# About the size of the atom.

1. Feb 6, 2012

### M. next

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)

2. Feb 6, 2012

### Staff: Mentor

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.

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

Last edited by a moderator: May 5, 2017
3. Feb 6, 2012

### M. next

Hello there, jedishrfu
Thanks, I ll be reading them.
But about the 3 points I mentioned, what do you think? Are they all logical?

4. Feb 6, 2012

### Staff: Mentor

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.

5. Feb 6, 2012

### M. next

Hmm.. That's why I was wondering, it doesn't even make sense..
If it so, Borek, how should I categorize radii?

6. Feb 6, 2012

7. Feb 7, 2012

Thanks