[LHM]

In which set of elements are the atomic radii most nearly the same?
A) Na, Mg, Al
B) Cs, Ba, La
C) Si, P ,S

So I originally thought it was B because I figured that since the atoms were so large that any additional electrons added would be more shielded from the nucleus than in any of the previous periods even though more protons are being added too. However, then I decided to check and using http://www.ptable.com/ and under the properties, it seemed like the atomic radii of Si, P, and S were more similar than the atomic radii of Cs, Ba, and La were. So what's the answer and why?

[Fluorine]

Check this link; http://en.wikipedia.org/wiki/Periodic_table_block

Silicon, phosphorus, and sulfur are all in the same period and electron configuration set (P-block). All of their electrons have access to the same extent, where A) and B) not all of the elements are in the same block, and their electrons have access to different levels.

Hopefully someone else can explain it better.