Relative Atomic Radii (Atoms and Ions)

lafalfa

Hi,

How would you determine the relative atomic sizes of a neutral atom and an ion?
I can't figure this out. It can't be that all neutral atoms are always smaller than all ions can it?

For example, how would I determine if Cl- is smaller or larger than S?.
(The answer is that Cl- is larger than S).

Is there a rule that applies to all comparisons of neutral atoms and ions?
For example, how would I determine if Na is smaller or larger than Cl-?

Thank you very much!

Borek

The only solid and strict rules here are that for identical number of electrons, radius goes down with increasing charge of nucleus, and for identical charge of nucleus, radius goes up with increasing number of electrons. When both change at the same time and in the same direction result is sometimes difficult to predict.

--
ChemBuddy chemical calculators - buffer calculator, stoichiometry calculator
www.ph-meter.info - ph meter, ph electrode

pzona

Lol, it's chemistry, of course there isn't a rule that applies to ALL situations. There are however guidelines that you can use to generalize (see Borek's post). You really have to look at the electron configuration of each ion/atom to be sure.

lafalfa

But then how would I conclude that Cl- is larger than S? I have to be able to conclude this somehow since I have this question for school.

Char. Limit

Anions are always *much* larger than the neutral atom. (i think the oxide ion is twice the size of an oxygen atom) So, even though S is *slightly* larger than Cl... Cl- is *much* larger than Cl.

That's the best I can do.

lafalfa

So when asked to order atoms/ions in terms of their size, would I immediately put all the negatively charged ions at one end, all the neutral atoms in the middle, and then all the positively charged ions at the other end, regardless of their relative positions on the periodic table?

Char. Limit

Well, I'm sure if you get far enough apart on the table (neutral cesium vs. fluoride ion) that won't always be the case, but mostly, yes.

I think.

alxm

Ions can have positive charge too, you know?


Anyway, original poster, the idea is that you learn the general trends seen on this chart.

Apart from those overall general trends, the radius of atoms is an ill-defined property. There's no need to know the exact numbers, except perhaps as a rough approximation.

Borek

Check if one of the "rules" I have listed won't give an answer. Do they have the same number of electrons? Or perhaps do they have the same number of protons in nucleus?

As alxm already suggested - check radius of K⁺ vs K, or Mg²⁺ vs Mg.

--
chemical calculators - buffer calculator, concentration calculator
www.titrations.info - all about titration methods

Char. Limit

Oops, I'm making mistakes everywhere I go, it seems. Let me fix that to "anions".