Ionic Radii Vs Atomic Radii?

Question

Explain how ionic radii influence crystal structure, and why they differ from atomic radii?

Answer 1

The atomic radius is the radius of the atom in its "normal" state; the ionic radius is the radius of the charged state of the atom, whether positive or negative.

For elements like Sodium that lose an electron to become an ion, the ionic radius is smaller--there are 11 protons pulling on only 10 electrons.
For elements like Chlorine there are more electrons that are repelling each other, so the ionic radius is larger.

The ionic radii of the elements in a crystal give them a "shape"; based on those shapes and how those "spheres" (we can think of them that way) pack themselves into the most efficient arrangement gives rise to the crystal structure.

Answer 2

the atomic radius is the radius of the uncharged atom, and once u make an ion out of it, it´s radius will change, sometimes dramatically.
take for example Na and Na+. in Na the last electron is on a 3s AO, which way far away from the 2p.
once u have a Na+, the 3s is empty, the ion has the stable configuration of a noble gas, but with more positive charge-as a result, the attraction of the nuclei on the electrons is bigger, the configurations-stable, and the whole shell shrinks.
the difference in atomic/ionic radius is more than twice!
http://pol.spurious.biz/projects/chemglobe/ptoe/_/11.php
this all leads to greater ion strength (the charge is spread over a smaller area, caused by the smaller ionic radius) therefore stronger bonds and more efficient packing in the crystal cell.

Answer 3

As you can see on my table the atomic radius increases from left to right. this is very well explained on my second link Down a group as the number Z of protons increases, the number of electrons increases also and occupy more external orbitals so when external orbitals are filled their size increases