Atomic Radii

What is Radius?

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

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 Atomic Radii?

  • Atomic radius is nothing but the total distance from the nucleus of an atom to the outermost orbital of it’s electron.
  • This is mostly similar to the idea of radius of a circle, where we can consider the nucleus to be the center 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 are 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 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 are 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 on the ion into consideration.

3. Metallic Radius:

The metallic radius is the radius of an atom joined by 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).

Stay tuned with Byju’s to learn more about atom, atomic radii and much more.

Practise This Question

A horse is harnessed to a cart. If the horse tries to pull the cart, the horse must exert a force on the cart. By Newton's third law the cart must then exert an equal and opposite force on the horse. Since the two forces are equal and opposite, they must add to zero, so Newton's second law tells us that the acceleration of the system must be zero and therefore no matter how hard the horse pulls, it can never move the cart.