Module 7 Day 1

Module 7 Overview

• Title: Periodic Properties

Periodic Properties Schedule

• Day 1 (9/23):

  • Introduction to Periodic Properties

  • Periodic Variations in Element Properties

  • Variation in Covalent Radius

  • Variation in Ionic Radii

• Day 2 (9/25):

  • Variation in Ionization Energies

  • Variation in Electron Affinities

Covalent Radii

• Definition:

  • Covalent radius = one-half the distance between the nuclei of two identical atoms joined by a covalent bond.

• Trends:

  • Down a Group:

    • Principal quantum number (n) increases

    • Electrons occupy orbitals farther from nucleus

    • Result: Atomic size & covalent radius increase

  • Across a Period:

    • Higher nuclear charge pulls electrons closer

    • Result: Atomic size & covalent radius decrease

Atomic Radius Trends

• Increases: Down a group

• Decreases: Across a period

Effective Nuclear Charge (Zeff)

• Definition: Net positive charge experienced by an electron.

  • Formula: Zeff = Z − S

    • Z = number of protons (nuclear charge)

    • S = shielding constant (core electron repulsion)

• Trend Across a Period:

  • Zeff increases as more protons are added while shielding remains nearly constant

  • Electrons are pulled closer to nucleus

• Trend Down a Group:

  • More electron shells increase shielding

  • Zeff remains relatively constant, but atomic size increases

Effective Nuclear Charge Values in Periodic Table

• Elemental Zeff:

  • Li: Zeff +1

  • Be: Zeff +2

  • B: Zeff +3

  • C: Zeff +4

  • N: Zeff +5

  • O: Zeff +6

  • F: Zeff +7

  • Actual Zeff increases in the series:

    • Li: 1.3

    • Be: 1.9

    • B: 2.4

    • C: 3.1

    • N: 3.8

    • O: 4.4

    • F: 5.1

Ionic Radius

• Cations:

  • Always smaller than neutral parent atom

  • Same number of protons but fewer electrons

  • Remaining electrons experience greater Zeff and are drawn closer to nucleus

  • Definition: Ionic radius describes size of an ion.

• Anions:

  • Larger than neutral parent atom

  • Same number of protons but more electrons

  • Greater electron repulsion increases distance from nucleus

Isoelectronic Series

• Definition: Atoms and ions that have the same electron configuration

• Size: Determined by the number of protons; more protons mean smaller radius

• Examples:

  • N3– , O2– , F– , Ne, Na+, Mg2+, and Al3+ (1s²2s²2p⁶)

  • P3– , S2– , Cl– , Ar, K+, Ca2+, and Sc3+ ([Ne]3s²3p⁶)

Trends in Atomic and Ionic Size

• Relative Sizes of Specimens:

  • Atomic and ionic sizes across groups, examples include increasing radius down a group and decreasing radius across a period.

Additional Questions and Concepts

• Effective Nuclear Charge Calculations:

  • Be explained in terms of protons and shielding e.g. Si, Cl, etc.

  • Atomic Radius Comparisons: e.g., Na vs Mg vs Al

  • Isoelectronic Comparisons: Assessment of ion sizes in series involving isoelectronic species

  • Unpaired Electrons in Fe: Understanding electron configurations.

• Valence Electrons: e.g., Determination of the number of valence electrons in Sb.