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

Answer

Atomic radius literally mens size of the atom. It can be measured either by X-ray diffraction 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 covalently bonded atoms of the saem element in a molecule. For example, the internulear distance between two chlorine atoms in chlorine molecule is `198 pm`. Therefore, the convalent radius of chlorine atom is `198//2 = 99 om (0.99 Å)`. In case of metals, atomic radius is called matallic radius. it is defined as one-half the distance between the two adjacent atoms in teh 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 (1.28 Å)`. Another type of atomic radius is also defined called vander waal's radius, mainly used for noble gases. It may be defined as half of the internuclear distance between two adjacent atoms of the same element belonging to two nearest neighbouring molecules of the same substance in solid state. Ionic radius mean size of the ions. 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 on remaining electrons increases and hence the ionic soze 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^(+)` is `95pm` while the atomic radius of sodium is `186 pm`. Ionic radius of `F^(-)` is `136 pm` while the atomic radius of fluorine atom is only `72 pm`.