Calculating Angles for Many Electron System w/ L=2, S=1, J=2

[neelakash]

Homework Statement

For a many electron system with L=2,S=1 and J=2 calculate the angle between the L and S vectors both according to the old quantum theory and quantum mechanics.

Homework Equations

The Attempt at a Solution

I suppose this problem is nothing but a simple application of cosine rule.The external angle is the answer.
But I am thinking twice as they mention: both according to the old quantum theory and quantum mechanics.

Am I missing something?

 

[quantumdude]

In the Bohr model (I think that qualifies as "old quantum theory"), we have $L=n\hbar$, where $n=1,2,3,...$. Contrast that with modern quantum mechanics.

Does that help?

 

[pam]

Homework Statement

For a many electron system with L=2,S=1 and J=2 calculate the angle between the L and S vectors both according to the old quantum theory and quantum mechanics.

In QM, use LS cos(theta)=L.S=[J(J+1)-L(L+1)-S(S+1)]/2.