Atomic Radii

What are Atomic Radii or Atomic Radius?

Atomic radius or Atomic Radii is the total distance from the nucleus of an atom to the outermost orbital of its electron.

We define the atomic radius of a chemical element as:

The mean or typical distance from the centre of the nucleus to the boundary of the surrounding shells of electrons.

Atomic radius is similar to the radius of a circle. The nucleus is analogous to be the centre of the circle and the outermost orbital of the electron to the outer edge of the circle. It is difficult to determine the atomic radii because of the uncertainty in the position of the outermost electron.

We use Heisenberg’s Uncertainty Principle to obtain a precise measurement of the radius. As per the principle, we determine the radius based on the distance between the nuclei of two bonded atoms. An atom will have different radii depending on the bond it forms, so there is no fixed radius of an atom. The radii of atoms are therefore determined by the bonds they form.

By watching the video, you will learn to simplify the concept of atomic radius to a level that you will never forget!

Types of Atomic Radius with Respect to the Types Of Bond

Based on the type of bond, atomic radius is divided into three types as follows:

• Covalent Radius
• Ionic Radius
• Metallic Radius

Covalent Radius

The covalent radius of an atom is the radius of an atom under the covalent bond with another atom(s) of a similar element. The covalent radius of an atom can be determined by measuring bond lengths between pairs of covalently-bonded atoms. If the two atoms are the same kind, the covalent radius is simply one half of the bond length. Whilst this is straightforward for some molecules such as Cl2 and O2, in other cases, one has to infer the covalent radius by measuring bond distances to atoms whose radii are already known (e.g., a C–X bond, in which the radius of C is known).

Ionic Radius

The ionic radius is the radius of an atom forming an ionic bond or an ion. The atomic bonds restrict the electrons and nucleus, and due to this, the ions or atoms don’t have a specific shape. The measuring unit for the ionic radius is Armstrong(A⁰) or picometers(pm). The characteristic radius ranges from 30 to 200 pm. The ionic radius is not static but differs with respect to the spin state of the electrons, coordination number and numerous other parameters. Ionic size grows with increasing coordination number. The ionic size also grows for an ion with a high spin state of an electron than an ion with a low spin state. The ion with a positive charge will have a smaller size than the ion with a negative charge if we consider the charge of the ion.

Metallic Radius

The metallic radius is the radius of an atom joined by the metallic bond. The metallic radius is half of the total distance between the nuclei of two adjacent atoms in a metallic cluster.

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Other terms related to Atomic Radius

Van Der Waals Radius

Van-der-Waals radii are determined from the contact distances between unbonded atoms in touching molecules or atoms.

Bohr Radius

The Bohr model of the atom predicted the radius of the lowest-energy electron orbit. Bohr’s radius is only relevant to atoms and ions with a single electron, such as hydrogen. Although the model itself is now obsolete, the Bohr radius for the hydrogen atom is still regarded as an essential physical constant.

Periodic Trends of Atomic Radii

An atom gets larger as the number of electronic shells increases. Therefore, the radius of the atom increases as one goes down a specific group in the periodic table of elements. The size of an atom generally decreases as one moves from left to right for a certain period. In general, the atomic radius decreases across a period and increases down a group. Francium has the largest atomic radius, while Helium has the lowest.

Frequently Asked Questions – FAQs

Q1

Does atomic radius increase across a period?

The size of an atom generally decreases as one moves from left to right for a certain period. In general, the atomic radius decreases across a period and increases down a group.

Q2

Why can’t atomic radii be measured directly?

It is difficult to determine the atomic radii because of the uncertainty in the position of the outermost electron.

Q3

What is the meaning of atomic radii?

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

Q4

Which element has the largest atomic radius?

Francium has the largest atomic radius.

Q5

What is Bohr radius?

The Bohr model of the atom predicted the radius of the lowest-energy electron orbit. Bohr’s radius is only relevant to atoms and ions with a single electron, such as hydrogen.

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