A GRE question on atomic physics

hr297
Posts: 1
Joined: Fri Mar 26, 2010 1:18 pm

A GRE question on atomic physics

Postby hr297 » Fri Mar 26, 2010 1:29 pm

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!

geshi
Posts: 200
Joined: Tue Dec 01, 2009 12:01 am

Re: A GRE question on atomic physics

Postby geshi » Sat Mar 27, 2010 12:46 am

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 ...

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grae313
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Joined: Tue May 29, 2007 8:46 pm

Re: A GRE question on atomic physics

Postby grae313 » Sat Mar 27, 2010 1:34 pm

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.

blackcat007
Posts: 378
Joined: Wed Mar 26, 2008 9:14 am

Re: A GRE question on atomic physics

Postby blackcat007 » Sat Mar 27, 2010 6:36 pm

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




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