Atomic radius

Definition and Explanation

The atomic radius of a chemical element is a measure of the size of its atoms. It is generally stated as being the total distance from an atom's nucleus to the outermost orbital of electrons.

It is the mean distance from the centre of the nucleus to the boundary electron cloud. Since the boundary is not well-defined (there is an uncertainty in the position of the outermost electron ), we have different atomic radii definitions which differ from each other to help explain the mean distance. Periodic trends emerge that help explain how atomic radii change.

Periodic Trends

Effective nuclear charge is one way of expressing the balance between attractive and repulsive forces of electrons when we think about the the outermost valence electrons in an atom . The net positive charge from the nucleus (Effective nuclear charge) felt by the outermost electrons affects the radius of an atom.

Generally, as the Effective nuclear charge of an atom decreases, the atomic radius will increase because there is more screening of the electrons from the nucleus, which decreases the attraction between the nucleus and the electron. This means that the electrons will have a position farther from the nucleus with decreased attraction.

• As the Effective nuclear charge of an atom increase, the atomic radius will decrease.

Types of Atomic Radii

Van der Waals Radius

Definition: It is half the minimum distance between the nuclei of two atoms of the element that are not bound to the same molecule.

• Van der Waals radius is used to define half of the distance between the closest approach of two non-bonded atoms of a given element .

The van der Waals radius, r_w, of an atom is the radius of an imaginary hard sphere which can be used to model the atom for many purposes. It is named after Johannes Diderik van der Waals, winner of the 1910 Nobel Prize in Physics, as he was the first to recognise that atoms were not simply points and to demonstrate the physical consequences of their size through the van der Waals equation of state .