A metallic radius is the measurement of an atom's size with regards to metal elements only. Metallic elements are most easily measured because of even distribution and distance between adjacent atoms, according to Purdue University. Scientists assume the radius of an atom is half the distance between an adjacent atom in a solid.
Metallic radii trend larger going down the periodic table. This is because there are more valence electrons, and the electron cloud is larger with heavier elements. The metallic radius gets smaller going from left to right across the periodic table because there are more protons in the nucleus of the atom. More protons hold more electrons in tighter orbits, so the measurement is smaller. Metallic radii are measured in elements such as lithium, magnesium, beryllium, calcium, potassium, cesium, barium and more. These are groups IA and IIA on the periodic table of the elements.
In general, an atomic radius quantifies the size of an atom. Different atoms have different sizes based upon how many electrons are in the cloud surrounding the nucleus. Some electrons are covalent, meaning they are shared in elements. Other electrons are ionic, meaning they are shed during chemical reactions. These electrons affect the radius of an atom because they extend out from the nucleus in different ways.