Ionic Radius – Definition & Trends in Periodic Table

What is Ionic Radius?

Ionic radius is the distance from the nucleus of an ion up to which it has an influence on its electron cloud.

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.

Ionic Radius Periodic Table

Ionic radius is the distance from the nucleus of an ion up to which it has an influence on its electron cloud.

For example,

  • Radius of potassium = 243pm.
  • Radius of potassium ion = 138pm.

Atoms and ions which consist of an equal number of electrons are considered isoelectronic species. For example, Both O2-, Mg2+ have 10 electrons but they don’t have the 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 a 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.

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

IonsConfigurationIonic radii(nm)IonsConfigurationIonic radii(nm)
Li+20.076F2,80.133
Na+2,80.102Cl2,8,80.181
K+2,8,80.138Br2,8,18,80.196

Let us understand the trends in the ionic radius of elements across a 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.

Period 3Na2+Mg2+Al3+P3-S2-Cl
Number of protons111213151617
Electronic configuration2,82,82,82,8,82,8,82,8,8
Ionic radius0.1020.0720.0540.2120.1840.181

Ions are formed as a result of the gain or loss of electrons. The 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.

Frequently Asked Questions

Q1

Define atomic radius.

Atomic radius is the mean or typical distance from the centre of the nucleus to the boundary of the surrounding shells of electrons.

Q2

Define ionic radius.

Ionic radius is the distance from the nucleus of an ion up to which it has an influence on its electron cloud.

Q3

Define cation and anion.

When an atom loses an electron it forms a cation and when it gains an electron it becomes an anion.

Q4

What are isoelectronic species?

Atoms and ions which consist of an equal number of electrons are considered electronic species.

Q5

Why the size of the anion is larger than 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.

Er. Neeraj K.Anand is a freelance mentor and writer who specializes in Engineering & Science subjects. Neeraj Anand received a B.Tech degree in Electronics and Communication Engineering from N.I.T Warangal & M.Tech Post Graduation from IETE, New Delhi. He has over 30 years of teaching experience and serves as the Head of Department of ANAND CLASSES. He concentrated all his energy and experiences in academics and subsequently grew up as one of the best mentors in the country for students aspiring for success in competitive examinations. In parallel, he started a Technical Publication "ANAND TECHNICAL PUBLISHERS" in 2002 and Educational Newspaper "NATIONAL EDUCATION NEWS" in 2014 at Jalandhar. Now he is a Director of leading publication "ANAND TECHNICAL PUBLISHERS", "ANAND CLASSES" and "NATIONAL EDUCATION NEWS". He has published more than hundred books in the field of Physics, Mathematics, Computers and Information Technology. Besides this he has written many books to help students prepare for IIT-JEE and AIPMT entrance exams. He is an executive member of the IEEE (Institute of Electrical & Electronics Engineers. USA) and honorary member of many Indian scientific societies such as Institution of Electronics & Telecommunication Engineers, Aeronautical Society of India, Bioinformatics Institute of India, Institution of Engineers. He has got award from American Biographical Institute Board of International Research in the year 2005.