Atomic Radius: Definition, Types, Variation, and Periodic Trends of Atomic Radii

Keywords: Periodic table, Atomic radii, Nucleus, Atom, Electrons, Ionic radius, Covalent radius, Metallic radius, Van Der Waals Radius, Bohr radius, Periodic trends

What is Atomic Radii or Atomic Radius?

[Click Here for Sample Questions]

The atomic radius is the distance between the nucleus's centre and the outermost shell containing electrons.

• In other words, it is the distance between the nucleus's core and the place where the electron cloud's density is greatest.
• The atomic radius is defined in basic chemistry as the smallest distance between the nuclei of an atom and the atom's outermost shell.
• The atomic radius of atoms generally decreases from left to right across a period.
• Similarly, the atomic radius of atoms increases from top to bottom within a group in the periodic table. 

Types of Atomic Radius

[Click Here for Sample Questions]

There are three types of atomic radii:

• Covalent Radius
• Ionic Radius
• Metallic Radius

Covalent Radius

In a molecule, a covalent radius is one-half the distance between the nuclei of two covalently linked atoms of the same element.

• As a result, r_covalent = \(\frac{1}{2}\) (internuclear distance between two bonded atoms).
• The bond length is the internuclear distance between two linked atoms.

Therefore,

r_covalent = \(\frac{1}{2}\)( bond length)

Ionic Radius

The ionic radius is the radius of an atom that forms an ionic bond or ion. Atomic bonds limit electrons and nuclei, resulting in ions or atoms with no definite structure.

• The ionic radius is measured in either Armstrong (Å) or picometers (pm).
• The average radius is between 30 and 200 pm. 
• The ionic radius is not constant, but varies according to the spin state of the electrons, coordination number, and a variety of other factors.
• The size of an ion increases as the number of coordination numbers increases.
• An ion with a high electron spin state has a larger ionic size than an ion with a low spin state.
• If we consider the ion's charge, the positive ion will be smaller than the negative ion.

Metallic Radius

A metal lattice or crystal is made up of positive kernels or metal ions that are organized in a certain pattern amid a sea of movable valence electrons. Each kernel is attracted by a number of mobile electrons at the same time, and each mobile electron is attracted by a number of metal ions.

• The metallic bond refers to the force of attraction between mobile electrons and positive kernels.
• In the metallic lattice, it is one-half the internuclear distance between two neighbouring metal ions.
• Because the valence electrons are movable in a metallic lattice, they are only weakly attracted by metal ions or kernels.
• A pair of electrons are strongly attracted by the nuclei of two atoms in a covalent connection.
• As a result, a metallic radius is always greater than its covalent radius.
• The metallic radius of sodium, for example, is 186 pm, but its covalent radius, as measured by its vapour, which exists as Na₂, is 154 pm.
• Potassium has a metallic radius of 231 pm and a covalent radius of 203 pm.

Other Terms related to Atomic Radius

In addition to the atomic radius and covalent radius, two more types of radii are known in chemistry that are based on the theories of two scientists: Vanderwall and Bohr. Other terms related to atomic radius are given below

Van der Waals Radius

It is half the distance between the nuclei of two identical non-bonded solitary atoms of two adjacent identical atoms belonging to two nearby molecules of a solid-state element.

• When an element is in the solid state, the size of the Van der Waals radius is determined by the atomic packing.
• In the solid state, for example, the internuclear distance between two adjacent chlorine atoms of two nearby molecules is 360 pm.
• As a result, the chlorine atom's Van der Waals radius is 180 pm.

Bohr Radius

The Bohr model of the atom predicted the radius of the lowest-energy electron orbit.

• Bohr's radius applies only to atoms and ions that have a single electron, such as hydrogen.
• Although the model itself is no longer used, the Bohr radius for the hydrogen atom is still considered an important physical constant.

Variation of Atomic Radii in the Periodic Table

[Click Here for Sample Questions]

The following are the variations of atomic radii in the periodic table

Variation within the Periodic Table

As we travel from left to right in a period, the Covalent and Van der Waals radii drop as the atomic number increases. The alkali metals on the far left of the periodic table are the biggest in a period.

• The halogens at the far right of the periodic table are the tiniest. Nitrogen has the lowest atomic size.
• After nitrogen, the atomic size of oxygen rises and subsequently falls for fluorine.
• Atoms of inert gases are bigger in size than those of the previous halogens.
• The nuclear charge grows by one unit in each consecutive element as we travel from left to right in a period, but the number of shells remains constant.
• This increased nuclear charge attracts electrons from all shells closer to the nucleus.
• As a result, each individual shell becomes smaller and smaller. As a result, as we travel from left to right in a period, the atomic radius decreases.
• As we get from halogens to inert gases, the atomic radius abruptly rises.
• This is due to the fact that noble gases have entirely filled orbitals. As a result, the inter-electronic is maximized.
• Because they do not form covalent bonds, we express atomic size in terms of vanderwall radius.
• The radius of van der Waals is greater than the radius of covalent bonding.
• As a result, the atomic size of the inert gas in a period is significantly larger than that of the previous halogen.

Variation within a Group

The atomic radii of elements rise as the atomic number in a group increases from top to bottom.

• The primary quantum number grows as we travel along with the group.
• At each following element, a new energy shell is added.
• The valence electrons are moving away from the nucleus.
• As a result, the nucleus's attraction to the electron weakens.
• As a result, the atomic radius grows.

Periodic Trends of Atomic Radii

[Click Here for Sample Questions]

In basic chemistry, we may see several patterns in the characteristics (physical and chemical) of elements as we move down a group or across a column or row in the current periodic table. For example, as we travel down a nonmetal group, the reactivity of the elements diminishes, but it increases when we move down the group of representative metals.

• When two atoms are combined, the distance between them may be used to calculate their atomic size.
• Another way to determine the atomic size of a nonmetallic element is to establish a single covalent connection between two atoms and measure the distance between the two atoms.
• The radius determined by this approach is referred to as the element's covalent radii.
• It is known as a metallic radius in the case of metal.
• It is defined as half the total distance between the nuclei of two adjacent metal ions connected by a metallic link.
• X-ray or other spectroscopic methods are used to determine an atom's atomic radius.
• Elements' atomic radii fluctuate in a predictable way across the periodic table.
• This pattern may be explained by examining the nuclear charge and energy level.
• In general, as we travel from left to right in a period, the atomic radius drops and rises as we move down a group.
• This is due to the fact that in periods, the valence electrons are all in the same outermost shell.
• Moving from left to right, the atomic number increases during the same time period, increasing the effective nuclear charge.
• The atomic radius of elements decreases as attractive forces rise.