7.5 Periodic Trends

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    Atomic Radius

    atomic radius trend

    Effective Nuclear Charge

    Zeff = Z – S              Z = # protons            S = # shielding (screening) electrons

    effective nuclear charge trend

    Bond Length

    The bond length is equal to the sum of the atomic (covalent bond) or ionic (ionic bond) radii.

    Ionic Radii

    ionic radii

    Isoelectronic Series

    S2- > Cl- > Ar > K+ > Ca2+

    Ionization Energy

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

    ionization energy trend

    Exceptions in the Ionization Energy Trend

    ionization energy exceptions

    Successive Ionization Energies (2nd, 3rd, 4th, etc.)

    Core electrons have SIGNIFICANTLY higher ionization energies than valence electrons.

    Ionization Energies of Boron
    B(g) → B+(g) + e- I.E.1 = 801 kJ/mol
    B+(g) → B2+(g) + e- I.E.2 = 2,427 kJ/mol
    B2+(g) → B3+(g) + e- I.E.3 = 3,660 kJ/mol
    B3+(g) → B4+(g) + e- I.E.4 = 25,026 kJ/mol

    Electron Affinity

    Electron affinity is the energy change for an atom in the gas phase gaining an electron.

    electron affinity trend

    Successive Electron Affinities

    The 1st electron affinity for most elements is negative, but any additional successive electron affinities are always positive.

    Electron Affinities of Oxygen
    O(g) + e- → O-(g) E.A.1 = -141 kJ/mol
    O-(g) + e- → O2-(g) E.A.1 = +744 kJ/mol