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    Atomic Electron Configurations, Chemical Periodicity

    Electron spin, Hund's Rule, Pauli exclusion principle, electron assignments, etc.

    Topic:  Atomic Electron Configurations and Chemical Periodicity

    Concept: a. Electron spin and Pauli exclusion principle

    Concept Overview:
    The addition quantum number, ms, the electon spin magnetic quantum number. This number tells the spin of the electron. It only has two values +1/2 and -1/2.

    An electron can only have two spins and each electron in an orbital must have different spins.

    Hund's Rule states when filling orbitals the orbitals of the same energy put one electron in each orbital until all orbitals are half filled then go back and pair them.

    Pauli exclusion principle - no 2 electrons in an atom can have the same 4 quantum numbers.

    Concept: b. Atomic subshell energies and electron assignments

    Concept Overview:

    Ionization energy, IE is the energy requied to remove an electron from an atom. Ionization energies generally increase across a period and decrease down a group.

    Atomic radius decreases across a period and increases down a group. It is opposite of IE.

    Electron affinity, AE is the atom's "liking" or affinity for electrons to form negative ions. EA generally becomes more negative (increase in affinity) across a period and less negative (decrease in affinity) down a group.

    Ion sizes the radius of a cation is always smaller than that of the atom from which it is derived and reverse for a anion, because the anion has more electons (attraction of the electrons to the nucleus is lessened) and the cation has less electrons (attraction of the electrons to the nucleus increses).

    Learn, practice, and know electon configuration. Especially the order to fill orbitals according to energy levels.

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