Why do atomic radii increase as you move down a column in the periodic table?

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The discussion centers on the concept of atomic radius, specifically the relationship between cation, atom, and anion sizes, which is summarized as Rcation < Ratom < Ranion. It is noted that atomic radius increases as one moves down a column in the periodic table due to the addition of electron shells. A key point of confusion arises regarding the relationship between atomic number (Z) and atomic radius, with the assertion that if the atomic number of one element is less than that of another, its radius should be greater. This claim is challenged, as atomic radius does not consistently correlate with atomic number across all elements. Examples are provided, such as sodium (Na), strontium (Sr), and potassium (K), which illustrate that atomic radius can vary independently of atomic number. The discussion also touches on the effects of electron addition and nuclear charge on atomic size, emphasizing that the relationship is not straightforward and may depend on specific subsets of elements. Additional resources are shared to clarify these concepts further.
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About the atomic radius,
as a starter it was mentioned (and correct me if am wrong) that:
Rcation < Ratom< Ranion (And we'll know the size of the atom based on these)
Then it was mentioned that: Radius of atom of a certain element increase if we walk straight downwards along a column in the periodic table.
And afterwards: if Z of any element is less than Z of any other one
then the radius of the first is greater than the 2nd.

How come? I really don't get it?
Any chemical, physical explanation (most importantly to the 3rd pt)
 
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cant say for these rules are alway true, if you consider H then removing its electron will leave just a nucleus so rcation<ratom but adding an electron to H it seems the radius would remain the same since the electron would be in the same orbital and shell.

Moving down a given column in the periodic table, you'd be adding many electrons to the next lower element so that new shells would be added increasing the radius of the atom.

For the last pt are you confusing Z with atomic weight?

Here's a brief article on it with a graph. The author says that when a new shell is added the radius clearly jumps up but as electrons are added the shell goes down due to increased attraction to the protons in the nucleus.

http://mmsphyschem.com/atmRad.htm

and here's another wiki article on it that describes things better:

http://en.wikipedia.org/wiki/Atomic_radius
 
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Hello there, jedishrfu
Thanks, I ll be reading them.
But about the 3 points I mentioned, what do you think? Are they all logical?
 
M. next said:
if Z of any element is less than Z of any other one then the radius of the first is greater than the 2nd

Clearly not true. Atomic radius is not a monotonic function of Z. Radius of 11Na 180 pm, of 38Sr 200 pm, of 19K is 220 pm.

It may be true if not ALL elements are taken into account, but just some subset of the periodic table.
 
Hmm.. That's why I was wondering, it doesn't even make sense..
If it so, Borek, how should I categorize radii?
 
Thanks
 

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