(LM15) A First Look at Periodic Trends Based on ENC

Periodic Trends Examined

  • Focus on periodic trends influenced by Effective Nuclear Charge (ENC).

  • Key trends include:

    • Atomic radius

    • Ionic radius

    • Metallic character

    • Electronegativity

Atomic Radius

  • Definition: The atomic radius refers to the size of a neutral atom, making it a widely accepted measure of atomic size.

  • Trends in atomic radius:

    • The trend resembles the graph for ENC presented in the previous module.

    • Increasing Atomic Radius:

      • Increases down the periodic table due to shielding.

    • Decreasing Atomic Radius:

      • Decreases moving right across the periodic table due to increasing ENC.

  • Overall trend: Atomic radius decreases from the lower left to the upper right of the periodic table.

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Ionic Radius

  • Definition: Ionic radius measures the size of an ion, which differs from the measurement of an atomic radius due to charge differences.

  • ENC explains trends in ionic radii.

    • The trend does not directly follow the periodic table due to the arrangement of neutral atoms.

    • Ionic Radius Trends:

      • Requires viewing the periodic table by isoelectronic species (atoms or ions with the same number of electrons and electron configurations).

      • Isoelectronic group example with 10 electrons:

        • Contains ions with the same number of electrons but different numbers of protons.

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  • Coulomb’s Law Explanation:

    • The more protons present in the nucleus, the greater the positive charge attracting the electrons, leading to a smaller ionic radius.

    • This principle applies to any isoelectronic series.

Electronegativity

  • Definition: Electronegativity is a measure of an atom's ability to attract and hold electrons in a chemical bond.

  • Trends in electronegativity:

    • Increasing Electronegativity:

      • Increases across a period (from left to right) due to increasing ENC and decreasing atomic radius, leading to a stronger attraction for bonding electrons.

    • Decreasing Electronegativity:

      • Decreases down a group due to increasing atomic size (shielding) and the greater distance between the nucleus and valence electrons, which reduces the attraction for bonding electrons.

  • The overall trend for electronegativity is that it increases from the lower left to the upper right of the periodic table (excluding noble gases, which generally do not form chemical bonds).

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  • ENC explains multiple trends with the same explanation involving shielding and ENC. All six trends below follow a diagonal up and to the right based on these concepts, including the two to be developed in the next module.

    • Atomic radius decreases up and to the right

    • Electron affinity increases up and to the right *

    • Ionization energy increases up and to the right*

    • Electronegativity increases up and to the right

    • Metallic character decreases up and to the right

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Quick sum:

  • Increasing Atomic Radius:

    • Increases down the periodic table due to shielding.

  • Decreasing Atomic Radius:

    • Decreases moving right across the periodic table due to increasing ENC.

  • ENC >1.5 = ionic