Ionic radius trend

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Fiona Grant 1I
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Joined: Fri Apr 06, 2018 11:03 am

Ionic radius trend

Postby Fiona Grant 1I » Sat May 05, 2018 2:48 pm

I understand the basic idea that ionic radii shows the same trends as atomic radii (radius increases down a group and decreases across a period), but I am confused as to how cations are smaller than the parent atom, and anions are larger. Wouldn't this contradict the atomic radii trend?

Natalie Noble 1G
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Re: Ionic radius trend

Postby Natalie Noble 1G » Sat May 05, 2018 2:49 pm

I too get confused by this concept here is what I think is the reasoning but I am not sure

The trend is the same as atomic radius (increasing down a group and decreasing across a period) just not when comparing the ion to its parent atom.

Cations are smaller than their parent atom because the valence electrons, which are furthest away from the nucleus, are lost. There is less electron-electron repulsion when an electron is lost so it is smaller.

Anions are larger than their parent atom because of the gain in electrons in the valence shell increases the electron-electron repulsion so the anion is larger.

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Re: Ionic radius trend

Postby NabilaNizam-1K » Sat May 05, 2018 3:07 pm

During the formation of a cation, electrons are lost from their shells and since the radius is directly affected by the number of shells, this causes the ion to possess less number of shells than their parent atom, hence they are smaller.
Meanwhile, during the formation of an anion, electrons are gained into a pre-existing shell. Increased number of electrons in the shell weakens the effective nuclear charge, leading to a larger ionic radius since the electrons experience less pull from the protons in the nucleus.

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