Periodic Trends In Ionic Radii In Modern Periodic Table

Ionic radii, what is it? Let’s figure out what do we mean by ionic radii and its variation in groups and periods of the periodic table (modern periodic table). Ions are formed when an atom loses or gains electrons. When an atom loses an electron it forms a cation and when it gains an electron it becomes an anion. The Ionic radius can be described as the distance between the nucleus of an ion and the outermost shell of the ion. The atomic size of a cation will be smaller than that of the parent atom. An anion is relatively larger in size than its parent atom. This is because when an atom gains electrons the total number of electrons increases which tends to create more repulsion between electrons and thus overshadows the net effective nuclear charge.

Periodic Trends

For example: Radius of potassium= 243pm.

Radius of potassium ion= 138pm.

Atoms and ions which consist of an equal number of electrons are considered as isoelectronic species. For example, Both O2-, Mg2+ have 10 electrons but they don’t have a same ionic radius as the effective nuclear charge in both of them is different. The radius of a cation will be smaller than that of the anion as a cation will have greater positive charge (i.e. number of protons) so it will attract the electrons in the outermost orbital with greater force and hence the smaller size.

Trends in ionic radius in groups:-

In periodic table while moving down in a group, atoms add extra shell (number of electrons) due to which ionic radius of elements increases down a group.

Ions Configuration Ionic radii(nm) Ions Configuration Ionic radii(nm)
Li+ 2 0.076 F 2,8 0.133
Na+ 2,8 0.102 Cl 2,8,8 0.181
K+ 2,8,8 0.138 Br 2,8,18,8 0.196


Trends in ionic radius across a period:-

Let us understand the trends in ionic radius of elements across period with an example. In period 3 we find that the atomic radius first decreases and then suddenly increases and then again it slowly decreases. This is because the starting elements in a period tend to form cations, and the elements towards the end of a period tend to form anions.

Trends in ionic radii of ions in period 3

Period 3 Na2+ Mg2+ Al3+ P3- S2- Cl
Number of protons 11 12 13 15 16 17
Electronic configuration 2,8 2,8 2,8 2,8,8 2,8,8 2,8,8
Ionic radius 0.102 0.072 0.054 0.212 0.184 0.181


Ions are formed as a result of gain or loss of electrons. Formation of ions plays a vital role in any chemical reaction to form a new substance. By knowing the ionic radii we can study various chemical bonds formed during a reaction. To know more about elements of the periodic table and its properties visit byju’

Practise This Question

Which of the following observations are incorrect for the noble gases - He, Ne, Ar, Kr, and Xe?