The ionic radius refers to the size of an ion in a crystal lattice. It is a measure of how far the outermost electrons are from the nucleus of the ion. The ionic radius can vary depending on whether the ion is a cation (positively charged) or an anion (negatively charged). Cations are generally smaller than their neutral atoms because they lose electrons, resulting in a stronger attraction between the remaining electrons and the nucleus. Factors such as the ion's charge and the coordination number, which is the number of ions surrounding a central ion, also influence the ionic radius. For example, a higher charge typically leads to a smaller ionic radius due to increased nuclear attraction. Understanding the ionic radius is important in fields like chemistry and materials science, as it affects properties such as solubility and crystal structure in compounds like sodium chloride or calcium fluoride.