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## Measure of the diameter of an atom's ion in a crystal lattice

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I've Got My I ON the Distance!

The ionic radius is the measure of an atom’s ion in a crystal structure. As a trend in the periodic table, ionic radii decrease across a period and increase down a group.

Track and Field is one of the most popular high school sports. Schools have teams of up to 100 members, all desiring the chance to race against competitors from rival schools while the cheer of many supporters roars in the background. As a sprinter is in the middle of his race in 2nd place, he notices how the distance between the 1st place opponent and him is gradually decreasing in length. If the two runners are nuclei of different atoms, then the distance between them represents 12\begin{align*}\frac{1}{2}\end{align*} of the ionic radii.

Credit: Peter Harris

This diagram demonstrates radii and is coincidentally in the shape of a track field! [Figure1]

Ionic radii are so microscopic that they are difficult to calculate. Measurements can range from 30 pm (0.3 Å) to 200 pm (2 Å) on average. The methods for measuring ionic radii include x-ray crystallography. When electrons are added to an atom, the ion becomes larger than the parent atom and the ionic radius increases. This is because the electron cloud now has the ability to increase in size and the ion can occupy more energy levels. The opposite is true when electrons are removed from an atom.

### Creative Applications

1. In the track and field scenario, how does a decrease in the distance between the runners relate to ionic radii?
2. Why are ionic radii important to understand?
3. What are the similarities and differences between atomic and ionic radii?