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Ionic Radius - When an atom either gains or loses an electron from its outer orbit in order to obtain a stable electronic configuration, it forms an ion. The ionic radius is hence referred to as the radial distance up to which the nucleus of an ion has an influence on its electron cloud. In this article, we will cover the ionic radius, electrical charge, and trends of ionic radius in the periodic table.
Ionic Radius: What is an ion?
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Ion is an atom, molecule, or particle with a net electrical charge. The gain or the loss of electrons in atom results in an ion. Hence, the removal/loss of electrons results in the formation of a cation. Similarly, gaining an electron leads to the formation of the anion.
Natural Atom
What is Ionic Radius?
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The ionic radius can be estimated by the measurement of distances between cations and anions in ionic crystals. Ionic radius is measured as the distance between the electron in the outermost shell of an ion and the center of the nucleus.
- In terms of size, the cation is usually smaller than its parent atom because of the fewer number of electrons with its nuclear charge being the same.
- The size of an anion is however larger than the parent atom because the addition of one or more electrons results in the creation of more repulsion between the electrons and a lesser nuclear charge.
For example:
Ionic radius of cation of Sodium (Na+) = 95 pm
Atomic Radius of Sodium = 186pm
Ionic Radius: Electrical Charge
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Atoms and ions that have the same number of electrons are termed isoelectronic species. For example, Mg2+, O2–, Na+, and F– have the same number of electrons (10). However, their ionic radius would be different due to their different nuclear charges. The anion will have a smaller radius as it will attract the electrons with greater force in the outermost orbital. Anions having a greater negative charge will further have a larger radius.
Read Also: Class 12 Gauss's Law
Ionic Radius Trends in Periodic Table
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While observing ionic radius in a periodic table, it has been noticed that moving down or across a modern periodic table helps in drawing significant analytical conclusions about the chemical reactions and reactivity of elements.
Check out more detailed notes of Classification of Elements and Periodicity in Properties.
Trends In Ionic Radius Down A Group
As we move down a group, atoms add an extra number of electrons or shells because of which the ionic radius of an element increases while moving down the group.
| Ions | Electronic Configuration | Ionic radius (nm) |
|---|---|---|
| F- | 2,8 | 0.133 |
| Cl- | 2,8,8 | 0.181 |
| K+ | 2,8,8 | 0.138 |
Trends In Ionic Radius Across a Period
While moving across a period, the nature of the elements changes significantly from metallic to non-metallic. The number of electrons keeps on increasing in the outermost shell until a noble gas is formed. Hence, the effective nuclear charge increases while moving across the period, and ionization potential, (which is the energy required for the removal of the outermost loosely bounded electron from an isolated gaseous atom), also increases. Thus, it is difficult to lose electrons as we move across. Hence, the ionic radius initially decreases and then increases and again decreases.
| Ions | Electronic Configuration | Ionic Radius(nm) |
|---|---|---|
| Na+ | 2,8 | 0.102 |
| Mg2+ | 2,8 | 0.072 |
| Al3+ | 2,8 | 0.054 |
| P3- | 2,8,8 | 0.212 |
| S2- | 2,8,8 | 0.184 |
Ionic Radius: Things to remember
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- Ionic radius is the distance between the electron in the outermost shell of an ion and the center of the nucleus.
- A cation has a less ionic radius and thus is usually smaller than its parent atom while an anion is larger in size.
- Atoms and ions that have the same number of electrons are termed isoelectronic species. For example, Mg2+, O2–, Na+, and F-.
- As we move down a group, the ionic radius increases.
- Across a period, the ionic radius initially decreases and then increases and again decreases.
Read More: Coulomb's Law Key Points
Ionic Radius: Sample Questions
Ques. What is an ion? What is the ionic radius? (2 marks)
Ans. - Ion refers to an atom, molecule, or particle with a net electrical charge. The gain or the loss of electrons in atom results in an ion. The loss of electrons results in the formation of a cation. Similarly, the gain of an electron leads to the formation of the anion.
Ionic radius is the distance between the electron in the outermost shell of an ion and the center of the nucleus.
Ques. What is the difference between the atomic and the ionic radius? (2 marks)
Ans. - One of the major differences between the ionic radius and atomic radius is that Atomic radius is basically the radius of a neutral atom. On the other hand, the Ionic radius is the radius of the electrically charged atom, referred to as an ion.
Atomic radius can be calculated as the distance between the nucleus of the atom and its electron cloud. Ionic radius is measured as the distance between two nuclei of two ions depending on their size.
Ques. What are trends in the periodic table? Give an example. (2 marks)
Ans. All the chemical elements have trends in the periodic table. This means when we move across or down the group in a periodic table, the atomic or ionic size of the element might increase and decrease which can be explained using the electronic configuration of elements. For example: across a period, the ionic radius initially decreases and then increases and again decreases.
Ques. What is the electronic configuration of an element? (2 marks)
Ans. The electronic configuration of an element depicts the distribution of electrons in its atomic orbit. For example, the electronic configuration of an atom is 1s2 2s2 2p6. This describes that 1s, 2s, and 2p subshells of the atom have 2, 2, and 6 electrons respectively.
Ques. What is the difference between a cation and an anion? Which one has the largest ionic radius? (2 marks)
Ans. An anion is an ion having one or more electrons. It acquires a negative charge while a Cation is an ion having lost one or more electrons. It acquires a positive charge.
As the cation loses an electron, it has a smaller ionic radius than its parent atom while an anion gains an electron to have a larger ionic radius.
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