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Question

For the 3d state of hydrogen, at what radius is the electron probability a maximum? Compare your answer with the radius of the Bohr orbit for (n=3).

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Answered 7 months ago
Answered 7 months ago
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The state (3d)(3d) has (n=3)(n=3) and (l=2)(l=2), so let's write the radial probability for this state

Pnl(r)dr=r2Rnl(r)2drP_{n l}(r) d r=r^{2}\left|R_{n l}(r)\right|^{2} d r

Pnl(r)=r2Rnl(r)2P_{n l}(r) =r^{2}\left|R_{n l}(r)\right|^{2}

P32(r)=r2R31(r)2P_{32}(r) =r^{2}\left|R_{31}(r)\right|^{2}

we can use Table 7.1\textbf{Table 7.1} to write (R32)(R_{32})

P32(r)=r2×[1(a0)3/248130r2a02er/3a0]2=(1(a0)3/248130)2e2r/3a0(r2a02)2r2\begin{align*} P_{32}(r) &=r^{2}\times \left[\frac{1}{\left(a_{0}\right)^{3 / 2}} \frac{4}{81 \sqrt{30}} \frac{r^{2}}{a_{0}^{2}} e^{-r / 3 a_{0}} \right]^{2}\\ &= \left(\dfrac{1}{\left(a_{0}\right)^{3 / 2}} \dfrac{4}{81 \sqrt{30}}\right)^{2} e^{-2 r / 3 a_{0}}\left(\frac{r^{2}}{a_{0}^{2}}\right)^{2} r^{2} \end{align*}

take the term (1(a0)3/248130)2\left(\dfrac{1}{\left(a_{0}\right)^{3 / 2}} \dfrac{4}{81 \sqrt{30}}\right)^{2} to be a constant (A2)(A^{2}). Hence we can write the probability as follows

P32(r)=A2(r6a04)e2r/3a0P_{32}(r) = A^{2}\left(\frac{r^{6}}{a_{0}^{4}}\right) e^{-2 r / 3 a_{0}}

In order to get the radius at which the electron probability is maximum, we need to find the first derivative and to solve for dPdr=0\dfrac{d P}{d r}=0

dPdr=A2[6r5a04e2r/3a02r63a05e2r/3a0]=0\dfrac{d P}{d r}=A^{2}\left[ \frac{6 r^{5}}{a_{0}^{4}} e^{-2 r / 3 a_{0}} -\frac{2 r^{6}}{3 a_{0}^{5}} e^{-2 r / 3 a_{0}}\right]=0

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