Atomic Radii

What is an atom?

  • An atom is made up of three tiny kinds of particles called subatomic particles: protons, neutrons, and electrons.
  • The protons and the neutrons make up the center of the atom called the nucleus and the electrons fly around above the nucleus in a small cloud.
  • The electrons carry a negative charge and the protons carry a positive charge.
  • In a neutral atom the number of protons and the number of electrons are equal. Often, but not always, the number of neutrons is the same, too.

What is Radius?

  • The radius of a circle is the distance from the centre point to the edge of the circle.
  • It’s the same distance anywhere on the circle because the circle has radial symmetry.

What is Atomic Radii?


Atomic radius or Atomic Radii is the total distance from the nucleus of an atom to the outermost orbital of its electron. We can define the atomic radius of a chemical element as:

The measure of the size of its atoms, usually the mean or typical distance from the center of the nucleus to the boundary of the surrounding shells of electrons.

  • This is mostly similar to the idea of the radius of a circle, where we can consider the nucleus to be the centre of the circle and the outermost orbital of the electron to be the outer edge of the circle.
  • It is really difficult to determine the atomic radii because of the uncertainty in the position of the outermost electron. We can not say the exact position of the electron.
  • Heisenberg Uncertainty Principle can be used to get a precise measurement of the radius but again it is not a completely correct one.
  • As per the principle, we determine the radius based on the distance between the nuclei of two bonded atoms. The radii of atoms are therefore determined by the bonds they form. An atom will have different radii depending on the bond it forms; so there is no fixed radius of an atom.
  • The different radius is van der Waals radius, ionic radius, metallic radius and covalent radius.

Types of Radius with Respect to the Types of Bonds

1. Van Der Waals Radius:

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

2. Ionic Radius:

  • The ionic radius is the radius of an atom forming an ionic bond or an ion.
  • The electrons and nucleus are restricted by the atomic bonds and due to this reason 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 – 200 pm.
  • The ionic radius is not static but differs with respect to 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 the high spin state of an electron than an ion with a low spin state of the electron.
  • The ion with a positive charge will have a smaller size than the ion with negative charge if we will take charge of the ion into consideration.

3. 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.

4. Covalent Radius:

  • The covalent radius of an atom is stated as the radius of an atom which is 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 of the same kind, then 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).