H atop forming a pyramid shape.

Practice Problems and Solutions

  • Lewis Structures Examples:

    • Draw the Lewis structure for H2O and NH3.

  • Energy Required to Break Bonds:

    • Question: Which bond requires more energy to break - single or triple bonds?

    • Answer: Triple bonds. As bond length decreases, bond strength increases.

  • Polarity of Bonds

    • Challenge: Identify the most polar bond among CO, PO, OF, and CP.

    • Analysis: Calculate differences in electronegativity; PO has the highest difference (1.4), making it the most polar.

Takeaways

  • Quaternary structures enable complex functions greater than tertiary structures.

  • Hemoglobin employs cooperative binding, while myoglobin uses non-cooperative binding.

Preparation for Next Class

  • Be proficient in using Lewis structures.

  • Understand the prediction of polarity versus nonpolarity in molecules.

  • Reflect on drawing molecular structures formally to consolidate understanding.