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Shells' arrangement

  1. May 18, 2011 #1

    Why does the 1st shell only can have 2 electrons in it ?

    Also what are sub-shells ?
  2. jcsd
  3. May 18, 2011 #2


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    The Pauli exclusion principle says electrons in atoms cannot share the same 4 quantum numbers. The fact that the electrons are in the 1st shell determines 3 of them (n=1, l=0, ml=0), and there are only two possible values for the 4th quantum number (ms=+1/2 and ms=+1/2). Got it?

    Well, for principal quantum numbers higher than n=1, there are multiple possibilities for the value of the angular momentum quantum number l (l=0 up to l=n-1). Each possible value of l is associated with a sub-shell: l=0 is called the s-subshell, l=1 is called the p-subshell, and so on.
  4. May 19, 2011 #3
    What is the quantum number ?

    and n,I and m ?
  5. May 19, 2011 #4


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    The quantum number is something you should already know about if you are asking the question in your original post :wink:. Seriously though, please look that up on wikipedia to get the basics down, then come back here and ask specific questions if you are confused.
  6. May 21, 2011 #5
    To know that, you need to learn quantum mechanics a little bit. Sub-shells are orbitals, determined by quantum numbers in Schrodinger's equations. n is the principal quantum number that determines the shell. l has something to do with angular momentum, m is the magnetic angular quantum number.

    In general, l must be less than n, while m must be equal to or less than l. In the first shell n=1 as it is the first shell, l only =0. In the second shell where n=2, l=0 or 1, so m=+1 or -1 or 0. For each set of parameters there is one orbital and in one orbital there are two electrons. Thus in first shell there is only on orbital possible, thus 2 electrons.

    I hope this would help you with your question.
  7. May 25, 2011 #6
    OK, thanks I'm going to learn quantum mechanics first.
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