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Homework Help: Chem Question

  1. Mar 2, 2005 #1
    How do you know whether a group of ions is isoelectronic? Also what does this notation mean:

    [tex] Ca: [Ar]4s^{2} [/tex]?

  2. jcsd
  3. Mar 2, 2005 #2


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    An atom or ion is isoelectonic with another atom or ion if the two contain equal numbers of electrons. This can easily be determined from the Atomic Numbers of the 2 species under consideration:
    {Number of Electrons} = {Atomic Number} - {Net Charge}

    Thus, for a neutral atom, the number of electrons equals the Atomic Number. For example, the neutral atom Ar with Atomic Number (18) has 18 electrons. For the negative ion Cl-1 with charge (-1) and Atomic Number (17), the number of electrons is {(17) - (-1)}=(18). For the positive ion Ca+2 with charge (+2) and Atomic Number (20), the number of electrons equals {(20) - (+2)}=(18).

    The terminology:
    [tex] Ca: [Ar]4s^{2} [/tex]
    indicates that the neutral Ca atom has the same electron energy/orbital structure as neutral Ar PLUS 2 ADDITIONAL ELECTRONS in the "s" Sublevel of the 4th Energy Level (Principle Quantum Number = 4). From this indication, it can be determined that if Ca loses its 2 valence electrons (i.e., the 2 4s2 electrons), the resulting Ca+2 ion will be isoelectronic with the neutral Ar atom.

    Last edited: Mar 2, 2005
  4. Mar 3, 2005 #3
    This has been said but not written so simply for clarity:

    [tex] Ca: 1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2} [/tex] means the same as [tex] Ca: [Ar]4s^{2} [/tex]

    It is just a short hand and easier to write but use the long hand in exams or you lose marks. :smile:

    The Bob (2004 ©)
  5. Mar 3, 2005 #4


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    Nope.It would be ridiculous,if one of the teachers would not accept the shorthanded notation...
    But i know that there are a lotta dumb profs out there...:yuck:

  6. Mar 3, 2005 #5
    Personally I do not understand why I cannot short hand but there is no mark for it in the mark scheme so it is better to be safe than sorry. :smile:

    The Bob (2004 ©)
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