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Homework Help: Calculating avg distance from nucleus

  1. Oct 15, 2009 #1
    1. The problem statement, all variables and given/known data

    Calculate the avg. distance from the Hydrogen nucleus for an electron in 2p.

    2. Relevant equations

    <r> = int[r^3 |R|^2]dr from 0->infinity

    For Hydrogen 2p, R = (1/a)^3/2 (1/(2*sqrt(sigma))*sigma*exp(-sigma/2)

    where sigma = r/a

    3. The attempt at a solution

    I get 1/4a^4 (24 a^5) = 6a but the answer's supposed to be 5a. (5a for 2p is supposed to be less than 6a, which is the avg distance for 2s.) What am I doing wrong??????
  2. jcsd
  3. Oct 16, 2009 #2


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    Homework Helper
    Gold Member

    That's only true when the wavefunction is spherically symmetric. In general,

    [tex]\langle r\rangle=\int_{\text{all space}}r|\psi(\textbf{r})|^2d^3\textbf{r}=\int_0^{\infty}\int_0^{\pi}\int_0^{2\pi}|\psi(\textbf{r})|^2r^3\sin\theta dr d\theta d\phi[/tex]

    That doesn't look quite right. Where did you get this from?
    Last edited: Oct 16, 2009
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