Question about atomic radii

[Saleemrodman1]

I came across a question on UWorld Chemistry (discrete), and I honestly have never been so confused, and am starting to question my knowledge!

Question: If a channel of a particular cell preferentially transmits smaller ionic species across the membrane, given the following biologically relevant, isoelectronic ions, the channel is LEAST likely to transmit:

A. S^2-

B. Cl-

C. K+

D. Ca2+

My answer = C = K+ and the reason is because if this channel prefers small species, I would assume the one it would least likely transmit would be the one with the largest atomic radius. Following this reasoning, I look on the periodic table and remember the trend that atomic radii decrease when going from left to right, and increase when going from top to bottom. In this case, K+ would be the largest because it is the left most, and therefore would be the one most discriminated.

Answer = A

I am lost.

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[morrisol]

All the choices have the same number of electrons; however the ratio of protons to electrons in each species is different.


The Sulfur ion has 18 electrons. The Potassium ion has 18 electrons.

The Sulfur ion has 16 protons. The Potassium ion has 19 protons.


Those 3 extra protons will pull the electrons in the potassium ion closer towards the nucleus and decrease the ionic radius.

but doesnt that go against the atomic radius trend?

 

[law2doc2018]

All the choices have the same number of electrons; however the ratio of protons to electrons in each species is different.


The Sulfur ion has 18 electrons. The Potassium ion has 18 electrons.

The Sulfur ion has 16 protons. The Potassium ion has 19 protons.


Those 3 extra protons will pull the electrons in the potassium ion closer towards the nucleus and decrease the ionic radius.

ddek is absolutely correct. Remember, we're comparing the size of IONS, not the elemental form, so we ditch normal atomic radii trends. This is known as an isoelectronic series, and most questions will include two cations and two anions that are in series around a noble gas. The trend is the isoelectronic species with the greatest number of protons will have the smallest radius.

 

[morrisol]

ddek is absolutely correct. Remember, we're comparing the size of IONS, not the elemental form, so we ditch normal atomic radii trends. This is known as an isoelectronic series, and most questions will include two cations and two anions that are in series around a noble gas. The trend is the isoelectronic species with the greatest number of protons will have the smallest radius.

thanks. i never knew about this concept so thank you for telling me about that

 

[Saleemrodman1]

thanks. i never knew about this concept so thank you for telling me about that

Same! When I first answered it, I wanted to throw my laptop out the window, cause I thought the MCAT was messing with me. Never knew about this idea.

 

[Saleemrodman1]

All the choices have the same number of electrons; however the ratio of protons to electrons in each species is different.


The Sulfur ion has 18 electrons. The Potassium ion has 18 electrons.

The Sulfur ion has 16 protons. The Potassium ion has 19 protons.


Those 3 extra protons will pull the electrons in the potassium ion closer towards the nucleus and decrease the ionic radius.

Thank you very much!

 

[law2doc2018]

Same! When I first answered it, I wanted to throw my laptop out the window, cause I thought the MCAT was messing with me. Never knew about this idea.

It was covered very briefly in an already condensed Gen Chem 1 summer semester. There was one multiple choice question, but somehow the concept stuck around the lower depths of my mind like a bad song you can't rid yourself of