Ok this is a relatively easy question but i am somehow failing to see the reasoning behind it. The question asks "which element has the smallest effective nuclear charge (Z eff.) on its outermost electrons?" The elements being compared are Li (lithium) and Rb (Rubidium). so...The correct answer is Lithium. WHY?!?!?

I know that effective nuclear charge is basically the amount of pull from the nucleus that is felt by an electron. Elements with small atomic radii have large Z eff. and those with large atomic radii have smaller Z eff. SO WHY THE HELL IS Li THE CORRECT ANSWER when Rb clearly has the larger atomic radii, therefore its outermost electrons are further away and experiencing Less pull from the nuc. AHHHHHH!

4/30/2006 2:22:04 PM

Think about the actual nuclear charge. Rb has many more protons, so it has a much larger actual nuclear charge. Zeff is different than the actual nuclear charge only because of shielding. Read (or reread) the section on shielding in your book. The result of inner and outer shell shielding effects is that your generalization:

Quote :

"Elements with small atomic radii have large Z eff. and those with large atomic radii have smaller Z eff. "

only works if both elements are in the same period (comparing boron to fluorine, for example.) As you go down a group, Zeff increases for the outermost electron because the number of protons increases dramatically. The overall trend is that the Zeff for the outermost electron increases as you go down and/or to the right on the periodic table.

4/30/2006 3:52:44 PM

THANK YOU

4/30/2006 3:55:51 PM

I wish I'm taking CH101. Those were the good old days.

4/30/2006 8:00:04 PM