Ask A Scientist Chemistry Archive

Question:  The atomic radius decreases from left to right on the periodic 
table except for the inert gases which are much larger than the halogens.  
Why is there a big increase for the inert gases?  This cannot be because 
the shell is full or there would always be an increase from left to right 
as the shell is filling.  What is the reason for the big increase?
ghodel

Answer:  Gee, I must be missing something. I'm looking at page 274 of Kotz 
and Purcells' "Chemistry and Chemical Reactivity" text (1987), which is 
the freshman Chem text they used at Yale when I was teaching there. And it 
says that although H < He and F < Ne, Cl > Ar, Br > Kr, I > Xe, and At = Rn. 
In fact, it is VERY DIFFICULT to decide the size or radius of an atom, and 
there  is no general agreement on the best way to do it! I quote from the 
text: "The fact that new tables of atomic radii are still being compiled 
suggests that there are fundamental difficulties involved and reminds us 
that we should treat any set of radii with extreme caution."

Now, in molecules like H2 and Cl2, the radius of the atom is usually 
defined experimentally to be half of the distances between the centers 
of the atoms. But obviously the size of the isolated atom is going to 
be different than is; the valence electrons are "spread out" over the whole 
molecule.  Now, this may or may not be a BIG problem, but it's STILL an 
APPROXIMATION. One way to see whether the guess is a good one is to try 
and use the guessed-at H radius and Cl radius to predict the bond length 
of HCl. The predicted value is 1.36 Angstroms, whereas the true value is 
1.27 angstroms. These differences are on the same order as the differences
you are citing, i.e., between Cl and Ar the difference is 0.05 Angstroms, 
which is actually the biggest difference. Thus, the difference between the 
two numbers is smaller than the uncertainty in either number. So, it's 
meaningless to argue whether one is larger than the other.  (and 
especially, why!). Those numbers aren't known with any great accuracy.
 
Now the reason why the radius decreases as you move across a period is 
the effective nuclear charge increases. As you go across, the nucleus 
has more and more protons, which means more positive charge. This pulls 
the electrons closer to the center, especially the core electrons 
(closest to the center).
 
I hope this helps. And I'm glad that you questioned authority and asked 
for more information. This is the mark of a truly independent thinker! 
If you still don't agree, please ask again and voice your objections!
 
Way to go!

-dr topper