Difference Between Atomic Radius and Ionic Radius

Atomic Radius vs Ionic Radius
 

We can define a radius for a circle or a ball. In that case, we say that the radius is the distance between the centre of the circle to a point in its circumference. Atoms and ions are also regarded as having a structure similar to a ball. Therefore, we can define a radius for them too. As in the general definition, for atoms and ions we say that the radius is the distance between the centre and the boundary.

Atomic Radius

Atomic radius is the distance from the centre of the nucleus to the boundary of the electron cloud. The atomic radius is in the Angstrom level. Although we define the atomic radius for a single atom, it is hard to measure it for a single atom. Therefore, normally the distance between the nuclei of two touching atoms is taken and divided by two, to get the atomic radius. Depending on the bonding between two atoms the radius can be categorized as metallic radius, covalent radius, Van der Waals radius, etc. Atomic radii increases as you go down in a column in the periodic table, because new layers of electrons are adding. From left to right in a row, atomic radii decrease (except for noble gases).

Ionic Radius

Atoms can gain or lose electrons and form negative or positive charged particles respectively. These particles are called ions. When neutral atoms remove one or more electrons, it forms positively charged cations. And when neutral atoms take up electrons, they form negatively charged anions. Ionic radius is the distance from the centre of a nucleus to the outer edge of the ion. However, most of the ions do not exist individually. Either they are bonded with another counter ion, or they have interactions with other ions, atoms or molecules. Because of this, the ionic radius of a single ion varies in different environments. Therefore, when ionic radii are compared, the ions in similar environments should be compared. There are trends in the ionic radii in the periodic table. As we go down in a column, additional orbitals are added to atoms; therefore, the respective ions also have additional electrons. Thus, from top to bottom the ionic radii increase. When we go from left to right across a row, there is a specific pattern of ionic radii change. For example, in the 3^rd row, sodium, magnesium and aluminum make +1, +2 and +3 cations respectively. The ionic radii of these three are gradually decreasing. As the number of protons are higher than the number of electrons, nucleus tends to pull the electrons more and more towards the centre, which result in decreased ionic radii. However, the anions in the 3^rd row have considerably higher ionic radii compared to the cationic radii. Starting from P^3- the ionic radii decrease to S^2- and to Cl^–. Reason for having a larger ionic radius in anions can be explained by addition of electrons into outer orbitals.