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Homework Help: Electron configuration quantum numbers

  1. May 30, 2007 #1
    I need help with this problem :)

    Problem: Give the quantum numbers (n, l, ml, ms) for each electron in one neutral carbonatom in ground state.

    My solution: I look at a table to get the electron configuration and for a carbonatom i get 1S^2 2s^2 2p^2

    For s
    n=1, l=0, ml=0, ms=+-1/2
    n=2, l=0, ml=0, ms=+-1/2
    n=2, l=1, ml=-1,0,1, ms=??
    For p
    n=2, l=0, ml=0, ms=??
    n=2, l=1, ml=-1,0,1, ms=??

    My question: From the two first shells from s i get 4 electrons, so i have to get 2 more, but from where do i get those 2? It must be som kind of rule but i can't find any. pls help :)
  2. jcsd
  3. May 30, 2007 #2
    you first two lines for the 1s ans 2s shells are correct

    since you have [itex] 2p^2[/itex] and there are only 2 electrons in the p orbital we fill electrons in the spaces with ascending order of ml value with spin +1/2 electrons first and then -1/2 electrons

    that is, only m=-1 and ml=0 are occupied with spin +1/2 electrons
  4. May 31, 2007 #3
    Firstly, you definitely shouldn't have to look up the electronic configuration of carbon in a table. For a lanthanide, that's ok. But you need to be able to figure out the electronic configurations of the first four periods by yourself. There's only a few basic rules you need to use (Aufbau principle, Pauli's Exclusion principle, Hund's Rule of Maximum Multiplicity, stability of half and fully filled shells). Maybe you should study your textbook more thoroughly.

    As for your question, it will be helpful to work digrammatically. Draw small boxes for each orbital, group them together by subshells and put each shell in its own row. Then start filling up electrons. Carbon has 6 electrons. First 2 go in 1s. Next 2 go in 2s. Next 2 go in 2p, and Hund's rule demands that they occupy two separate 2p orbitals with same spin.

    For 1s2, you have (n,l,m,s) = (1,0,0,+1/2) and (1,0,0,-1/2)
    For 2s2, (2,0,0,+1/2) and (2,0,0,-1/2)
    For 2p2, (2,1,-1,+1/2) and (2,1,0,+1/2)

    You are mistaken in your understanding of what s and p mean. They are labels to denote subshells, the value of l. Whatever the shell, s always means l=0 and p always means l=1. After that, m varies from -l to +l. Really, this is all very basic stuff that forms the foundation of chemistry. You really should read your textbook of introductory chemistry more thoroughly.

  5. May 31, 2007 #4

    Thanks for the help, for this problem i think i understand how it works. But in generall i'm very confused.
    The course i have is quantum physics, and my book doesn't say anything about Aufbau principle and hund's rule. We are just studing the Schrödinger equation and from that learning about quantum number and so on.
    I read about hund's rule but i don't get it very well. You were talking about s gives l=0 and p gives l=1 does this mean that this continues like this and d gives l=2 and so on? and we must pick m from -1 to 1 or in the d case from -2 to 2? If so it is very good thing to remember. Which rule och princip gives this? is it hund's rule?
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