What do atomic radius and ionic radius really mean to you?

Explanation

Atomic radius means size of the atom. It can be measured either by `X-ray` or by spectroscopic methods. In case of non-metals, atomic radius is called covalent radius. It is defined as one-half the distance between the nuclei of two covalent bonded atoms of the same element in a molecule. For example, the internuclear distance between to chlorine atoms in chlorine molecule is `198"pm"`. Therefore, the covalent radius of chlorine atom is `198//2=99 "pm" (0.99 Å)`. In case of metals, atomic radius is called metallic radius. It is defined as one-half the distance between the two adjacent atoms in the crystal lattice. For example, the distance between two adjacent copper atoms in solid copper is `256 "pm"`, therefore, the metallic radius of copper is `256//2=128 "pm" (1.28Å )`.
Ionic radius means size of the ion. An ion can be a cation or an anion. The size of a cation is always smaller than that of the parent atom because loss of one or more electrons increases the effective nuclear charge. As a result, force of attraction increases and hence the ionic size decreases. On the other hand, the size of the anion is larger than the parent atom because the addition of one or more electrons decreases the effective nuclear charge. As a result, The force of attraction decreases and hence the ionic size increases. For example,the ionic radius of `Na^(o+)` is 95 pm while the atomic radius of sodium is 186 pm. On the other hand, ionic radius of fluoride ion is `136 pm` whereas the atomic radius of flourine atom is only 72 pm.