Which has more effect on the atomic radius?

[Postictal Raiden]

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My mcat partner and I have a little dispute over which has a larger effect on the atomic radius. My friend believes that protons have more effect on the radius than electrons. I, on the other hand, believe that it's a little more complicated than that. I believe that protons do have the greater effect within the same period, but moving downward on the periodic table electrons have more effect.

Who is right?

 

[FormerPhysicist]

There is a "shielding" that occurs from inner electrons acting on outer electrons, which is why atomic radii expand when going down the table. Maybe I'm reading your question wrong, but it seems like you're both right, in a way.

 

[drspacheman]

Yeah you're both right. It has to do with effective nuclear charge, which takes into account the relative proton and electron numbers.

 

[Shjanzey]

But for the most part it should be electrons that effect it most, because you have contraction of radius when the p and d orbitals are half filled, and fully filled. Protons do not vary for an atom, but you can have loss and gain of electrons. That variance lends one to think atomic radii is more dependent on the electrons...especially when the measure of atomic radius also includes the electron cloud. If you change energy levels, then you have gone down (or up) a shell and have experienced a significant change in the atomic radius.

 

[gettheleadout]

But for the most part it should be electrons that effect it most, because you have contraction of radius when the p and d orbitals are half filled, and fully filled. Protons do not vary for an atom, but you can have loss and gain of electrons. That variance lends one to think atomic radii is more dependent on the electrons...especially when the measure of atomic radius also includes the electron cloud. If you change energy levels, then you have gone down (or up) a shell and have experienced a significant change in the atomic radius.

I would actually disagree for a couple reasons.

First, atomic radius is in fact a uniform trend and half-filled and filled stability of subshells does not have an effect on atomic radius.

Second, in comparing isoelectronic species, addition or subtraction of a proton will always decrease or increase the atomic radius, respectively. Atomic radius is primarily determined by effective nuclear charge, and because shielding effects are due to core electrons and new electrons are added (as you move up in atomic number) to the valence shell, the only instances where addition of an electron + proton pair (to move up to the next neutral element) results in an increase in atomic radius is in the case of the electron occupying a new primary energy level. 90%+ of the time, adding both decreases atomic radius.

Of course the real answer is neither determines it alone, because net nuclear charge due to protons and distance to valence shell due to electron energy level both determine effective nuclear charge, but if the question demanded a choice, I'd pick protons.

And of course, this is a poor question and would not be asked in this form on the MCAT, but I definitely wanted to point out the correction about half-filled and filled stability.

 

[Shjanzey]

Second, in comparing isoelectronic species, addition or subtraction of a proton will always decrease or increase the atomic radius, respectively.

This is an extreme example that should only occur in a radioactive decay situation. I sort of assumed that the proton would be constant when considering my answer. If you are assuming that protons are easily (emphasis on easily) gained or lost, then I agree with your assessment as well.

First, atomic radius is in fact a uniform trend and half-filled and filled stability of subshells does not have an effect on atomic radius.

I had to go back and re-read to make sure I understood. I think I may have confused electronegativity and bond length with this subject 😳

 

[gettheleadout]

This is an extreme example that should only occur in a radioactive decay situation. I sort of assumed that the proton would be constant when considering my answer. If you are assuming that protons are easily (emphasis on easily) gained or lost, then I agree with your assessment as well.



I had to go back and re-read to make sure I understood. I think I may have confused electronegativity and bond length with this subject 😳

I was using the isoelectronic example just as a lead-in to my point that 90% of the time, when you add both a proton and electron, radius decreases; this means protons dominate most of the time.

I probably didn't articulate the isoelectronic example well given my intention, but I was trying to look at the effect of protons individually. I guess that's irrelevant since adding electrons individually would also always increase size, so ignore those cases entirely as focus on my main argument: when adding one of each together, protons dominate most of the time.