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A GRE question on atomic physics

Posted: Fri Mar 26, 2010 1:29 pm
by hr297
In the Stern-Gerlach effect, observed when a
beam of atoms is passed through an inhomogeneous
magnetic field, which of the following is true?
(A) The atoms must be ionized first.
(B) No deflection is observed for hydrogen atoms.
(C) Only one deflection is observed for hydrogen
atoms.
(D) Two distinct deflections are observed for sodium
(Z = 11) atoms.
(E) Three distinct deflections are observed for
magnesium (Z = 12) atoms.

I know E is right, but I don't understand why D is not. :x
Could anyone explain this to me? Thanks in advance!

Re: A GRE question on atomic physics

Posted: Sat Mar 27, 2010 12:46 am
by geshi
To be honest, I'm slightly confused. I think it's the opposite: D is correct and E is wrong. I suppose someone brighter than I will have to answer this ...

Re: A GRE question on atomic physics

Posted: Sat Mar 27, 2010 1:34 pm
by grae313
D is correct, E is not.

Any atom with a single unpaired electron will be deflected into two distinct spots for spin up and spin down. Magnesium in its ground state has all electrons paired, so S=0 and it is a singlet state.

Re: A GRE question on atomic physics

Posted: Sat Mar 27, 2010 6:36 pm
by blackcat007
just to add to what grae said.. no of lines in a stern gerlach exp = 2s+1

for Na s=1/2 thus 2 lines
for Mg s=0 thus 1 line ie no deflection