Question

What is the metallic radius?

Answer 1

Everything is written on the image below:

As you can see, metallic radius is defined as one-half of the distance between the nuclei of two atoms in crystal or between two adjacent metal ions in the metallic lattice.

Metallic radii:
- decrease across the period due to increase in the effective nuclear charge.
- increase down the group due to increase in principal quantum number.

If you need more than this, you can find here much more:
http://en.wikipedia.org/wiki/Metallic_bonding