GenChem-ch09-lecture-slides

Periodicity and Ionic Bonding

Chapter Overview

  • Chapter 9: Periodicity and Ionic Bonding outlines essential concepts in chemistry, particularly in relation to electron configuration, atomic and ionic sizes, and ionization energies.

Chapter Structure

  • Section 9.1: Valence Electrons

  • Section 9.2: Atomic and Ionic Sizes

  • Section 9.3: Ionization Energy and Electron Affinity

  • Section 9.4: Ionic Bonding

  • Section 9.5: Lattice Energy

Section 9.1: Valence Electrons

Electron Configurations

  • Valence electrons are the highest occupied energy level electrons, crucial for bonding.

  • Group 1A (1) Metals: 1 outer s electron.

  • Group 2A (2) Metals: 2 outer s electrons.

  • All other electrons serve as core electrons.

Characteristics of Valence Electrons

  • Periodicity of chemical properties often arises from similar numbers of valence electrons.

  • Main Group Elements: Number of valence electrons equals the group number labeled "A."

  • Elements with the same electron configurations in outer energy levels exhibit similar chemical properties.

Example 9.1: Electron Configurations of Oxygen Family

  • Electron Configurations:

    • O: 1s² 2s² 2p⁴

    • S: 1s² 2s² 2p⁶ 3s² 3p⁴

    • Se: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴

    • Te: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁴

  • Shares the ns² np⁴ configuration in highest energy levels.

Section 9.2: Atomic and Ionic Sizes

Understanding Atomic Size Trends

  • Atomic Radius: Decreases across a period due to increasing positive charge attracting electrons closer to the nucleus.

  • Increases down a group due to the addition of electron shells (n).

Effective Nuclear Charge (Z_eff)

  • Z_eff is the net positive charge experienced by an electron in a multi-electron atom.

  • Calculated by: Z_eff = Z - S (where S = shielding electrons).

  • Core electrons shield valence electrons more effectively than other valence electrons do.

Cation and Anion Sizes

  • Ionic Radius:

  • Cations are smaller than their parent atoms due to loss of electrons increasing nucleus’ effective attraction on remaining electrons.

  • Anions are larger than their parent atoms due to addition of electrons that increase electron-electron repulsion.

Section 9.3: Ionization Energy and Electron Affinity

Ionization Energy (IE)

  • IE is the energy required to remove an electron from an atom in the gas phase.

  • Generally increases across a period and decreases down a group.

  • Second ionization energies are always higher than the first due to increased positive charge on the ion.

Electron Affinity (EA)

  • EA is the energy change associated with adding an electron to a neutral atom in the gas phase.

  • Most elements have negative electron affinity values indicating that energy is released when forming anions (exothermic).

Section 9.4: Ionic Bonding

Ionic Bond Formation

  • Metals transfer electrons to nonmetals leading to the formation of cations and anions that attract each other (ionic bonds).

  • Example: Sodium transfers an electron to chlorine, forming Na⁺ and Cl⁻.

Ionic Compounds Structure

  • Ionic compounds exist in large three-dimensional lattices rather than individual ion pairs.

  • Chemical formulas show the simplest whole-number ratio of ions present.

Section 9.5: Lattice Energy

Lattice Energy Concept

  • Lattice energy is the energy released when gaseous ions form a solid ionic compound.

  • Determined using Hess’s law and the Born-Haber cycle, which include reactions such as ionization energy and electron affinity.

Trends in Lattice Energy

  • Lattice energy increases with higher ionic charges and decreases as ionic sizes increase.

  • The equation for lattice energy: E_el = k(q1*q2)/d, where q1 and q2 are ionic charges and d is bond length.