Atomic Radius – Definition, Types, Periodic Trends

An atom comprises three main subatomic particles: electrons, neutrons, and protons. At the same time, the neutrons and protons lie inside an area at the centre of the atom known as the nucleus; the electrons orbit around this nucleus in a circular path. Atomic radius refers to the total distance from the centre of the nucleus to the orbit that lies at the greatest distance from the nucleus. This blog is focused on the discussion of atomic radius and its characteristics.

What are Atomic Radii or Atomic Radius?

The atomic radius definition: Atomic radius can be defined as the total distance between the centre of the atomic nucleus to the outermost orbit its electrons are revolving in. 

The atomic radius of a chemical element is defined as the average distance from the atom’s nucleus to the boundary of the shells of electrons surrounding it. Atomic radius can be compared to the radius of a circle, where the nucleus corresponds to the centre of the circle, while the outermost orbit can be compared to the outer edge of the circle. Finding atomic radius is difficult as there is uncertainty about the position of the outermost orbit of electrons. 

A precise measurement of the atomic radius can be obtained by using Heisenberg’s Uncertainty Principle, according to which the radius is determined based on the distance between two atoms bonded to each other. Atomic radii are different depending on the type of bond an atom forms. So, an atom has no fixed radius, and the radii depend upon the type of bonds the atoms form. In general, the sodium atomic radius is 227 pm, while the aluminium atomic radius is 143 pm.

Types of Atomic Radius Concerning the Types of Bond

Depending on the type of bond an atom forms, atomic radii are divided into the following three types:

• Ionic radius
• Covalent radius
• Metallic radius

Covalent Radius

Covalent atomic radius definition: This is the radius of an atom that is measured when the atom is covalently bonded with another atom of a similar element. It can be determined by measuring the lengths between the pairs of atoms that are covalently bonded. If the two atoms in the pair are similar, the covalent radius will equal half of the bond’s total length. While this is straightforward for some molecules like CI2 and O2, in some cases, the covalent radius needs to be inferred by measuring the bond lengths of atoms with already known radii.

Ionic Radius

Ionic atomic radius definition: This is the atomic radius that forms an ionic bond. The atomic bonds limit the electrons. Because of this, these ions or atoms lack a particular shape. The unit in which ionic radius is measured in picometers(pm) or Armstrong (Å). Characteristic ionic radii range from 30-200 pm. There is no one ionic radius; rather, it varies concerning the electron spin state, coordination number, and many other factors. With increasing coordination numbers, the size of the ion increases. It also increases for an ion that has a higher spin state of an electron than one with a lower spin state. Considering the charge of an ion, positively charged ions are smaller in size than negatively charged ones. 

Metallic Radius

Metallic atomic radius definition: This is the radius an atom forms when metallic bonds bond with it. It is equal to half of the distance between adjacent atomic nuclei in a metallic cluster. 

Other Terms Related to Atomic Radius

Van Der Waals Radius

We can determine the Van-der-Waals radii from the contact distances between atoms that are not bonded in atoms or molecules touching each other.

Bohr Radius

The Bohr atomic model predicted the radius of the electron orbit with the lowest energy. This radius only applies to ions and atoms that have a single electron. For example, hydrogen atoms. Although the Bohr model of an atom is now obsolete, the radius of the hydrogen atom, as Bohr stated, is still considered an essential physical constant. 

Periodic Trends in Atomic Radii

As the number of electronic shells in an atom increases, it keeps getting larger. Therefore, as we move down a specific group in the periodic table, the radius of the atoms goes on increasing. So the elements at the bottom have the largest atomic radius. While moving from left to right for a certain period, the atomic size generally decreases. Generally, the radius of atoms goes on decreasing while we move across a period and goes on increasing while we move down a group in the periodic table. Therefore, the largest atomic radius belongs to Francium, while the lowest belongs to Helium.

Trends in the Periodic Table

Many trends can be observed in the physical and chemical properties of elements while moving across a row or a column or moving down a group in the modern periodic table. Suppose, while moving down a group of nonmetals, the reactivity of the elements goes on decreasing while it goes on increasing while we go down a group of representative elements. 

By combining two atoms, their atomic size can be estimated by determining the distance between the two atoms. Another method of measuring the atomic size of non-metallic elements is by forming a single covalent bond across two atoms and then measuring the distance between them. The resultant radius of the atom of the element is called the covalent radius. In the case of metallic elements, the radius is called the metallic radius. It can be defined as half of the distance between the nuclei of two adjoining metallic ions bonded by a metallic bond. 

We can measure the atomic radius of an atom by X-ray or other spectroscopy techniques. The variation of the atomic radius in the periodic table follows a fixed pattern. This trend can be explained by considering the energy level and the nuclear charge. 

Generally, there is a decrease in the atomic radius while moving from left to right in a period, while there is an increase in the same while moving down a group. This is due to the fact that the valence electrons in the element’s periods are the same in the outermost shell. There is an increase in the atomic number within the same period when we move from left to right, due to which the effective nuclear charge increases. The atomic radius of elements is decreased due to the increase in attractive forces. So the elements on the left have the largest atomic radius, while those on the right have the smallest atomic radius.

The following is a table summarising the major phenomena influencing an element’s atomic radius:

Factor	Principle	Increase with	Tend to	Effect on Radius
Nuclear charge	An attractive force that acts because of protons in a nucleus on the electrons.	Atomic number	increase on moving from left to right in a period.	The atomic radius is decreased.
Electron shells	Quantum mechanics.	Azimuthal and principal quantum numbers.	Increase when moving down a column.	The atomic radius decreases.
Shielding	The repulsive force is impacted by the inner electrons on the electrons in the outermost shell.	The number of electrons in the inner shells.	Decrease the effect of the first factor.	The atomic radius is increased.

Lanthanide Contraction

In the periodic table, the elements that immediately follow the lanthanides have the smallest atomic radius. The atomic radii are almost similar to those of the elements above them. Therefore, lutetium is smaller than yttrium, and the atomic radius of tantalum is similar to niobium’s atomic radius of hafnium and zirconium is almost the same. This phenomenon is called lanthanide contraction, and its effect is noticeable up to platinum, following which it is masked by the inert pair effect. 

Conclusion

The atomic radius is an important concept in chemistry as the properties of elements depend on this concept. We hope this article helped you gain a basic understanding of the atomic radius, its characteristics, and the trends it follows while we move up or down the periodic table. Interestingly, the atomic radius increases or decreases when the force of attraction between the protons and electrons varies. 

Frequently Asked Questions

Q) Is there an increase in the atomic radius across a period?

A) Generally, the atomic size decreases as we move from left to right in a period of the periodic table. So the elements on the left have the largest atomic radius, while those on the right have the smallest atomic radius. The size of the atom goes on decreasing across a period and increasing down a group. 

Q) Why can’t we measure the atomic radius directly?

A) We can’t directly measure the atomic radius because the position of the outermost electrons in orbit is uncertain. 

Q) Define the Bohr radius.

The Bohr model of an atom predicted the radius of the electron orbit with the lowest energy. The Bohr radius only applies to ions and atoms with a single electron in their shell, like the hydrogen atom.